Fix: ensure unpacked **kwargs have string-compatible keys (#20706)

Author: Vikash-Kumar-23

mypy/checkexpr.py

@@ -5432,6 +5432,13 @@ def visit_dict_expr(self, e: DictExpr) -> Type:
         expected_types: list[Type] = []
         for key, value in e.items:
             if key is None:
+                # This is a **expr unpacking. Check that the expression has string keys.
+                value_type = get_proper_type(self.accept(value))
+                if not self.is_valid_keyword_var_arg(value_type):
+                    is_mapping = is_subtype(
+                        value_type, self.chk.named_type("_typeshed.SupportsKeysAndGetItem")
+                    )
+                    self.msg.invalid_keyword_var_arg(value_type, is_mapping, value)
                 args.append(value)
                 expected_types.append(
                     self.chk.named_generic_type("_typeshed.SupportsKeysAndGetItem", [kt, vt])