ty_python_semantic: add union type context to function call type errors

This context gets added only when calling a function through a union
type.

crates/ruff_benchmark/benches/ty.rs

@@ -59,26 +59,37 @@ type KeyDiagnosticFields = (
     Severity,
 );
 
+// left: [
+// (Lint(LintName("invalid-argument-type")), Some("/src/tomllib/_parser.py"), Some(8224..8254), "Argument to function `skip_until` is incorrect", Error),
+// (Lint(LintName("invalid-argument-type")), Some("/src/tomllib/_parser.py"), Some(16914..16948), "Argument to function `skip_until` is incorrect", Error),
+// (Lint(LintName("invalid-argument-type")), Some("/src/tomllib/_parser.py"), Some(17319..17363), "Argument to function `skip_until` is incorrect", Error),
+// ]
+//right: [
+// (Lint(LintName("invalid-argument-type")), Some("/src/tomllib/_parser.py"), Some(8224..8254), "Argument to this function is incorrect", Error),
+// (Lint(LintName("invalid-argument-type")), Some("/src/tomllib/_parser.py"), Some(16914..16948), "Argument to this function is incorrect", Error),
+// (Lint(LintName("invalid-argument-type")), Some("/src/tomllib/_parser.py"), Some(17319..17363), "Argument to this function is incorrect", Error),
+// ]
+
 static EXPECTED_TOMLLIB_DIAGNOSTICS: &[KeyDiagnosticFields] = &[
     (
         DiagnosticId::lint("invalid-argument-type"),
         Some("/src/tomllib/_parser.py"),
         Some(8224..8254),
-        "Argument to this function is incorrect",
+        "Argument to function `skip_until` is incorrect",
         Severity::Error,
     ),
     (
         DiagnosticId::lint("invalid-argument-type"),
         Some("/src/tomllib/_parser.py"),
         Some(16914..16948),
-        "Argument to this function is incorrect",
+        "Argument to function `skip_until` is incorrect",
         Severity::Error,
     ),
     (
         DiagnosticId::lint("invalid-argument-type"),
         Some("/src/tomllib/_parser.py"),
         Some(17319..17363),
-        "Argument to this function is incorrect",
+        "Argument to function `skip_until` is incorrect",
         Severity::Error,
     ),
 ];

crates/ty_python_semantic/resources/mdtest/annotations/new_types.md

@@ -12,7 +12,7 @@ X = GenericAlias(type, ())
 A = NewType("A", int)
 # TODO: typeshed for `typing.GenericAlias` uses `type` for the first argument. `NewType` should be special-cased
 # to be compatible with `type`
-# error: [invalid-argument-type] "Argument to this function is incorrect: Expected `type`, found `NewType`"
+# error: [invalid-argument-type] "Argument to function `__new__` is incorrect: Expected `type`, found `NewType`"
 B = GenericAlias(A, ())
 
 def _(

crates/ty_python_semantic/resources/mdtest/call/annotation.md

@@ -9,8 +9,8 @@ def _(c: Callable[[], int]):
 def _(c: Callable[[int, str], int]):
     reveal_type(c(1, "a"))  # revealed: int
 
-    # error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["a"]`"
-    # error: [invalid-argument-type] "Argument to this function is incorrect: Expected `str`, found `Literal[1]`"
+    # error: [invalid-argument-type] "Argument is incorrect: Expected `int`, found `Literal["a"]`"
+    # error: [invalid-argument-type] "Argument is incorrect: Expected `str`, found `Literal[1]`"
     reveal_type(c("a", 1))  # revealed: int
 ```
 

crates/ty_python_semantic/resources/mdtest/call/callable_instance.md

@@ -85,7 +85,7 @@ class C:
 
 c = C()
 
-# error: 15 [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["foo"]`"
+# error: 15 [invalid-argument-type] "Argument to bound method `__call__` is incorrect: Expected `int`, found `Literal["foo"]`"
 reveal_type(c("foo"))  # revealed: int
 ```
 
@@ -99,7 +99,7 @@ class C:
 
 c = C()
 
-# error: 13 [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `C`"
+# error: 13 [invalid-argument-type] "Argument to bound method `__call__` is incorrect: Expected `int`, found `C`"
 reveal_type(c())  # revealed: int
 ```
 

crates/ty_python_semantic/resources/mdtest/call/constructor.md

@@ -99,8 +99,8 @@ def _(flag: bool) -> None:
             def __new__(cls, x: int, y: int = 1): ...
 
     reveal_type(Foo(1))  # revealed: Foo
-    # error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["1"]`"
-    # error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["1"]`"
+    # error: [invalid-argument-type] "Argument to function `__new__` is incorrect: Expected `int`, found `Literal["1"]`"
+    # error: [invalid-argument-type] "Argument to function `__new__` is incorrect: Expected `int`, found `Literal["1"]`"
     reveal_type(Foo("1"))  # revealed: Foo
     # error: [missing-argument] "No argument provided for required parameter `x` of function `__new__`"
     # error: [missing-argument] "No argument provided for required parameter `x` of function `__new__`"
@@ -232,8 +232,8 @@ def _(flag: bool) -> None:
             def __init__(self, x: int, y: int = 1): ...
 
     reveal_type(Foo(1))  # revealed: Foo
-    # error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["1"]`"
-    # error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["1"]`"
+    # error: [invalid-argument-type] "Argument to bound method `__init__` is incorrect: Expected `int`, found `Literal["1"]`"
+    # error: [invalid-argument-type] "Argument to bound method `__init__` is incorrect: Expected `int`, found `Literal["1"]`"
     reveal_type(Foo("1"))  # revealed: Foo
     # error: [missing-argument] "No argument provided for required parameter `x` of bound method `__init__`"
     # error: [missing-argument] "No argument provided for required parameter `x` of bound method `__init__`"

crates/ty_python_semantic/resources/mdtest/call/function.md

@@ -77,7 +77,7 @@ def _(flag: bool):
 def f(x: int) -> int:
     return 1
 
-# error: 15 [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["foo"]`"
+# error: 15 [invalid-argument-type] "Argument to function `f` is incorrect: Expected `int`, found `Literal["foo"]`"
 reveal_type(f("foo"))  # revealed: int
 ```
 
@@ -87,7 +87,7 @@ reveal_type(f("foo"))  # revealed: int
 def f(x: int, /) -> int:
     return 1
 
-# error: 15 [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["foo"]`"
+# error: 15 [invalid-argument-type] "Argument to function `f` is incorrect: Expected `int`, found `Literal["foo"]`"
 reveal_type(f("foo"))  # revealed: int
 ```
 
@@ -97,7 +97,7 @@ reveal_type(f("foo"))  # revealed: int
 def f(*args: int) -> int:
     return 1
 
-# error: 15 [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["foo"]`"
+# error: 15 [invalid-argument-type] "Argument to function `f` is incorrect: Expected `int`, found `Literal["foo"]`"
 reveal_type(f("foo"))  # revealed: int
 ```
 
@@ -107,7 +107,7 @@ reveal_type(f("foo"))  # revealed: int
 def f(x: int) -> int:
     return 1
 
-# error: 15 [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["foo"]`"
+# error: 15 [invalid-argument-type] "Argument to function `f` is incorrect: Expected `int`, found `Literal["foo"]`"
 reveal_type(f(x="foo"))  # revealed: int
 ```
 
@@ -117,7 +117,7 @@ reveal_type(f(x="foo"))  # revealed: int
 def f(*, x: int) -> int:
     return 1
 
-# error: 15 [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["foo"]`"
+# error: 15 [invalid-argument-type] "Argument to function `f` is incorrect: Expected `int`, found `Literal["foo"]`"
 reveal_type(f(x="foo"))  # revealed: int
 ```
 
@@ -127,7 +127,7 @@ reveal_type(f(x="foo"))  # revealed: int
 def f(**kwargs: int) -> int:
     return 1
 
-# error: 15 [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["foo"]`"
+# error: 15 [invalid-argument-type] "Argument to function `f` is incorrect: Expected `int`, found `Literal["foo"]`"
 reveal_type(f(x="foo"))  # revealed: int
 ```
 
@@ -137,8 +137,8 @@ reveal_type(f(x="foo"))  # revealed: int
 def f(x: int = 1, y: str = "foo") -> int:
     return 1
 
-# error: 15 [invalid-argument-type] "Argument to this function is incorrect: Expected `str`, found `Literal[2]`"
-# error: 20 [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["bar"]`"
+# error: 15 [invalid-argument-type] "Argument to function `f` is incorrect: Expected `str`, found `Literal[2]`"
+# error: 20 [invalid-argument-type] "Argument to function `f` is incorrect: Expected `int`, found `Literal["bar"]`"
 reveal_type(f(y=2, x="bar"))  # revealed: int
 ```
 

crates/ty_python_semantic/resources/mdtest/call/getattr_static.md

@@ -115,7 +115,7 @@ inspect.getattr_static()
 # error: [missing-argument] "No argument provided for required parameter `attr`"
 inspect.getattr_static(C())
 
-# error: [invalid-argument-type] "Argument to this function is incorrect: Expected `str`, found `Literal[1]`"
+# error: [invalid-argument-type] "Argument to function `getattr_static` is incorrect: Expected `str`, found `Literal[1]`"
 inspect.getattr_static(C(), 1)
 
 # error: [too-many-positional-arguments] "Too many positional arguments to function `getattr_static`: expected 3, got 4"

crates/ty_python_semantic/resources/mdtest/call/invalid_syntax.md

@@ -24,7 +24,7 @@ to the valid order:
 def f(**kw: int, x: str) -> int:
     return 1
 
-# error: 15 [invalid-argument-type] "Argument to this function is incorrect: Expected `str`, found `Literal[1]`"
+# error: 15 [invalid-argument-type] "Argument to function `f` is incorrect: Expected `str`, found `Literal[1]`"
 reveal_type(f(1))  # revealed: int
 ```
 
@@ -38,7 +38,7 @@ def f(x: int = 1, y: str) -> int:
     return 1
 
 reveal_type(f(y="foo"))  # revealed: int
-# error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["foo"]`"
+# error: [invalid-argument-type] "Argument to function `f` is incorrect: Expected `int`, found `Literal["foo"]`"
 # error: [missing-argument] "No argument provided for required parameter `y` of function `f`"
 reveal_type(f("foo"))  # revealed: int
 ```

crates/ty_python_semantic/resources/mdtest/call/methods.md

@@ -350,7 +350,7 @@ class D:
         # This function is wrongly annotated, it should be `type[D]` instead of `D`
         pass
 
-# error: [invalid-argument-type] "Argument to this function is incorrect: Expected `D`, found `<class 'D'>`"
+# error: [invalid-argument-type] "Argument to bound method `f` is incorrect: Expected `D`, found `<class 'D'>`"
 D.f()
 ```
 

crates/ty_python_semantic/resources/mdtest/call/subclass_of.md

@@ -20,7 +20,7 @@ class C:
 def _(subclass_of_c: type[C]):
     reveal_type(subclass_of_c(1))  # revealed: C
 
-    # error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["a"]`"
+    # error: [invalid-argument-type] "Argument to bound method `__init__` is incorrect: Expected `int`, found `Literal["a"]`"
     reveal_type(subclass_of_c("a"))  # revealed: C
     # error: [missing-argument] "No argument provided for required parameter `x` of bound method `__init__`"
     reveal_type(subclass_of_c())  # revealed: C

crates/ty_python_semantic/resources/mdtest/call/union.md

@@ -94,7 +94,7 @@ def _(flag: bool):
     else:
         f = f2
 
-    # error: [invalid-argument-type] "Argument to this function is incorrect: Expected `str`, found `Literal[3]`"
+    # error: [invalid-argument-type] "Argument to function `f2` is incorrect: Expected `str`, found `Literal[3]`"
     x = f(3)
     reveal_type(x)  # revealed: int | str
 ```

crates/ty_python_semantic/resources/mdtest/decorators.md

@@ -207,7 +207,7 @@ first argument:
 def wrong_signature(f: int) -> str:
     return "a"
 
-# error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `def f(x) -> Unknown`"
+# error: [invalid-argument-type] "Argument to function `wrong_signature` is incorrect: Expected `int`, found `def f(x) -> Unknown`"
 @wrong_signature
 def f(x): ...
 

crates/ty_python_semantic/resources/mdtest/diagnostics/union_call.md

@@ -99,9 +99,9 @@ def _(n: int):
     else:
         f = PossiblyNotCallable()
     # error: [too-many-positional-arguments]
-    # error: [invalid-argument-type] "Argument to this function is incorrect: Expected `str`, found `Literal[3]`"
+    # error: [invalid-argument-type] "Argument to function `f2` is incorrect: Expected `str`, found `Literal[3]`"
     # error: [missing-argument]
-    # error: [invalid-argument-type] "Argument to this function is incorrect: Argument type `Literal[3]` does not satisfy upper bound of type variable `T`"
+    # error: [invalid-argument-type] "Argument to function `f4` is incorrect: Argument type `Literal[3]` does not satisfy upper bound of type variable `T`"
     # error: [call-non-callable] "Object of type `Literal[5]` is not callable"
     # error: [no-matching-overload]
     # error: [call-non-callable] "Object of type `PossiblyNotCallable` is not callable (possibly unbound `__call__` method)"

crates/ty_python_semantic/resources/mdtest/doc/public_type_undeclared_symbols.md

@@ -42,7 +42,7 @@ def f(w: Wrapper) -> None:
     v: int | None = w.value
 
     # This function call is incorrect, because `w.value` could be `None`. We therefore emit the following
-    # error: "Argument to this function is incorrect: Expected `int`, found `Unknown | None`"
+    # error: "Argument to function `accepts_int` is incorrect: Expected `int`, found `Unknown | None`"
     c = accepts_int(w.value)
 ```
 

crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md

@@ -123,11 +123,11 @@ reveal_type(Bounded[int]())  # revealed: Bounded[int]
 reveal_type(Bounded[IntSubclass]())  # revealed: Bounded[IntSubclass]
 
 # TODO: update this diagnostic to talk about type parameters and specializations
-# error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `str`"
+# error: [invalid-argument-type] "Argument to class `Bounded` is incorrect: Expected `int`, found `str`"
 reveal_type(Bounded[str]())  # revealed: Unknown
 
 # TODO: update this diagnostic to talk about type parameters and specializations
-# error:  [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `int | str`"
+# error:  [invalid-argument-type] "Argument to class `Bounded` is incorrect: Expected `int`, found `int | str`"
 reveal_type(Bounded[int | str]())  # revealed: Unknown
 
 reveal_type(BoundedByUnion[int]())  # revealed: BoundedByUnion[int]
@@ -156,7 +156,7 @@ reveal_type(Constrained[str]())  # revealed: Constrained[str]
 reveal_type(Constrained[int | str]())  # revealed: Constrained[int | str]
 
 # TODO: update this diagnostic to talk about type parameters and specializations
-# error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int | str`, found `object`"
+# error: [invalid-argument-type] "Argument to class `Constrained` is incorrect: Expected `int | str`, found `object`"
 reveal_type(Constrained[object]())  # revealed: Unknown
 ```
 

crates/ty_python_semantic/resources/mdtest/generics/pep695/classes.md

@@ -98,11 +98,11 @@ reveal_type(Bounded[int]())  # revealed: Bounded[int]
 reveal_type(Bounded[IntSubclass]())  # revealed: Bounded[IntSubclass]
 
 # TODO: update this diagnostic to talk about type parameters and specializations
-# error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `str`"
+# error: [invalid-argument-type] "Argument to class `Bounded` is incorrect: Expected `int`, found `str`"
 reveal_type(Bounded[str]())  # revealed: Unknown
 
 # TODO: update this diagnostic to talk about type parameters and specializations
-# error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `int | str`"
+# error: [invalid-argument-type] "Argument to class `Bounded` is incorrect: Expected `int`, found `int | str`"
 reveal_type(Bounded[int | str]())  # revealed: Unknown
 
 reveal_type(BoundedByUnion[int]())  # revealed: BoundedByUnion[int]
@@ -129,7 +129,7 @@ reveal_type(Constrained[str]())  # revealed: Constrained[str]
 reveal_type(Constrained[int | str]())  # revealed: Constrained[int | str]
 
 # TODO: update this diagnostic to talk about type parameters and specializations
-# error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int | str`, found `object`"
+# error: [invalid-argument-type] "Argument to class `Constrained` is incorrect: Expected `int | str`, found `object`"
 reveal_type(Constrained[object]())  # revealed: Unknown
 ```
 

crates/ty_python_semantic/resources/mdtest/generics/scoping.md

@@ -84,7 +84,7 @@ class C[T]:
 c: C[int] = C[int]()
 c.m1(1)
 c.m2(1)
-# error: [invalid-argument-type] "Argument to this function is incorrect: Expected `int`, found `Literal["string"]`"
+# error: [invalid-argument-type] "Argument to bound method `m2` is incorrect: Expected `int`, found `Literal["string"]`"
 c.m2("string")
 ```
 

crates/ty_python_semantic/resources/mdtest/properties.md

@@ -146,7 +146,7 @@ class C:
     @property
     def attr(self) -> int:
         return 1
-    # error: [invalid-argument-type] "Argument to this function is incorrect: Expected `(Any, Any, /) -> None`, found `def attr(self) -> None`"
+    # error: [invalid-argument-type] "Argument to bound method `setter` is incorrect: Expected `(Any, Any, /) -> None`, found `def attr(self) -> None`"
     @attr.setter
     def attr(self) -> None:
         pass
@@ -156,7 +156,7 @@ class C:
 
 ```py
 class C:
-    # error: [invalid-argument-type] "Argument to this function is incorrect: Expected `((Any, /) -> Any) | None`, found `def attr(self, x: int) -> int`"
+    # error: [invalid-argument-type] "Argument to class `property` is incorrect: Expected `((Any, /) -> Any) | None`, found `def attr(self, x: int) -> int`"
     @property
     def attr(self, x: int) -> int:
         return 1

crates/ty_python_semantic/resources/mdtest/snapshots/functions.md_-_Generic_functions___Legacy_syntax_-_Inferring_a_bound_typevar.snap

@@ -68,7 +68,7 @@ info[revealed-type]: Revealed type
 ```
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `f` is incorrect
   --> src/mdtest_snippet.py:12:15
    |
 10 | reveal_type(f(True))  # revealed: Literal[True]

crates/ty_python_semantic/resources/mdtest/snapshots/functions.md_-_Generic_functions___Legacy_syntax_-_Inferring_a_constrained_typevar.snap

@@ -83,7 +83,7 @@ info[revealed-type]: Revealed type
 ```
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `f` is incorrect
   --> src/mdtest_snippet.py:13:15
    |
 11 | reveal_type(f(None))  # revealed: None

crates/ty_python_semantic/resources/mdtest/snapshots/functions.md_-_Generic_functions___PEP_695_syntax_-_Inferring_a_bound_typevar.snap

@@ -65,7 +65,7 @@ info[revealed-type]: Revealed type
 ```
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `f` is incorrect
  --> src/mdtest_snippet.py:9:15
   |
 7 | reveal_type(f(True))  # revealed: Literal[True]

crates/ty_python_semantic/resources/mdtest/snapshots/functions.md_-_Generic_functions___PEP_695_syntax_-_Inferring_a_constrained_typevar.snap

@@ -80,7 +80,7 @@ info[revealed-type]: Revealed type
 ```
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `f` is incorrect
   --> src/mdtest_snippet.py:10:15
    |
  8 | reveal_type(f(None))  # revealed: None

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Basic.snap

@@ -21,7 +21,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `foo` is incorrect
  --> src/mdtest_snippet.py:4:5
   |
 2 |     return x * x

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Calls_to_methods.snap

@@ -23,7 +23,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to bound method `square` is incorrect
  --> src/mdtest_snippet.py:6:10
   |
 5 | c = C()

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Different_files.snap

@@ -27,7 +27,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `foo` is incorrect
  --> src/mdtest_snippet.py:3:13
   |
 1 | import package

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Different_source_order.snap

@@ -22,7 +22,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `foo` is incorrect
  --> src/mdtest_snippet.py:2:9
   |
 1 | def bar():

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Many_parameters.snap

@@ -21,7 +21,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `foo` is incorrect
  --> src/mdtest_snippet.py:4:8
   |
 2 |     return x * y * z

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Many_parameters_across_multiple_lines.snap

@@ -25,7 +25,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `foo` is incorrect
  --> src/mdtest_snippet.py:8:8
   |
 6 |     return x * y * z

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Many_parameters_with_multiple_invalid_arguments.snap

@@ -24,7 +24,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `foo` is incorrect
  --> src/mdtest_snippet.py:7:5
   |
 5 | # error: [invalid-argument-type]
@@ -44,7 +44,7 @@ info: `invalid-argument-type` is enabled by default
 ```
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `foo` is incorrect
  --> src/mdtest_snippet.py:7:10
   |
 5 | # error: [invalid-argument-type]
@@ -64,7 +64,7 @@ info: `invalid-argument-type` is enabled by default
 ```
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `foo` is incorrect
  --> src/mdtest_snippet.py:7:15
   |
 5 | # error: [invalid-argument-type]

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Test_calling_a_function_whose_type_is_vendored_from_`typeshed`.snap

@@ -20,7 +20,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `loads` is incorrect
  --> src/mdtest_snippet.py:3:12
   |
 1 | import json

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Tests_for_a_variety_of_argument_types_-_Keyword_only_arguments.snap

@@ -21,7 +21,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `foo` is incorrect
  --> src/mdtest_snippet.py:4:11
   |
 2 |     return x * y * z

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Tests_for_a_variety_of_argument_types_-_Mix_of_arguments.snap

@@ -21,7 +21,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `foo` is incorrect
  --> src/mdtest_snippet.py:4:11
   |
 2 |     return x * y * z

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Tests_for_a_variety_of_argument_types_-_One_keyword_argument.snap

@@ -21,7 +21,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `foo` is incorrect
  --> src/mdtest_snippet.py:4:11
   |
 2 |     return x * y * z

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Tests_for_a_variety_of_argument_types_-_Only_positional.snap

@@ -21,7 +21,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `foo` is incorrect
  --> src/mdtest_snippet.py:4:8
   |
 2 |     return x * y * z

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Tests_for_a_variety_of_argument_types_-_Synthetic_arguments.snap

@@ -23,7 +23,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to bound method `__call__` is incorrect
  --> src/mdtest_snippet.py:6:3
   |
 5 | c = C()

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Tests_for_a_variety_of_argument_types_-_Variadic_arguments.snap

@@ -21,7 +21,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `foo` is incorrect
  --> src/mdtest_snippet.py:4:14
   |
 2 |     return len(numbers)

crates/ty_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Tests_for_a_variety_of_argument_types_-_Variadic_keyword_arguments.snap

@@ -21,7 +21,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/invalid_argu
 # Diagnostics
 
 ```
-error[invalid-argument-type]: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `foo` is incorrect
  --> src/mdtest_snippet.py:4:20
   |
 2 |     return len(numbers)

crates/ty_python_semantic/resources/mdtest/snapshots/union_call.md_-_Calling_a_union_of_function_types_-_A_smaller_scale_example.snap

@@ -31,7 +31,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/union_call.m
 # Diagnostics
 
 ```
-error: lint:invalid-argument-type: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `f2` is incorrect
   --> src/mdtest_snippet.py:14:11
    |
 12 |     # error: [too-many-positional-arguments]
@@ -48,12 +48,14 @@ info: Function defined here
   |     ^^ --------- Parameter declared here
 5 |     return 0
   |
-info: `lint:invalid-argument-type` is enabled by default
+info: Union variant `def f2(name: str) -> int` is incompatible with this call site
+info: Attempted to call union type `(def f1() -> int) | (def f2(name: str) -> int)`
+info: `invalid-argument-type` is enabled by default
 
 ```
 
 ```
-error: lint:too-many-positional-arguments: Too many positional arguments to function `f1`: expected 0, got 1
+error[too-many-positional-arguments]: Too many positional arguments to function `f1`: expected 0, got 1
   --> src/mdtest_snippet.py:14:11
    |
 12 |     # error: [too-many-positional-arguments]
@@ -61,6 +63,8 @@ error: lint:too-many-positional-arguments: Too many positional arguments to func
 14 |     x = f(3)
    |           ^
    |
-info: `lint:too-many-positional-arguments` is enabled by default
+info: Union variant `def f1() -> int` is incompatible with this call site
+info: Attempted to call union type `(def f1() -> int) | (def f2(name: str) -> int)`
+info: `too-many-positional-arguments` is enabled by default
 
 ```

crates/ty_python_semantic/resources/mdtest/snapshots/union_call.md_-_Calling_a_union_of_function_types_-_Multiple_variants_but_only_one_is_invalid.snap

@@ -30,7 +30,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/union_call.m
 # Diagnostics
 
 ```
-error: lint:invalid-argument-type: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `f2` is incorrect
   --> src/mdtest_snippet.py:13:11
    |
 11 |         f = f2
@@ -47,6 +47,8 @@ info: Function defined here
   |     ^^ --------- Parameter declared here
 5 |     return 0
   |
-info: `lint:invalid-argument-type` is enabled by default
+info: Union variant `def f2(name: str) -> int` is incompatible with this call site
+info: Attempted to call union type `(def f1(a: int) -> int) | (def f2(name: str) -> int)`
+info: `invalid-argument-type` is enabled by default
 
 ```

crates/ty_python_semantic/resources/mdtest/snapshots/union_call.md_-_Calling_a_union_of_function_types_-_Try_to_cover_all_possible_reasons_-_Cover_keyword_argument_related_reasons.snap

@@ -31,7 +31,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/union_call.m
 # Diagnostics
 
 ```
-error: lint:parameter-already-assigned: Multiple values provided for parameter `name` of function `f1`
+error[parameter-already-assigned]: Multiple values provided for parameter `name` of function `f1`
   --> src/mdtest_snippet.py:14:18
    |
 12 |     # error: [parameter-already-assigned]
@@ -39,12 +39,14 @@ error: lint:parameter-already-assigned: Multiple values provided for parameter `
 14 |     y = f("foo", name="bar", unknown="quux")
    |                  ^^^^^^^^^^
    |
-info: `lint:parameter-already-assigned` is enabled by default
+info: Union variant `def f1(name: str) -> int` is incompatible with this call site
+info: Attempted to call union type `(def f1(name: str) -> int) | (def any(*args, **kwargs) -> int)`
+info: `parameter-already-assigned` is enabled by default
 
 ```
 
 ```
-error: lint:unknown-argument: Argument `unknown` does not match any known parameter of function `f1`
+error[unknown-argument]: Argument `unknown` does not match any known parameter of function `f1`
   --> src/mdtest_snippet.py:14:30
    |
 12 |     # error: [parameter-already-assigned]
@@ -52,6 +54,8 @@ error: lint:unknown-argument: Argument `unknown` does not match any known parame
 14 |     y = f("foo", name="bar", unknown="quux")
    |                              ^^^^^^^^^^^^^^
    |
-info: `lint:unknown-argument` is enabled by default
+info: Union variant `def f1(name: str) -> int` is incompatible with this call site
+info: Attempted to call union type `(def f1(name: str) -> int) | (def any(*args, **kwargs) -> int)`
+info: `unknown-argument` is enabled by default
 
 ```

crates/ty_python_semantic/resources/mdtest/snapshots/union_call.md_-_Calling_a_union_of_function_types_-_Try_to_cover_all_possible_reasons_-_Cover_non-keyword_related_reasons.snap

@@ -53,9 +53,9 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/union_call.m
 39 |     else:
 40 |         f = PossiblyNotCallable()
 41 |     # error: [too-many-positional-arguments]
-42 |     # error: [invalid-argument-type] "Argument to this function is incorrect: Expected `str`, found `Literal[3]`"
+42 |     # error: [invalid-argument-type] "Argument to function `f2` is incorrect: Expected `str`, found `Literal[3]`"
 43 |     # error: [missing-argument]
-44 |     # error: [invalid-argument-type] "Argument to this function is incorrect: Argument type `Literal[3]` does not satisfy upper bound of type variable `T`"
+44 |     # error: [invalid-argument-type] "Argument to function `f4` is incorrect: Argument type `Literal[3]` does not satisfy upper bound of type variable `T`"
 45 |     # error: [call-non-callable] "Object of type `Literal[5]` is not callable"
 46 |     # error: [no-matching-overload]
 47 |     # error: [call-non-callable] "Object of type `PossiblyNotCallable` is not callable (possibly unbound `__call__` method)"
@@ -65,7 +65,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/union_call.m
 # Diagnostics
 
 ```
-error: lint:call-non-callable: Object of type `Literal[5]` is not callable
+error[call-non-callable]: Object of type `Literal[5]` is not callable
   --> src/mdtest_snippet.py:48:9
    |
 46 |     # error: [no-matching-overload]
@@ -73,12 +73,14 @@ error: lint:call-non-callable: Object of type `Literal[5]` is not callable
 48 |     x = f(3)
    |         ^^^^
    |
-info: `lint:call-non-callable` is enabled by default
+info: Union variant `Literal[5]` is incompatible with this call site
+info: Attempted to call union type `(def f1() -> int) | (def f2(name: str) -> int) | (def f3(a: int, b: int) -> int) | (def f4(x: T) -> int) | Literal[5] | Unknown | (<method-wrapper `__get__` of `f`>) | PossiblyNotCallable`
+info: `call-non-callable` is enabled by default
 
 ```
 
 ```
-error: lint:call-non-callable: Object of type `PossiblyNotCallable` is not callable (possibly unbound `__call__` method)
+error[call-non-callable]: Object of type `PossiblyNotCallable` is not callable (possibly unbound `__call__` method)
   --> src/mdtest_snippet.py:48:9
    |
 46 |     # error: [no-matching-overload]
@@ -86,12 +88,14 @@ error: lint:call-non-callable: Object of type `PossiblyNotCallable` is not calla
 48 |     x = f(3)
    |         ^^^^
    |
-info: `lint:call-non-callable` is enabled by default
+info: Union variant `PossiblyNotCallable` is incompatible with this call site
+info: Attempted to call union type `(def f1() -> int) | (def f2(name: str) -> int) | (def f3(a: int, b: int) -> int) | (def f4(x: T) -> int) | Literal[5] | Unknown | (<method-wrapper `__get__` of `f`>) | PossiblyNotCallable`
+info: `call-non-callable` is enabled by default
 
 ```
 
 ```
-error: lint:missing-argument: No argument provided for required parameter `b` of function `f3`
+error[missing-argument]: No argument provided for required parameter `b` of function `f3`
   --> src/mdtest_snippet.py:48:9
    |
 46 |     # error: [no-matching-overload]
@@ -99,12 +103,14 @@ error: lint:missing-argument: No argument provided for required parameter `b` of
 48 |     x = f(3)
    |         ^^^^
    |
-info: `lint:missing-argument` is enabled by default
+info: Union variant `def f3(a: int, b: int) -> int` is incompatible with this call site
+info: Attempted to call union type `(def f1() -> int) | (def f2(name: str) -> int) | (def f3(a: int, b: int) -> int) | (def f4(x: T) -> int) | Literal[5] | Unknown | (<method-wrapper `__get__` of `f`>) | PossiblyNotCallable`
+info: `missing-argument` is enabled by default
 
 ```
 
 ```
-error: lint:no-matching-overload: No overload of method wrapper `__get__` of function `f` matches arguments
+error[no-matching-overload]: No overload of method wrapper `__get__` of function `f` matches arguments
   --> src/mdtest_snippet.py:48:9
    |
 46 |     # error: [no-matching-overload]
@@ -112,12 +118,14 @@ error: lint:no-matching-overload: No overload of method wrapper `__get__` of fun
 48 |     x = f(3)
    |         ^^^^
    |
-info: `lint:no-matching-overload` is enabled by default
+info: Union variant `<method-wrapper `__get__` of `f`>` is incompatible with this call site
+info: Attempted to call union type `(def f1() -> int) | (def f2(name: str) -> int) | (def f3(a: int, b: int) -> int) | (def f4(x: T) -> int) | Literal[5] | Unknown | (<method-wrapper `__get__` of `f`>) | PossiblyNotCallable`
+info: `no-matching-overload` is enabled by default
 
 ```
 
 ```
-error: lint:invalid-argument-type: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `f2` is incorrect
   --> src/mdtest_snippet.py:48:11
    |
 46 |     # error: [no-matching-overload]
@@ -134,12 +142,14 @@ info: Function defined here
   |     ^^ --------- Parameter declared here
 7 |     return 0
   |
-info: `lint:invalid-argument-type` is enabled by default
+info: Union variant `def f2(name: str) -> int` is incompatible with this call site
+info: Attempted to call union type `(def f1() -> int) | (def f2(name: str) -> int) | (def f3(a: int, b: int) -> int) | (def f4(x: T) -> int) | Literal[5] | Unknown | (<method-wrapper `__get__` of `f`>) | PossiblyNotCallable`
+info: `invalid-argument-type` is enabled by default
 
 ```
 
 ```
-error: lint:invalid-argument-type: Argument to this function is incorrect
+error[invalid-argument-type]: Argument to function `f4` is incorrect
   --> src/mdtest_snippet.py:48:11
    |
 46 |     # error: [no-matching-overload]
@@ -156,12 +166,14 @@ info: Type variable defined here
    |        ^^^^^^
 13 |     return 0
    |
-info: `lint:invalid-argument-type` is enabled by default
+info: Union variant `def f4(x: T) -> int` is incompatible with this call site
+info: Attempted to call union type `(def f1() -> int) | (def f2(name: str) -> int) | (def f3(a: int, b: int) -> int) | (def f4(x: T) -> int) | Literal[5] | Unknown | (<method-wrapper `__get__` of `f`>) | PossiblyNotCallable`
+info: `invalid-argument-type` is enabled by default
 
 ```
 
 ```
-error: lint:too-many-positional-arguments: Too many positional arguments to function `f1`: expected 0, got 1
+error[too-many-positional-arguments]: Too many positional arguments to function `f1`: expected 0, got 1
   --> src/mdtest_snippet.py:48:11
    |
 46 |     # error: [no-matching-overload]
@@ -169,6 +181,8 @@ error: lint:too-many-positional-arguments: Too many positional arguments to func
 48 |     x = f(3)
    |           ^
    |
-info: `lint:too-many-positional-arguments` is enabled by default
+info: Union variant `def f1() -> int` is incompatible with this call site
+info: Attempted to call union type `(def f1() -> int) | (def f2(name: str) -> int) | (def f3(a: int, b: int) -> int) | (def f4(x: T) -> int) | Literal[5] | Unknown | (<method-wrapper `__get__` of `f`>) | PossiblyNotCallable`
+info: `too-many-positional-arguments` is enabled by default
 
 ```

crates/ty_python_semantic/src/types/call/bind.rs

@@ -24,7 +24,7 @@ use crate::types::{
     FunctionType, KnownClass, KnownFunction, KnownInstanceType, MethodWrapperKind,
     PropertyInstanceType, TupleType, UnionType, WrapperDescriptorKind,
 };
-use ruff_db::diagnostic::{Annotation, Severity, SubDiagnostic};
+use ruff_db::diagnostic::{Annotation, Diagnostic, Severity, SubDiagnostic};
 use ruff_python_ast as ast;
 
 /// Binding information for a possible union of callables. At a call site, the arguments must be
@@ -199,8 +199,19 @@ impl<'db> Bindings<'db> {
             }
         }
 
+        // If this is not a union, then report a diagnostic for any
+        // errors as normal.
+        if let Some(binding) = self.single_element() {
+            binding.report_diagnostics(context, node, None);
+            return;
+        }
+
         for binding in self {
-            binding.report_diagnostics(context, node);
+            let union_diag = UnionDiagnostic {
+                callable_type: self.callable_type(),
+                binding,
+            };
+            binding.report_diagnostics(context, node, Some(&union_diag));
         }
     }
 
@@ -1043,23 +1054,34 @@ impl<'db> CallableBinding<'db> {
         Type::unknown()
     }
 
-    fn report_diagnostics(&self, context: &InferContext<'db>, node: ast::AnyNodeRef) {
+    fn report_diagnostics(
+        &self,
+        context: &InferContext<'db>,
+        node: ast::AnyNodeRef,
+        union_diag: Option<&UnionDiagnostic<'_, '_>>,
+    ) {
         if !self.is_callable() {
             if let Some(builder) = context.report_lint(&CALL_NON_CALLABLE, node) {
-                builder.into_diagnostic(format_args!(
+                let mut diag = builder.into_diagnostic(format_args!(
                     "Object of type `{}` is not callable",
                     self.callable_type.display(context.db()),
                 ));
+                if let Some(union_diag) = union_diag {
+                    union_diag.add_union_context(context.db(), &mut diag);
+                }
             }
             return;
         }
 
         if self.dunder_call_is_possibly_unbound {
             if let Some(builder) = context.report_lint(&CALL_NON_CALLABLE, node) {
-                builder.into_diagnostic(format_args!(
+                let mut diag = builder.into_diagnostic(format_args!(
                     "Object of type `{}` is not callable (possibly unbound `__call__` method)",
                     self.callable_type.display(context.db()),
                 ));
+                if let Some(union_diag) = union_diag {
+                    union_diag.add_union_context(context.db(), &mut diag);
+                }
             }
             return;
         }
@@ -1067,7 +1089,7 @@ impl<'db> CallableBinding<'db> {
         let callable_description = CallableDescription::new(context.db(), self.callable_type);
         if self.overloads.len() > 1 {
             if let Some(builder) = context.report_lint(&NO_MATCHING_OVERLOAD, node) {
-                builder.into_diagnostic(format_args!(
+                let mut diag = builder.into_diagnostic(format_args!(
                     "No overload{} matches arguments",
                     if let Some(CallableDescription { kind, name }) = callable_description {
                         format!(" of {kind} `{name}`")
@@ -1075,6 +1097,9 @@ impl<'db> CallableBinding<'db> {
                         String::new()
                     }
                 ));
+                if let Some(union_diag) = union_diag {
+                    union_diag.add_union_context(context.db(), &mut diag);
+                }
             }
             return;
         }
@@ -1086,6 +1111,7 @@ impl<'db> CallableBinding<'db> {
                 node,
                 self.signature_type,
                 callable_description.as_ref(),
+                union_diag,
             );
         }
     }
@@ -1385,9 +1411,10 @@ impl<'db> Binding<'db> {
         node: ast::AnyNodeRef,
         callable_ty: Type<'db>,
         callable_description: Option<&CallableDescription>,
+        union_diag: Option<&UnionDiagnostic<'_, '_>>,
     ) {
         for error in &self.errors {
-            error.report_diagnostic(context, node, callable_ty, callable_description);
+            error.report_diagnostic(context, node, callable_ty, callable_description, union_diag);
         }
     }
 
@@ -1539,12 +1566,13 @@ pub(crate) enum BindingError<'db> {
 }
 
 impl<'db> BindingError<'db> {
-    pub(super) fn report_diagnostic(
+    fn report_diagnostic(
         &self,
         context: &InferContext<'db>,
         node: ast::AnyNodeRef,
         callable_ty: Type<'db>,
         callable_description: Option<&CallableDescription>,
+        union_diag: Option<&UnionDiagnostic<'_, '_>>,
     ) {
         match self {
             Self::InvalidArgumentType {
@@ -1561,7 +1589,14 @@ impl<'db> BindingError<'db> {
                 let provided_ty_display = provided_ty.display(context.db());
                 let expected_ty_display = expected_ty.display(context.db());
 
-                let mut diag = builder.into_diagnostic("Argument to this function is incorrect");
+                let mut diag = builder.into_diagnostic(format_args!(
+                    "Argument{} is incorrect",
+                    if let Some(CallableDescription { kind, name }) = callable_description {
+                        format!(" to {kind} `{name}`")
+                    } else {
+                        String::new()
+                    }
+                ));
                 diag.set_primary_message(format_args!(
                     "Expected `{expected_ty_display}`, found `{provided_ty_display}`"
                 ));
@@ -1575,6 +1610,9 @@ impl<'db> BindingError<'db> {
                     );
                     diag.sub(sub);
                 }
+                if let Some(union_diag) = union_diag {
+                    union_diag.add_union_context(context.db(), &mut diag);
+                }
             }
 
             Self::TooManyPositionalArguments {
@@ -1584,7 +1622,7 @@ impl<'db> BindingError<'db> {
             } => {
                 let node = Self::get_node(node, *first_excess_argument_index);
                 if let Some(builder) = context.report_lint(&TOO_MANY_POSITIONAL_ARGUMENTS, node) {
-                    builder.into_diagnostic(format_args!(
+                    let mut diag = builder.into_diagnostic(format_args!(
                         "Too many positional arguments{}: expected \
                         {expected_positional_count}, got {provided_positional_count}",
                         if let Some(CallableDescription { kind, name }) = callable_description {
@@ -1593,13 +1631,16 @@ impl<'db> BindingError<'db> {
                             String::new()
                         }
                     ));
+                    if let Some(union_diag) = union_diag {
+                        union_diag.add_union_context(context.db(), &mut diag);
+                    }
                 }
             }
 
             Self::MissingArguments { parameters } => {
                 if let Some(builder) = context.report_lint(&MISSING_ARGUMENT, node) {
                     let s = if parameters.0.len() == 1 { "" } else { "s" };
-                    builder.into_diagnostic(format_args!(
+                    let mut diag = builder.into_diagnostic(format_args!(
                         "No argument{s} provided for required parameter{s} {parameters}{}",
                         if let Some(CallableDescription { kind, name }) = callable_description {
                             format!(" of {kind} `{name}`")
@@ -1607,6 +1648,9 @@ impl<'db> BindingError<'db> {
                             String::new()
                         }
                     ));
+                    if let Some(union_diag) = union_diag {
+                        union_diag.add_union_context(context.db(), &mut diag);
+                    }
                 }
             }
 
@@ -1616,7 +1660,7 @@ impl<'db> BindingError<'db> {
             } => {
                 let node = Self::get_node(node, *argument_index);
                 if let Some(builder) = context.report_lint(&UNKNOWN_ARGUMENT, node) {
-                    builder.into_diagnostic(format_args!(
+                    let mut diag = builder.into_diagnostic(format_args!(
                         "Argument `{argument_name}` does not match any known parameter{}",
                         if let Some(CallableDescription { kind, name }) = callable_description {
                             format!(" of {kind} `{name}`")
@@ -1624,6 +1668,9 @@ impl<'db> BindingError<'db> {
                             String::new()
                         }
                     ));
+                    if let Some(union_diag) = union_diag {
+                        union_diag.add_union_context(context.db(), &mut diag);
+                    }
                 }
             }
 
@@ -1633,7 +1680,7 @@ impl<'db> BindingError<'db> {
             } => {
                 let node = Self::get_node(node, *argument_index);
                 if let Some(builder) = context.report_lint(&PARAMETER_ALREADY_ASSIGNED, node) {
-                    builder.into_diagnostic(format_args!(
+                    let mut diag = builder.into_diagnostic(format_args!(
                         "Multiple values provided for parameter {parameter}{}",
                         if let Some(CallableDescription { kind, name }) = callable_description {
                             format!(" of {kind} `{name}`")
@@ -1641,6 +1688,9 @@ impl<'db> BindingError<'db> {
                             String::new()
                         }
                     ));
+                    if let Some(union_diag) = union_diag {
+                        union_diag.add_union_context(context.db(), &mut diag);
+                    }
                 }
             }
 
@@ -1657,7 +1707,14 @@ impl<'db> BindingError<'db> {
                 let argument_type = error.argument_type();
                 let argument_ty_display = argument_type.display(context.db());
 
-                let mut diag = builder.into_diagnostic("Argument to this function is incorrect");
+                let mut diag = builder.into_diagnostic(format_args!(
+                    "Argument{} is incorrect",
+                    if let Some(CallableDescription { kind, name }) = callable_description {
+                        format!(" to {kind} `{name}`")
+                    } else {
+                        String::new()
+                    }
+                ));
                 diag.set_primary_message(format_args!(
                     "Argument type `{argument_ty_display}` does not satisfy {} of type variable `{}`",
                     match error {
@@ -1671,12 +1728,15 @@ impl<'db> BindingError<'db> {
                 let mut sub = SubDiagnostic::new(Severity::Info, "Type variable defined here");
                 sub.annotate(Annotation::primary(typevar_range.into()));
                 diag.sub(sub);
+                if let Some(union_diag) = union_diag {
+                    union_diag.add_union_context(context.db(), &mut diag);
+                }
             }
 
             Self::InternalCallError(reason) => {
                 let node = Self::get_node(node, None);
                 if let Some(builder) = context.report_lint(&CALL_NON_CALLABLE, node) {
-                    builder.into_diagnostic(format_args!(
+                    let mut diag = builder.into_diagnostic(format_args!(
                         "Call{} failed: {reason}",
                         if let Some(CallableDescription { kind, name }) = callable_description {
                             format!(" of {kind} `{name}`")
@@ -1684,6 +1744,9 @@ impl<'db> BindingError<'db> {
                             String::new()
                         }
                     ));
+                    if let Some(union_diag) = union_diag {
+                        union_diag.add_union_context(context.db(), &mut diag);
+                    }
                 }
             }
         }
@@ -1708,3 +1771,39 @@ impl<'db> BindingError<'db> {
         }
     }
 }
+
+/// Contains additional context for union specific diagnostics.
+///
+/// This is used when a function call is inconsistent with one or more variants
+/// of a union. This can be used to attach sub-diagnostics that clarify that
+/// the error is part of a union.
+struct UnionDiagnostic<'b, 'db> {
+    /// The type of the union.
+    callable_type: Type<'db>,
+    /// The specific binding that failed.
+    binding: &'b CallableBinding<'db>,
+}
+
+impl UnionDiagnostic<'_, '_> {
+    /// Adds context about any relevant union function types to the given
+    /// diagnostic.
+    fn add_union_context(&self, db: &'_ dyn Db, diag: &mut Diagnostic) {
+        let sub = SubDiagnostic::new(
+            Severity::Info,
+            format_args!(
+                "Union variant `{callable_ty}` is incompatible with this call site",
+                callable_ty = self.binding.callable_type.display(db),
+            ),
+        );
+        diag.sub(sub);
+
+        let sub = SubDiagnostic::new(
+            Severity::Info,
+            format_args!(
+                "Attempted to call union type `{}`",
+                self.callable_type.display(db)
+            ),
+        );
+        diag.sub(sub);
+    }
+}