Fix error on instance property and init-only variable with the same name in a dataclass

mypy/nodes.py

@@ -3232,12 +3232,18 @@ def get_method(self, name: str) -> FuncBase | Decorator | None:
         for cls in self.mro:
             if name in cls.names:
                 node = cls.names[name].node
-                if isinstance(node, FuncBase):
-                    return node
-                elif isinstance(node, Decorator):  # Two `if`s make `mypyc` happy
-                    return node
-                else:
-                    return None
+            elif possible_redefinitions := sorted(
+                [n for n in cls.names.keys() if n.startswith(f"{name}-redefinition")]
+            ):
+                node = cls.names[possible_redefinitions[-1]].node
+            else:
+                continue
+            if isinstance(node, FuncBase):
+                return node
+            elif isinstance(node, Decorator):  # Two `if`s make `mypyc` happy
+                return node
+            else:
+                return None
         return None
 
     def calculate_metaclass_type(self) -> mypy.types.Instance | None:

mypy/semanal.py

@@ -6561,7 +6561,12 @@ def add_symbol_table_node(
             if not is_same_symbol(old, new):
                 if isinstance(new, (FuncDef, Decorator, OverloadedFuncDef, TypeInfo)):
                     self.add_redefinition(names, name, symbol)
-                if not (isinstance(new, (FuncDef, Decorator)) and self.set_original_def(old, new)):
+                if isinstance(old, Var) and is_init_only(old):
+                    if old.has_explicit_value:
+                        self.fail("InitVar with default value cannot be redefined", context)
+                elif not (
+                    isinstance(new, (FuncDef, Decorator)) and self.set_original_def(old, new)
+                ):
                     self.name_already_defined(name, context, existing)
         elif name not in self.missing_names[-1] and "*" not in self.missing_names[-1]:
             names[name] = symbol
@@ -7575,3 +7580,10 @@ def halt(self, reason: str = ...) -> NoReturn:
     return isinstance(stmt, PassStmt) or (
         isinstance(stmt, ExpressionStmt) and isinstance(stmt.expr, EllipsisExpr)
     )
+
+
+def is_init_only(node: Var) -> bool:
+    return (
+        isinstance(type := get_proper_type(node.type), Instance)
+        and type.type.fullname == "dataclasses.InitVar"
+    )

test-data/unit/check-dataclasses.test

@@ -2476,6 +2476,61 @@ class Child(Base):
     y: int
 [builtins fixtures/dataclasses.pyi]
 
+[case testDataclassesInitVarsWithProperty]
+from dataclasses import InitVar, dataclass, field
+
+@dataclass
+class Test:
+    foo: InitVar[str]
+    _foo: str = field(init=False)
+
+    def __post_init__(self, foo: str) -> None:
+        self._foo = foo
+
+    @property
+    def foo(self) -> str:
+        return self._foo
+
+    @foo.setter
+    def foo(self, value: str) -> None:
+        self._foo = value
+
+reveal_type(Test)  # N: Revealed type is "def (foo: builtins.str) -> __main__.Test"
+test = Test(42)  # E: Argument 1 to "Test" has incompatible type "int"; expected "str"
+test = Test("foo")
+test.foo
+[builtins fixtures/dataclasses.pyi]
+
+[case testDataclassesDefaultValueInitVarWithProperty]
+from dataclasses import InitVar, dataclass, field
+
+@dataclass
+class Test:
+    foo: InitVar[str] = "foo"
+    _foo: str = field(init=False)
+
+    def __post_init__(self, foo: str) -> None:
+        self._foo = foo
+
+    @property  # E: InitVar with default value cannot be redefined
+    def foo(self) -> str:
+        return self._foo
+
+[builtins fixtures/dataclasses.pyi]
+
+[case testDataclassesWithProperty]
+from dataclasses import dataclass
+
+@dataclass
+class Test:
+    @property
+    def foo(self) -> str:
+        return "a"
+
+    @foo.setter
+    def foo(self, value: str) -> None:
+        pass
+[builtins fixtures/dataclasses.pyi]
 
 [case testDataclassInheritanceWorksWithExplicitOverridesAndOrdering]
 # flags: --enable-error-code explicit-override