Fix false positive when using properties (#5574)

Author: ilevkivskyi

mypy/semanal.py

@@ -3293,7 +3293,7 @@ def add_symbol(self, name: str, node: SymbolTableNode,
             self.locals[-1][name] = node
         elif self.type:
             existing = self.type.names.get(name)
-            if existing and existing.node != node.node:
+            if existing and isinstance(existing.node, TypeInfo) and existing.node != node.node:
                 self.name_already_defined(name, context, existing)
                 return
             self.type.names[name] = node

test-data/unit/check-classes.test

@@ -5087,3 +5087,33 @@ c = D()
 from a import Other
 class B: ...
 [out]
+
+[case testAllowPropertyAndInit1]
+class C:
+    def __init__(self, x: int) -> None:
+        self.x = x
+    @property
+    def x(self) -> int: pass
+    @x.setter
+    def x(self, x: int) -> None: pass
+[builtins fixtures/property.pyi]
+[out]
+
+[case testAllowPropertyAndInit2]
+class C:
+    @property
+    def x(self) -> int: pass
+    @x.setter
+    def x(self, x: int) -> None: pass
+    def __init__(self, x: int) -> None:
+        self.x = x
+[builtins fixtures/property.pyi]
+
+[case testAllowPropertyAndInit3]
+class C:
+    def __init__(self, x: int) -> None:
+        self.x = x  # type: ignore
+    @property  # Should be no error here
+    def x(self) -> int: pass
+[builtins fixtures/property.pyi]
+[out]