Do not assume that args of decorated functions can be cleanly mapped to their nodes (#20203)

Fixes #20059. If any non-trivial decorator is present, avoid trying to
pick the parameter corresponding to i-th parameter of the callable type.

Author: sterliakov

mypy/checker.py

@@ -2578,7 +2578,18 @@ def erase_override(t: Type) -> Type:
                         continue
                     if not is_subtype(original_arg_type, erase_override(override_arg_type)):
                         context: Context = node
-                        if isinstance(node, FuncDef) and not node.is_property:
+                        if (
+                            isinstance(node, FuncDef)
+                            and not node.is_property
+                            and (
+                                not node.is_decorated  # fast path
+                                # allow trivial decorators like @classmethod and @override
+                                or not (sym := node.info.get(node.name))
+                                or not isinstance(sym.node, Decorator)
+                                or not sym.node.decorators
+                            )
+                        ):
+                            # If there's any decorator, we can no longer map arguments 1:1 reliably.
                             arg_node = node.arguments[i + override.bound()]
                             if arg_node.line != -1:
                                 context = arg_node

test-data/unit/check-classes.test

@@ -585,6 +585,58 @@ class B(A):
     @dec
     def f(self) -> int: pass
 
+[case testOverrideWithDecoratorReturningCallable]
+from typing import Any, Callable, TypeVar
+
+class Base:
+    def get(self, a: str) -> None: ...
+
+def dec(fn: Any) -> Callable[[Any, int], None]: ...
+
+class Derived(Base):
+    @dec
+    def get(self) -> None: ...  # E: Argument 1 of "get" is incompatible with supertype "Base"; supertype defines the argument type as "str" \
+                                # N: This violates the Liskov substitution principle \
+                                # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
+[builtins fixtures/tuple.pyi]
+
+[case testOverrideWithDecoratorReturningCallable2]
+# flags: --pretty
+from typing import Any, Callable, TypeVar
+
+_C = TypeVar("_C", bound=Callable[..., Any])
+
+def infer_signature(f: _C) -> Callable[[Any], _C]: ...
+
+class Base:
+    def get(self, a: str, b: str, c: str) -> None: ...
+    def post(self, a: str, b: str) -> None: ...
+
+# Third argument incompatible
+def get(self, a: str, b: str, c: int) -> None: ...
+
+# Second argument incompatible - still should not map to **kwargs
+def post(self, a: str, b: int) -> None: ...
+
+class Derived(Base):
+    @infer_signature(get)
+    def get(self, *args: Any, **kwargs: Any) -> None: ...
+
+    @infer_signature(post)
+    def post(self, *args: Any, **kwargs: Any) -> None: ...
+[builtins fixtures/tuple.pyi]
+[out]
+main:20: error: Argument 3 of "get" is incompatible with supertype "Base"; supertype defines the argument type as "str"
+        def get(self, *args: Any, **kwargs: Any) -> None: ...
+        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+main:20: note: This violates the Liskov substitution principle
+main:20: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
+main:23: error: Argument 2 of "post" is incompatible with supertype "Base"; supertype defines the argument type as "str"
+        def post(self, *args: Any, **kwargs: Any) -> None: ...
+        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+main:23: note: This violates the Liskov substitution principle
+main:23: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
+
 [case testOverrideWithDecoratorReturningInstance]
 def dec(f) -> str: pass