Add warning if redefined dataclass InitVar has a default value

Author: roberfi

mypy/semanal.py

@@ -6211,9 +6211,11 @@ 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 (
-                    is_init_only(old)
-                    or (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]:
@@ -7200,9 +7202,8 @@ def halt(self, reason: str = ...) -> NoReturn:
     )
 
 
-def is_init_only(node: SymbolNode | None) -> bool:
+def is_init_only(node: Var) -> bool:
     return (
-        isinstance(node, Var)
-        and isinstance(type := get_proper_type(node.type), Instance)
+        isinstance(type := get_proper_type(node.type), Instance)
         and type.type.fullname == "dataclasses.InitVar"
     )

test-data/unit/check-dataclasses.test

@@ -2501,6 +2501,23 @@ 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