Reject promotions when checking against protocols. (#18360)

Fixes #18359
Fixes #18257

The change is simple but may be way too general. Making it more precise
should be straightforward, but I want to see the Mypy Primer results and
hear other opinions first.

mypy/subtypes.py

@@ -499,7 +499,7 @@ def visit_instance(self, left: Instance) -> bool:
                 return True
             if type_state.is_cached_negative_subtype_check(self._subtype_kind, left, right):
                 return False
-            if not self.subtype_context.ignore_promotions:
+            if not self.subtype_context.ignore_promotions and not right.type.is_protocol:
                 for base in left.type.mro:
                     if base._promote and any(
                         self._is_subtype(p, self.right) for p in base._promote

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

@@ -187,3 +187,19 @@ if isinstance(x, (float, complex)):
 else:
     reveal_type(x)  # N: Revealed type is "builtins.int"
 [builtins fixtures/primitives.pyi]
+
+[case testRejectPromotionsForProtocols]
+from typing import Protocol
+
+class H(Protocol):
+    def hex(self, /) -> str: ...
+
+f: H = 1.0
+o: H = object()  # E: Incompatible types in assignment (expression has type "object", variable has type "H")
+c: H = 1j   # E: Incompatible types in assignment (expression has type "complex", variable has type "H")
+i: H = 1  # E: Incompatible types in assignment (expression has type "int", variable has type "H")
+b: H = False  # E: Incompatible types in assignment (expression has type "bool", variable has type "H")
+
+class N(float): ...
+n: H = N()
+[builtins fixtures/primitives.pyi]

test-data/unit/fixtures/primitives.pyi

@@ -22,6 +22,7 @@ class int:
 class float:
     def __float__(self) -> float: pass
     def __add__(self, x: float) -> float: pass
+    def hex(self) -> str: pass
 class complex:
     def __add__(self, x: complex) -> complex: pass
 class bool(int): pass