Remove special case for Union type context (#20610)

Reverts some of #2715. The test cases introduced in that PR still pass

Fixes #20608

Test case minimised by A5rocks

Codex assisted in diagnosing the issue

Author: hauntsaninja

mypy/checkexpr.py

@@ -5941,7 +5941,12 @@ def visit_conditional_expr(self, e: ConditionalExpr, allow_none_return: bool = F
             ctx_else_type = self.analyze_cond_branch(
                 else_map, e.else_expr, context=ctx, allow_none_return=allow_none_return
             )
-            ctx = make_simplified_union([ctx_if_type, ctx_else_type])
+            if has_ambiguous_uninhabited_component(ctx_if_type):
+                ctx = ctx_else_type
+            elif has_ambiguous_uninhabited_component(ctx_else_type):
+                ctx = ctx_if_type
+            else:
+                ctx = make_simplified_union([ctx_if_type, ctx_else_type])
 
         if_type = self.analyze_cond_branch(
             if_map, e.if_expr, context=ctx, allow_none_return=allow_none_return
@@ -6636,6 +6641,20 @@ def visit_uninhabited_type(self, t: UninhabitedType) -> bool:
         return True
 
 
+def has_ambiguous_uninhabited_component(t: Type) -> bool:
+    return t.accept(HasAmbiguousUninhabitedComponentsQuery())
+
+
+class HasAmbiguousUninhabitedComponentsQuery(types.BoolTypeQuery):
+    """Visitor for querying whether a type has an ambiguous UninhabitedType component."""
+
+    def __init__(self) -> None:
+        super().__init__(types.ANY_STRATEGY)
+
+    def visit_uninhabited_type(self, t: UninhabitedType) -> bool:
+        return t.ambiguous
+
+
 def arg_approximate_similarity(actual: Type, formal: Type) -> bool:
     """Return if caller argument (actual) is roughly compatible with signature arg (formal).
 

mypy/constraints.py

@@ -20,7 +20,6 @@
     ArgKind,
     TypeInfo,
 )
-from mypy.type_visitor import ALL_STRATEGY, BoolTypeQuery
 from mypy.types import (
     TUPLE_LIKE_INSTANCE_NAMES,
     AnyType,
@@ -397,13 +396,14 @@ 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 +652,6 @@ 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