[ty] don't union in default type for annotated parameters (#21208)

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

@@ -1,12 +1,9 @@
 # Function parameter types
 
 Within a function scope, the declared type of each parameter is its annotated type (or Unknown if
-not annotated). The initial inferred type is the union of the declared type with the type of the
-default value expression (if any). If both are fully static types, this union should simplify to the
-annotated type (since the default value type must be assignable to the annotated type, and for fully
-static types this means subtype-of, which simplifies in unions). But if the annotated type is
-Unknown or another non-fully-static type, the default value type may still be relevant as lower
-bound.
+not annotated). The initial inferred type is the annotated type of the parameter, if any. If there
+is no annotation, it is the union of `Unknown` with the type of the default value expression (if
+any).
 
 The variadic parameter is a variadic tuple of its annotated type; the variadic-keywords parameter is
 a dictionary from strings to its annotated type.
@@ -41,13 +38,13 @@ def g(*args, **kwargs):
 
 ## Annotation is present but not a fully static type
 
-The default value type should be a lower bound on the inferred type.
+If there is an annotation, we respect it fully and don't union in the default value type.
 
 ```py
 from typing import Any
 
 def f(x: Any = 1):
-    reveal_type(x)  # revealed: Any | Literal[1]
+    reveal_type(x)  # revealed: Any
 ```
 
 ## Default value type must be assignable to annotated type
@@ -64,7 +61,7 @@ def f(x: int = "foo"):
 from typing import Any
 
 def g(x: Any = "foo"):
-    reveal_type(x)  # revealed: Any | Literal["foo"]
+    reveal_type(x)  # revealed: Any
 ```
 
 ## Stub functions

crates/ty_python_semantic/resources/mdtest/ty_extensions.md

@@ -99,7 +99,7 @@ static_assert(is_assignable_to(int, Unknown))
 def explicit_unknown(x: Unknown, y: tuple[str, Unknown], z: Unknown = 1) -> None:
     reveal_type(x)  # revealed: Unknown
     reveal_type(y)  # revealed: tuple[str, Unknown]
-    reveal_type(z)  # revealed: Unknown | Literal[1]
+    reveal_type(z)  # revealed: Unknown
 ```
 
 `Unknown` can be subclassed, just like `Any`:

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -2423,15 +2423,8 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
     ///
     /// The declared type is the annotated type, if any, or `Unknown`.
     ///
-    /// The inferred type is the annotated type, unioned with the type of the default value, if
-    /// any. If both types are fully static, this union is a no-op (it should simplify to just the
-    /// annotated type.) But in a case like `f(x=None)` with no annotated type, we want to infer
-    /// the type `Unknown | None` for `x`, not just `Unknown`, so that we can error on usage of `x`
-    /// that would not be valid for `None`.
-    ///
-    /// If the default-value type is not assignable to the declared (annotated) type, we ignore the
-    /// default-value type and just infer the annotated type; this is the same way we handle
-    /// assignments, and allows an explicit annotation to override a bad inference.
+    /// The inferred type is the annotated type, if any. If there is no annotation, it is the union
+    /// of `Unknown` and the type of the default value, if any.
     ///
     /// Parameter definitions are odd in that they define a symbol in the function-body scope, so
     /// the Definition belongs to the function body scope, but the expressions (annotation and
@@ -2460,23 +2453,17 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             .map(|default| self.file_expression_type(default));
         if let Some(annotation) = parameter.annotation.as_ref() {
             let declared_ty = self.file_expression_type(annotation);
-            let declared_and_inferred_ty = if let Some(default_ty) = default_ty {
-                if default_ty.is_assignable_to(self.db(), declared_ty) {
-                    DeclaredAndInferredType::MightBeDifferent {
-                        declared_ty: TypeAndQualifiers::declared(declared_ty),
-                        inferred_ty: UnionType::from_elements(self.db(), [declared_ty, default_ty]),
-                    }
-                } else if (self.in_stub()
-                    || self.in_function_overload_or_abstractmethod()
-                    || self
-                        .class_context_of_current_method()
-                        .is_some_and(|class| class.is_protocol(self.db())))
-                    && default
-                        .as_ref()
-                        .is_some_and(|d| d.is_ellipsis_literal_expr())
+            if let Some(default_ty) = default_ty {
+                if !default_ty.is_assignable_to(self.db(), declared_ty)
+                    && !((self.in_stub()
+                        || self.in_function_overload_or_abstractmethod()
+                        || self
+                            .class_context_of_current_method()
+                            .is_some_and(|class| class.is_protocol(self.db())))
+                        && default
+                            .as_ref()
+                            .is_some_and(|d| d.is_ellipsis_literal_expr()))
                 {
-                    DeclaredAndInferredType::are_the_same_type(declared_ty)
-                } else {
                     if let Some(builder) = self
                         .context
                         .report_lint(&INVALID_PARAMETER_DEFAULT, parameter_with_default)
@@ -2488,15 +2475,12 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                             declared_ty.display(self.db())
                         ));
                     }
-                    DeclaredAndInferredType::are_the_same_type(declared_ty)
                 }
-            } else {
-                DeclaredAndInferredType::are_the_same_type(declared_ty)
-            };
+            }
             self.add_declaration_with_binding(
                 parameter.into(),
                 definition,
-                &declared_and_inferred_ty,
+                &DeclaredAndInferredType::are_the_same_type(declared_ty),
             );
         } else {
             let ty = if let Some(default_ty) = default_ty {