[red-knot] support typing.Union in type annotations (#14499)

Fix #14498

## Summary

This PR adds `typing.Union` support

## Test Plan

I created new tests in mdtest.

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/red_knot_python_semantic/resources/mdtest/annotations/union.md

@@ -0,0 +1,61 @@
+# Union
+
+## Annotation
+
+`typing.Union` can be used to construct union types same as `|` operator.
+
+```py
+from typing import Union
+
+a: Union[int, str]
+a1: Union[int, bool]
+a2: Union[int, Union[float, str]]
+a3: Union[int, None]
+a4: Union[Union[float, str]]
+a5: Union[int]
+a6: Union[()]
+
+def f():
+    # revealed: int | str
+    reveal_type(a)
+    # Since bool is a subtype of int we simplify to int here. But we do allow assigning boolean values (see below).
+    # revealed: int
+    reveal_type(a1)
+    # revealed: int | float | str
+    reveal_type(a2)
+    # revealed: int | None
+    reveal_type(a3)
+    # revealed: float | str
+    reveal_type(a4)
+    # revealed: int
+    reveal_type(a5)
+    # revealed: Never
+    reveal_type(a6)
+```
+
+## Assignment
+
+```py
+from typing import Union
+
+a: Union[int, str]
+a = 1
+a = ""
+a1: Union[int, bool]
+a1 = 1
+a1 = True
+# error: [invalid-assignment] "Object of type `Literal[b""]` is not assignable to `int | str`"
+a = b""
+```
+
+## Typing Extensions
+
+```py
+from typing_extensions import Union
+
+a: Union[int, str]
+
+def f():
+    # revealed: int | str
+    reveal_type(a)
+```

crates/red_knot_python_semantic/src/types.rs

@@ -1809,6 +1809,8 @@ pub enum KnownInstanceType<'db> {
     Literal,
     /// The symbol `typing.Optional` (which can also be found as `typing_extensions.Optional`)
     Optional,
+    /// The symbol `typing.Union` (which can also be found as `typing_extensions.Union`)
+    Union,
     /// A single instance of `typing.TypeVar`
     TypeVar(TypeVarInstance<'db>),
     // TODO: fill this enum out with more special forms, etc.
@@ -1819,6 +1821,7 @@ impl<'db> KnownInstanceType<'db> {
         match self {
             KnownInstanceType::Literal => "Literal",
             KnownInstanceType::Optional => "Optional",
+            KnownInstanceType::Union => "Union",
             KnownInstanceType::TypeVar(_) => "TypeVar",
         }
     }
@@ -1826,7 +1829,9 @@ impl<'db> KnownInstanceType<'db> {
     /// Evaluate the known instance in boolean context
     pub const fn bool(self) -> Truthiness {
         match self {
-            Self::Literal | Self::Optional | Self::TypeVar(_) => Truthiness::AlwaysTrue,
+            Self::Literal | Self::Optional | Self::TypeVar(_) | Self::Union => {
+                Truthiness::AlwaysTrue
+            }
         }
     }
 
@@ -1835,6 +1840,7 @@ impl<'db> KnownInstanceType<'db> {
         match self {
             Self::Literal => "typing.Literal",
             Self::Optional => "typing.Optional",
+            Self::Union => "typing.Union",
             Self::TypeVar(typevar) => typevar.name(db),
         }
     }
@@ -1844,6 +1850,7 @@ impl<'db> KnownInstanceType<'db> {
         match self {
             Self::Literal => KnownClass::SpecialForm,
             Self::Optional => KnownClass::SpecialForm,
+            Self::Union => KnownClass::SpecialForm,
             Self::TypeVar(_) => KnownClass::TypeVar,
         }
     }
@@ -1864,6 +1871,7 @@ impl<'db> KnownInstanceType<'db> {
         match (module.name().as_str(), instance_name) {
             ("typing" | "typing_extensions", "Literal") => Some(Self::Literal),
             ("typing" | "typing_extensions", "Optional") => Some(Self::Optional),
+            ("typing" | "typing_extensions", "Union") => Some(Self::Union),
             _ => None,
         }
     }

crates/red_knot_python_semantic/src/types/infer.rs

@@ -4567,6 +4567,13 @@ impl<'db> TypeInferenceBuilder<'db> {
                 let param_type = self.infer_type_expression(parameters);
                 UnionType::from_elements(self.db, [param_type, Type::none(self.db)])
             }
+            KnownInstanceType::Union => match parameters {
+                ast::Expr::Tuple(t) => UnionType::from_elements(
+                    self.db,
+                    t.iter().map(|elt| self.infer_type_expression(elt)),
+                ),
+                _ => self.infer_type_expression(parameters),
+            },
             KnownInstanceType::TypeVar(_) => Type::Todo,
         }
     }

crates/red_knot_python_semantic/src/types/mro.rs

@@ -373,6 +373,7 @@ impl<'db> ClassBase<'db> {
             Type::KnownInstance(known_instance) => match known_instance {
                 KnownInstanceType::TypeVar(_)
                 | KnownInstanceType::Literal
+                | KnownInstanceType::Union
                 | KnownInstanceType::Optional => None,
             },
         }