From 84673c26a6cbbfef68b7539c0f8a9566e5c7d667 Mon Sep 17 00:00:00 2001
From: Christoph Tyralla <c.tyralla@bjoernsen.de>
Date: Sun, 29 Dec 2024 15:29:29 +0100
Subject: [PATCH] Reject promotions when checking against protocols.

Fixes #18359
---
 mypy/subtypes.py                         |  2 +-
 test-data/unit/check-type-promotion.test | 16 ++++++++++++++++
 test-data/unit/fixtures/primitives.pyi   |  1 +
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/mypy/subtypes.py b/mypy/subtypes.py
index a26aaf798b58f..666bdebed7429 100644
--- a/mypy/subtypes.py
+++ b/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
diff --git a/test-data/unit/check-type-promotion.test b/test-data/unit/check-type-promotion.test
index d98d0c60e1644..1b69174a45454 100644
--- a/test-data/unit/check-type-promotion.test
+++ b/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]
diff --git a/test-data/unit/fixtures/primitives.pyi b/test-data/unit/fixtures/primitives.pyi
index e7d3e12bd5e6a..fc220a4e2ee0c 100644
--- a/test-data/unit/fixtures/primitives.pyi
+++ b/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