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

@@ -224,6 +224,10 @@ def visit_instance(self, t: Instance) -> Type:
             return t.copy_modified(args=args)
 
         if t.type.fullname == "builtins.tuple":
+            if not args:
+                # After expansion, args is empty (e.g. Unpack[tuple[()]] expanded
+                # to nothing). Return the empty fixed-length tuple tuple[()].
+                return TupleType([], fallback=t.copy_modified(args=[]))
             # Normalize Tuple[*Tuple[X, ...], ...] -> Tuple[X, ...]
             arg = args[0]
             if isinstance(arg, UnpackType):
@@ -519,7 +523,15 @@ def expand_type_list_with_unpack(self, typs: list[Type]) -> list[Type]:
             if isinstance(item, UnpackType) and isinstance(item.type, TypeVarTupleType):
                 items.extend(self.expand_unpack(item))
             else:
-                items.append(item.accept(self))
+                expanded = item.accept(self)
+                if isinstance(expanded, UnpackType) and isinstance(
+                    expanded.type, TupleType
+                ):
+                    # Inline Unpack[tuple[X, Y]] -> X, Y
+                    # This also handles Unpack[tuple[()]] -> nothing
+                    items.extend(expanded.type.items)
+                    continue
+                items.append(expanded)
         return items
 
     def expand_type_tuple_with_unpack(self, typs: tuple[Type, ...]) -> list[Type]:
@@ -530,7 +542,15 @@ def expand_type_tuple_with_unpack(self, typs: tuple[Type, ...]) -> list[Type]:
             if isinstance(item, UnpackType) and isinstance(item.type, TypeVarTupleType):
                 items.extend(self.expand_unpack(item))
             else:
-                items.append(item.accept(self))
+                expanded = item.accept(self)
+                if isinstance(expanded, UnpackType) and isinstance(
+                    expanded.type, TupleType
+                ):
+                    # Inline Unpack[tuple[X, Y]] -> X, Y
+                    # This also handles Unpack[tuple[()]] -> nothing
+                    items.extend(expanded.type.items)
+                    continue
+                items.append(expanded)
         return items
 
     def visit_tuple_type(self, t: TupleType) -> Type:

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]