Make decorator inference in semanal consistent with checker

python · ilevkivskyi · Apr 14, 2026 · Mar 27, 2026 · Mar 27, 2026 · Mar 27, 2026
commit d2c6312664523b388758b4b73fec42814107acfe
diff --git a/mypy/semanal_infer.py b/mypy/semanal_infer.py
@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 from mypy.nodes import ARG_POS, CallExpr, Decorator, Expression, FuncDef, RefExpr, Var
-from mypy.semanal_shared import SemanticAnalyzerInterface
+from mypy.semanal_shared import SemanticAnalyzerInterface, set_callable_name
 from mypy.typeops import function_type
 from mypy.types import (
     AnyType,
@@ -58,7 +58,8 @@ def infer_decorator_signature_if_simple(
     if decorator_preserves_type:
         # No non-identity decorators left. We can trivially infer the type
         # of the function here.
-        dec.var.type = function_type(dec.func, analyzer.named_type("builtins.function"))
+        sig = function_type(dec.func, analyzer.named_type("builtins.function"))
+        dec.var.type = set_callable_name(sig, dec.func)
     if dec.decorators:
         return_type = calculate_return_type(dec.decorators[0])
         if return_type and isinstance(return_type, AnyType):
@@ -72,6 +73,8 @@ def infer_decorator_signature_if_simple(
             orig_sig = function_type(dec.func, analyzer.named_type("builtins.function"))
             sig.name = orig_sig.items[0].name
             dec.var.type = sig
+            if isinstance(sig, CallableType):
+                sig.definition = dec
 
 
 def is_identity_signature(sig: Type) -> bool:

diff --git a/test-data/unit/check-functions.test b/test-data/unit/check-functions.test
@@ -1122,7 +1122,7 @@ class A:
 [case testForwardReferenceToDynamicallyTypedStaticMethod]
 def f(self) -> None:
     A.x(1).y
-    A.x() # E: Missing positional argument "x" in call to "x"
+    A.x() # E: Missing positional argument "x" in call to "x" of "A"
 
 class A:
     @staticmethod
@@ -1144,7 +1144,7 @@ class A:
 [case testForwardReferenceToDynamicallyTypedClassMethod]
 def f(self) -> None:
     A.x(1).y
-    A.x() # E: Missing positional argument "a" in call to "x"
+    A.x() # E: Missing positional argument "a" in call to "x" of "A"
 
 class A:
     @classmethod

diff --git a/test-data/unit/check-protocols.test b/test-data/unit/check-protocols.test
@@ -4558,7 +4558,30 @@ def bad() -> Proto:
                    # N:     Expected: \
                    # N:         def f(self) -> int \
                    # N:     Got: \
-                   # N:         def f() -> str \
+                   # N:         def f(self) -> str \
+
+class Impl:
+    @defer
+    def f(self) -> int: ...
+
+[case testProtocolCheckDefersNode2]
+from typing import Any, Callable, Protocol, TypeVar
+
+class Proto(Protocol):
+    def f(self) -> int:
+        ...
+
+T = TypeVar("T")
+def defer(f: Callable[[T], int]) -> Callable[[T], list[int]]:
+    ...
+
+def bad() -> Proto:
+    return Impl()  # E: Incompatible return value type (got "Impl", expected "Proto") \
+                   # N: Following member(s) of "Impl" have conflicts: \
+                   # N:     Expected: \
+                   # N:         def f(self) -> int \
+                   # N:     Got: \
+                   # N:         def f(self) -> list[int] \
 
 class Impl:
     @defer