Remove special case for Union type context

Kind of reverts #2715. The test cases in that PR still pass

Fixes #20608

Author: hauntsaninja

mypy/constraints.py

@@ -397,13 +397,11 @@ def _infer_constraints(
     # be a supertype of the potential subtype, some item of the Union
     # must be a supertype of it.
     if direction == SUBTYPE_OF and isinstance(actual, UnionType):
-        # If some of items is not a complete type, disregard that.
-        items = simplify_away_incomplete_types(actual.items)
         # We infer constraints eagerly -- try to find constraints for a type
         # variable if possible. This seems to help with some real-world
         # use cases.
         return any_constraints(
-            [infer_constraints_if_possible(template, a_item, direction) for a_item in items],
+            [infer_constraints_if_possible(template, a_item, direction) for a_item in actual.items],
             eager=True,
         )
     if direction == SUPERTYPE_OF and isinstance(template, UnionType):
@@ -652,31 +650,6 @@ def _is_similar_constraints(x: list[Constraint], y: list[Constraint]) -> bool:
     return True
 
 
-def simplify_away_incomplete_types(types: Iterable[Type]) -> list[Type]:
-    complete = [typ for typ in types if is_complete_type(typ)]
-    if complete:
-        return complete
-    else:
-        return list(types)
-
-
-def is_complete_type(typ: Type) -> bool:
-    """Is a type complete?
-
-    A complete doesn't have uninhabited type components or (when not in strict
-    optional mode) None components.
-    """
-    return typ.accept(CompleteTypeVisitor())
-
-
-class CompleteTypeVisitor(BoolTypeQuery):
-    def __init__(self) -> None:
-        super().__init__(ALL_STRATEGY)
-
-    def visit_uninhabited_type(self, t: UninhabitedType) -> bool:
-        return False
-
-
 class ConstraintBuilderVisitor(TypeVisitor[list[Constraint]]):
     """Visitor class for inferring type constraints."""
 

test-data/unit/check-inference-context.test

@@ -965,6 +965,24 @@ reveal_type(c.f([])) # N: Revealed type is "builtins.list[builtins.int]"
 reveal_type(c.f(None)) # N: Revealed type is "builtins.list[builtins.int] | None"
 [builtins fixtures/list.pyi]
 
+[case testUnionWithTupleNeverBranch]
+# https://github.com/python/mypy/issues/20608
+from __future__ import annotations
+from typing import Callable, Never, Generic, TypeVar
+
+T = TypeVar('T')
+class ValueType(Generic[T]): pass
+class Bundle(Generic[T]): pass
+
+F = TypeVar('F')
+
+def fail_value_type_inference(self_bundle: ValueType[F]) -> tuple[Never, Bundle[ValueType[F]]] | tuple[int, Bundle[ValueType[int]]]:
+    raise
+
+x = fail_value_type_inference
+x = fail_value_type_inference
+[builtins fixtures/tuple.pyi]
+
 [case testGenericMethodCalledInGenericContext]
 from typing import TypeVar, Generic