Prohibit dangerous case

mypy/checkexpr.py

@@ -1638,6 +1638,27 @@ def check_callable_call(
                 callee.type_object().name, abstract_attributes, context
             )
 
+        var_arg = callee.var_arg()
+        if var_arg and isinstance(var_arg.typ, UnpackType):
+            # It is hard to support multiple variadic unpacks (except for old-style *args: int),
+            # fail gracefully to avoid crashes later.
+            seen_unpack = False
+            for arg, arg_kind in zip(args, arg_kinds):
+                if arg_kind != ARG_STAR:
+                    continue
+                arg_type = get_proper_type(self.accept(arg))
+                if not isinstance(arg_type, TupleType) or any(
+                    isinstance(t, UnpackType) for t in arg_type.items
+                ):
+                    if seen_unpack:
+                        self.msg.fail(
+                            "Passing multiple variadic unpacks in a call is not supported",
+                            context,
+                            code=codes.CALL_ARG,
+                        )
+                        return AnyType(TypeOfAny.from_error), callee
+                    seen_unpack = True
+
         formal_to_actual = map_actuals_to_formals(
             arg_kinds,
             arg_names,

test-data/unit/check-typevar-tuple.test

@@ -371,6 +371,9 @@ def args_to_tuple(*args: Unpack[Ts]) -> Tuple[Unpack[Ts]]:
     reveal_type(args_to_tuple(1, *args))  # N: Revealed type is "Tuple[Literal[1]?, Unpack[Ts`-1]]"
     reveal_type(args_to_tuple(*args, 'a'))  # N: Revealed type is "Tuple[Unpack[Ts`-1], Literal['a']?]"
     reveal_type(args_to_tuple(1, *args, 'a'))  # N: Revealed type is "Tuple[Literal[1]?, Unpack[Ts`-1], Literal['a']?]"
+    args_to_tuple(*args, *args)  # E: Passing multiple variadic unpacks in a call is not supported
+    ok = (1, 'a')
+    reveal_type(args_to_tuple(*ok, *ok))  # N: Revealed type is "Tuple[builtins.int, builtins.str, builtins.int, builtins.str]"
     if int():
         return args
     else:
@@ -381,6 +384,7 @@ vt: Tuple[int, ...]
 reveal_type(args_to_tuple(1, *vt))  # N: Revealed type is "Tuple[Literal[1]?, Unpack[builtins.tuple[builtins.int, ...]]]"
 reveal_type(args_to_tuple(*vt, 'a'))  # N: Revealed type is "Tuple[Unpack[builtins.tuple[builtins.int, ...]], Literal['a']?]"
 reveal_type(args_to_tuple(1, *vt, 'a'))  # N: Revealed type is "Tuple[Literal[1]?, Unpack[builtins.tuple[builtins.int, ...]], Literal['a']?]"
+args_to_tuple(*vt, *vt)  # E: Passing multiple variadic unpacks in a call is not supported
 [builtins fixtures/tuple.pyi]
 
 [case testTypeVarTuplePep646TypeVarStarArgs]