Fix inheriting from generic @Frozen attrs class (#15700)

Fixes #15658.

mypy/plugins/attrs.py

@@ -803,7 +803,7 @@ def _make_frozen(ctx: mypy.plugin.ClassDefContext, attributes: list[Attribute])
         else:
             # This variable belongs to a super class so create new Var so we
             # can modify it.
-            var = Var(attribute.name, ctx.cls.info[attribute.name].type)
+            var = Var(attribute.name, attribute.init_type)
             var.info = ctx.cls.info
             var._fullname = f"{ctx.cls.info.fullname}.{var.name}"
             ctx.cls.info.names[var.name] = SymbolTableNode(MDEF, var)

test-data/unit/check-plugin-attrs.test

@@ -2253,3 +2253,27 @@ c = attrs.assoc(c, name=42)  # E: Argument "name" to "assoc" of "C" has incompat
 
 [builtins fixtures/plugin_attrs.pyi]
 [typing fixtures/typing-medium.pyi]
+
+[case testFrozenInheritFromGeneric]
+from typing import Generic, TypeVar
+from attrs import field, frozen
+
+T = TypeVar('T')
+
+def f(s: str) -> int:
+    ...
+
+@frozen
+class A(Generic[T]):
+    x: T
+    y: int = field(converter=f)
+
+@frozen
+class B(A[int]):
+    pass
+
+b = B(42, 'spam')
+reveal_type(b.x)  # N: Revealed type is "builtins.int"
+reveal_type(b.y)  # N: Revealed type is "builtins.int"
+
+[builtins fixtures/plugin_attrs.pyi]