Use upper bounds as fallback solutions for inference (#16184)

Fixes #13220

This looks a bit ad-hoc, but it is probably the least disruptive
solution possible.

Author: ilevkivskyi

mypy/solve.py

@@ -109,6 +109,13 @@ def solve_constraints(
             else:
                 candidate = AnyType(TypeOfAny.special_form)
             res.append(candidate)
+
+    if not free_vars:
+        # Most of the validation for solutions is done in applytype.py, but here we can
+        # quickly test solutions w.r.t. to upper bounds, and use the latter (if possible),
+        # if solutions are actually not valid (due to poor inference context).
+        res = pre_validate_solutions(res, original_vars, constraints)
+
     return res, free_vars
 
 
@@ -487,3 +494,31 @@ def check_linear(scc: set[TypeVarId], lowers: Bounds, uppers: Bounds) -> bool:
 def get_vars(target: Type, vars: list[TypeVarId]) -> set[TypeVarId]:
     """Find type variables for which we are solving in a target type."""
     return {tv.id for tv in get_all_type_vars(target)} & set(vars)
+
+
+def pre_validate_solutions(
+    solutions: list[Type | None],
+    original_vars: Sequence[TypeVarLikeType],
+    constraints: list[Constraint],
+) -> list[Type | None]:
+    """Check is each solution satisfies the upper bound of the corresponding type variable.
+
+    If it doesn't satisfy the bound, check if bound itself satisfies all constraints, and
+    if yes, use it instead as a fallback solution.
+    """
+    new_solutions: list[Type | None] = []
+    for t, s in zip(original_vars, solutions):
+        if s is not None and not is_subtype(s, t.upper_bound):
+            bound_satisfies_all = True
+            for c in constraints:
+                if c.op == SUBTYPE_OF and not is_subtype(t.upper_bound, c.target):
+                    bound_satisfies_all = False
+                    break
+                if c.op == SUPERTYPE_OF and not is_subtype(c.target, t.upper_bound):
+                    bound_satisfies_all = False
+                    break
+            if bound_satisfies_all:
+                new_solutions.append(t.upper_bound)
+                continue
+        new_solutions.append(s)
+    return new_solutions

test-data/unit/check-inference.test

@@ -3542,6 +3542,14 @@ T = TypeVar("T")
 def type_or_callable(value: T, tp: Union[Type[T], Callable[[int], T]]) -> T: ...
 reveal_type(type_or_callable(A("test"), A))  # N: Revealed type is "__main__.A"
 
+[case testUpperBoundAsInferenceFallback]
+from typing import Callable, TypeVar, Any, Mapping, Optional
+T = TypeVar("T", bound=Mapping[str, Any])
+def raises(opts: Optional[T]) -> T: pass
+def assertRaises(cb: Callable[..., object]) -> None: pass
+assertRaises(raises)  # OK
+[builtins fixtures/dict.pyi]
+
 [case testJoinWithAnyFallback]
 from unknown import X  # type: ignore[import]