Fix IndexError when unpacking empty tuple in type alias

When expanding type arguments for a builtins.tuple instance, the
normalization code accessed args[0] without checking if args was
non-empty. This caused an IndexError when Unpack[tuple[()]] was used
in a type alias, since the empty tuple expansion produces zero args.

Fixes #20913

Author: bxff

mypy/expandtype.py

@@ -223,7 +223,7 @@ def visit_instance(self, t: Instance) -> Type:
             # See: https://github.com/python/mypy/issues/16649
             return t.copy_modified(args=args)
 
-        if t.type.fullname == "builtins.tuple":
+        if t.type.fullname == "builtins.tuple" and len(args) > 0:
             # Normalize Tuple[*Tuple[X, ...], ...] -> Tuple[X, ...]
             arg = args[0]
             if isinstance(arg, UnpackType):

test-data/unit/check-python312.test

@@ -2237,3 +2237,17 @@ class D[*Ts](Generic[Unpack[Us]]):  # E: Generic[...] base class is redundant \
                                     # E: Can only use one type var tuple in a class def
     pass
 [builtins fixtures/tuple.pyi]
+
+[case testUnpackEmptyTupleInTypeAliasNoCrash]
+# https://github.com/python/mypy/issues/20913
+from typing import Unpack
+
+class C[*Ts]:
+    pass
+
+type T[T, *Ts] = C[*Ts]
+
+x: T[bool, Unpack[tuple[()]]]
+reveal_type(x)  # N: Revealed type is "__main__.C[()]"
+[builtins fixtures/tuple.pyi]
+[typing fixtures/typing-full.pyi]