Fix crash caused by protocols-promote interference (#5297)

Fixes #5291

Author: ilevkivskyi

mypy/constraints.py

@@ -312,6 +312,8 @@ def visit_instance(self, template: Instance) -> List[Constraint]:
             actual = actual.as_anonymous().fallback
         if isinstance(actual, Instance):
             instance = actual
+            erased = erase_typevars(template)
+            assert isinstance(erased, Instance)
             # We always try nominal inference if possible,
             # it is much faster than the structural one.
             if (self.direction == SUBTYPE_OF and
@@ -343,8 +345,11 @@ def visit_instance(self, template: Instance) -> List[Constraint]:
                     # This is a conservative way break the inference cycles.
                     # It never produces any "false" constraints but gives up soon
                     # on purely structural inference cycles, see #3829.
+                    # Note that we use is_protocol_implementation instead of is_subtype
+                    # because some type may be considered a subtype of a protocol
+                    # due to _promote, but still not implement the protocol.
                     not any(is_same_type(template, t) for t in template.type.inferring) and
-                    mypy.subtypes.is_subtype(instance, erase_typevars(template))):
+                    mypy.subtypes.is_protocol_implementation(instance, erased)):
                 template.type.inferring.append(template)
                 self.infer_constraints_from_protocol_members(res, instance, template,
                                                              original_actual, template)
@@ -353,7 +358,7 @@ def visit_instance(self, template: Instance) -> List[Constraint]:
             elif (instance.type.is_protocol and self.direction == SUBTYPE_OF and
                   # We avoid infinite recursion for structural subtypes also here.
                   not any(is_same_type(instance, i) for i in instance.type.inferring) and
-                  mypy.subtypes.is_subtype(erase_typevars(template), instance)):
+                  mypy.subtypes.is_protocol_implementation(erased, instance)):
                 instance.type.inferring.append(instance)
                 self.infer_constraints_from_protocol_members(res, instance, template,
                                                              template, instance)

test-data/unit/check-type-promotion.test

@@ -37,3 +37,15 @@ y = 0.0
 y = 1
 y.x # E: "int" has no attribute "x"
 [builtins fixtures/primitives.pyi]
+
+[case testTypePromotionsDontInterfereWithProtocols]
+from typing import TypeVar, Union, Protocol
+
+class SupportsFloat(Protocol):
+    def __float__(self) -> float: pass
+
+T = TypeVar('T')
+def f(x: Union[SupportsFloat, T]) -> Union[SupportsFloat, T]: pass
+f(0)  # should not crash
+[builtins fixtures/primitives.pyi]
+[out]

test-data/unit/fixtures/primitives.pyi

@@ -11,7 +11,8 @@ class type:
 
 class int:
     def __add__(self, i: int) -> int: pass
-class float: pass
+class float:
+    def __float__(self) -> float: pass
 class complex: pass
 class bool(int): pass
 class str: