Fix false positive when @Property getter and setter are non-adjacent

`fix_function_overloads` in fastparse.py groups consecutive same-named
decorators into an `OverloadedFuncDef`. When a property getter and its
setter/deleter are separated by other method definitions, the setter was
left as an isolated `Decorator` instead of being folded into the
property's `OverloadedFuncDef`.  This caused mypy to report:

  error: "Callable[[A], T]" has no attribute "setter"  [attr-defined]
  error: Name "x" already defined on line N  [no-redef]
  error: Property "x" defined in "A" is read-only  [misc]

Add a second pass `_merge_non_adjacent_property_overloads` that runs
after the existing loop.  It scans the output list for any `@x.setter`
or `@x.deleter` `Decorator` nodes whose property getter was seen earlier
in the same scope, and merges them into the getter's `OverloadedFuncDef`
(promoting a lone getter `Decorator` to an `OverloadedFuncDef` when
needed).  The adjacent case is unaffected because the setter/deleter is
already inside the `OverloadedFuncDef` after the first pass.

Fixes #1465.

Author: PGrayCS

mypy/fastparse.py

@@ -761,7 +761,72 @@ def fix_function_overloads(self, stmts: list[Statement]) -> list[Statement]:
             ret.append(last_if_overload)
         elif last_if_stmt is not None:
             ret.append(last_if_stmt)
-        return ret
+        return self._merge_non_adjacent_property_overloads(ret)
+
+    def _merge_non_adjacent_property_overloads(
+        self, stmts: list[Statement]
+    ) -> list[Statement]:
+        """Merge non-adjacent @x.setter and @x.deleter into their property getter.
+
+        fix_function_overloads only groups *consecutive* same-named decorators.
+        When a property getter and its setter/deleter are separated by other
+        statements, the setter/deleter ends up as an isolated Decorator in the
+        output.  This second pass finds those stray components and folds them
+        back into the property's OverloadedFuncDef (or promotes the lone getter
+        Decorator to an OverloadedFuncDef).
+
+        See https://github.com/python/mypy/issues/1465.
+        """
+        # Build a map: property name -> index of its getter / OverloadedFuncDef in stmts.
+        prop_getter_pos: dict[str, int] = {}
+        for i, stmt in enumerate(stmts):
+            if isinstance(stmt, Decorator):
+                if any(isinstance(d, NameExpr) and d.name == "property" for d in stmt.decorators):
+                    prop_getter_pos[stmt.name] = i
+            elif isinstance(stmt, OverloadedFuncDef) and stmt.items:
+                first = stmt.items[0]
+                if isinstance(first, Decorator) and any(
+                    isinstance(d, NameExpr) and d.name == "property" for d in first.decorators
+                ):
+                    prop_getter_pos[first.name] = i
+
+        if not prop_getter_pos:
+            return stmts
+
+        # Find setter/deleter Decorators whose getter appeared earlier, then merge.
+        result: list[Statement | None] = list(stmts)
+        for i, stmt in enumerate(stmts):
+            if not isinstance(stmt, Decorator):
+                continue
+            # Identify @prop_name.setter or @prop_name.deleter
+            prop_name: str | None = None
+            for d in stmt.decorators:
+                if (
+                    isinstance(d, MemberExpr)
+                    and isinstance(d.expr, NameExpr)
+                    and d.name in {"setter", "deleter"}
+                ):
+                    prop_name = d.expr.name
+                    break
+            if prop_name is None:
+                continue
+            getter_pos = prop_getter_pos.get(prop_name)
+            if getter_pos is None or getter_pos >= i:
+                # No matching getter found earlier in the same scope.
+                continue
+            # This is a non-adjacent setter/deleter; merge it into the getter's node.
+            existing = result[getter_pos]
+            if isinstance(existing, Decorator):
+                # Promote the lone getter Decorator to an OverloadedFuncDef.
+                ovl = OverloadedFuncDef([existing, stmt])
+                ovl.set_line(existing)
+                result[getter_pos] = ovl
+            elif isinstance(existing, OverloadedFuncDef):
+                existing.items.append(stmt)
+                existing.unanalyzed_items.append(stmt)
+            result[i] = None  # Remove from its original (non-adjacent) position.
+
+        return [s for s in result if s is not None]
 
     def _check_ifstmt_for_overloads(
         self, stmt: IfStmt, current_overload_name: str | None = None

test-data/unit/check-classes.test

@@ -1812,6 +1812,59 @@ a.f = a.f # E: Property "f" defined in "A" is read-only
 a.f.x # E: "int" has no attribute "x"
 [builtins fixtures/property.pyi]
 
+[case testPropertyWithNonAdjacentSetter]
+# Regression test for https://github.com/python/mypy/issues/1465
+# @f.setter need not be immediately after @property def f.
+class A:
+    @property
+    def f(self) -> int:
+        return 1
+    def other(self) -> None:
+        pass
+    @f.setter
+    def f(self, x: int) -> None:
+        pass
+a = A()
+a.f = a.f
+a.f = 1
+a.f = ''  # E: Incompatible types in assignment (expression has type "str", variable has type "int")
+reveal_type(a.f)  # N: Revealed type is "builtins.int"
+[builtins fixtures/property.pyi]
+
+[case testPropertyWithNonAdjacentDeleter]
+class A:
+    @property
+    def f(self) -> int:
+        return 1
+    def other(self) -> None:
+        pass
+    @f.deleter
+    def f(self) -> None:
+        pass
+a = A()
+a.f = a.f  # E: Property "f" defined in "A" is read-only
+[builtins fixtures/property.pyi]
+
+[case testPropertyWithNonAdjacentSetterAndDeleter]
+class A:
+    @property
+    def f(self) -> int:
+        return 1
+    def other(self) -> None:
+        pass
+    @f.setter
+    def f(self, x: int) -> None:
+        pass
+    def another(self) -> None:
+        pass
+    @f.deleter
+    def f(self) -> None:
+        pass
+a = A()
+a.f = 1
+a.f = ''  # E: Incompatible types in assignment (expression has type "str", variable has type "int")
+[builtins fixtures/property.pyi]
+
 -- Descriptors
 -- -----------