gh-20542: Fix crash in split_for_callable when passing values to variadic generics

mypy/checkexpr.py

@@ -4963,10 +4963,13 @@ class C(Generic[T, Unpack[Ts]]): ...
                     )
                     return [AnyType(TypeOfAny.from_error)] * len(vars)
 
-        if not vars or not any(isinstance(v, TypeVarTupleType) for v in vars):
-            return list(args)
         # TODO: in future we may want to support type application to variadic functions.
-        assert t.is_type_obj()
+        if (
+            not vars
+            or not any(isinstance(v, TypeVarTupleType) for v in vars)
+            or not t.is_type_obj()
+        ):
+            return list(args)
         info = t.type_object()
         # We reuse the logic from semanal phase to reduce code duplication.
         fake = Instance(info, args, line=ctx.line, column=ctx.column)

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

@@ -2754,3 +2754,15 @@ class X(Generic[Unpack[Ts]]):
 def c(t: X[Concatenate[int, ...]]) -> None:  # E: Cannot use "[int, VarArg(Any), KwArg(Any)]" for TypeVarTuple, only for ParamSpec
     reveal_type(t)  # N: Revealed type is "__main__.X[Unpack[builtins.tuple[Any, ...]]]"
 [builtins fixtures/tuple.pyi]
+
+[case testTypeVarTupleLiteralValueAsTypeArgument]
+from typing import TypeVarTuple, Unpack, Callable, TypeVar
+
+Ts = TypeVarTuple('Ts')
+T = TypeVar('T')
+
+def func(d: Callable[[Unpack[Ts]], T]) -> T: ...
+
+y = func[1, int] # E: Type application is only supported for generic classes \
+                 # E: Invalid type: try using Literal[1] instead?
+[builtins fixtures/tuple.pyi]