Fix union callees with functools.partial (#17903)

Fixes #17741.

mypy/plugins/functools.py

@@ -18,6 +18,7 @@
     Type,
     TypeOfAny,
     UnboundType,
+    UnionType,
     get_proper_type,
 )
 
@@ -130,7 +131,19 @@ def partial_new_callback(ctx: mypy.plugin.FunctionContext) -> Type:
     if isinstance(get_proper_type(ctx.arg_types[0][0]), Overloaded):
         # TODO: handle overloads, just fall back to whatever the non-plugin code does
         return ctx.default_return_type
-    fn_type = ctx.api.extract_callable_type(ctx.arg_types[0][0], ctx=ctx.default_return_type)
+    return handle_partial_with_callee(ctx, callee=ctx.arg_types[0][0])
+
+
+def handle_partial_with_callee(ctx: mypy.plugin.FunctionContext, callee: Type) -> Type:
+    if not isinstance(ctx.api, mypy.checker.TypeChecker):  # use internals
+        return ctx.default_return_type
+
+    if isinstance(callee_proper := get_proper_type(callee), UnionType):
+        return UnionType.make_union(
+            [handle_partial_with_callee(ctx, item) for item in callee_proper.items]
+        )
+
+    fn_type = ctx.api.extract_callable_type(callee, ctx=ctx.default_return_type)
     if fn_type is None:
         return ctx.default_return_type
 

test-data/unit/check-functools.test

@@ -346,15 +346,32 @@ fn1: Union[Callable[[int], int], Callable[[int], int]]
 reveal_type(functools.partial(fn1, 2)())  # N: Revealed type is "builtins.int"
 
 fn2: Union[Callable[[int], int], Callable[[int], str]]
-reveal_type(functools.partial(fn2, 2)())  # N: Revealed type is "builtins.object"
+reveal_type(functools.partial(fn2, 2)())  # N: Revealed type is "Union[builtins.int, builtins.str]"
 
 fn3: Union[Callable[[int], int], str]
 reveal_type(functools.partial(fn3, 2)())  # E: "str" not callable \
-                                          # E: "Union[Callable[[int], int], str]" not callable \
                                           # N: Revealed type is "builtins.int" \
                                           # E: Argument 1 to "partial" has incompatible type "Union[Callable[[int], int], str]"; expected "Callable[..., int]"
 [builtins fixtures/tuple.pyi]
 
+[case testFunctoolsPartialUnionOfTypeAndCallable]
+import functools
+from typing import Callable, Union, Type
+from typing_extensions import TypeAlias
+
+class FooBar:
+    def __init__(self, arg1: str) -> None:
+        pass
+
+def f1(t: Union[Type[FooBar], Callable[..., 'FooBar']]) -> None:
+    val = functools.partial(t)
+
+FooBarFunc: TypeAlias = Callable[..., 'FooBar']
+
+def f2(t: Union[Type[FooBar], FooBarFunc]) -> None:
+    val = functools.partial(t)
+[builtins fixtures/tuple.pyi]
+
 [case testFunctoolsPartialExplicitType]
 from functools import partial
 from typing import Type, TypeVar, Callable