Fix function argument inference for constrained type variables within unions (Fixes #3644).

mypy/constraints.py

@@ -420,8 +420,12 @@ def filter_satisfiable(option: list[Constraint] | None) -> list[Constraint] | No
         return option
     satisfiable = []
     for c in option:
-        # TODO: add similar logic for TypeVar values (also in various other places)?
-        if mypy.subtypes.is_subtype(c.target, c.origin_type_var.upper_bound):
+        if isinstance(c.origin_type_var, TypeVarType) and c.origin_type_var.values:
+            if any(
+                mypy.subtypes.is_subtype(c.target, value) for value in c.origin_type_var.values
+            ):
+                satisfiable.append(c)
+        elif mypy.subtypes.is_subtype(c.target, c.origin_type_var.upper_bound):
             satisfiable.append(c)
     if not satisfiable:
         return None

test-data/unit/check-unions.test

@@ -1171,6 +1171,25 @@ def foo(
 foo([1])
 [builtins fixtures/list.pyi]
 
+[case testGenericUnionMemberWithTypeVarConstraints]
+
+from typing import Generic, TypeVar, Union
+
+T = TypeVar('T', str, int)
+
+class C(Generic[T]): ...
+
+def f(s: Union[T, C[T]]) -> T:  ...
+
+ci: C[int]
+cs: C[str]
+
+reveal_type(f(1))  # N: Revealed type is "builtins.int"
+reveal_type(f(''))  # N: Revealed type is "builtins.str"
+reveal_type(f(ci))  # N: Revealed type is "builtins.int"
+reveal_type(f(cs))  # N: Revealed type is "builtins.str"
+
+
 [case testNestedInstanceTypeAliasUnsimplifiedUnion]
 from typing import TypeVar, Union, Iterator, List, Any
 T = TypeVar("T")