Report error when type alias has multiple TypeVarTuples (PEP 695)

mypy/semanal.py

@@ -4020,6 +4020,8 @@ def analyze_alias(
         for td in tvar_defs:
             if isinstance(td, TypeVarTupleType):
                 if variadic:
+                    if not python_3_12_type_alias:
+                        self.fail("Can only use one TypeVarTuple in a type alias", rvalue)
                     continue
                 variadic = True
             new_tvar_defs.append(td)
@@ -5641,6 +5643,15 @@ def visit_type_alias_stmt(self, s: TypeAliasStmt) -> None:
             return
         all_type_params_names = [p.name for p in s.type_args]
 
+        # Check for multiple TypeVarTuples in type alias definition
+        has_type_var_tuple = False
+        for p in s.type_args:
+            if p.kind == TYPE_VAR_TUPLE_KIND:
+                if has_type_var_tuple:
+                    self.fail("Can only use one TypeVarTuple in a type alias", s)
+                    break
+                has_type_var_tuple = True
+
         try:
             existing = self.current_symbol_table().get(s.name.name)
             if existing and not (

test-data/unit/check-python312.test

@@ -2237,3 +2237,26 @@ 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 testPEP695MultipleTypeVarTuplesTypeAlias]
+type TA1[*Ts1, *Ts2] = tuple[*Ts1] | tuple[*Ts2]  # E: Can only use one TypeVarTuple in a type alias
+type TA2[T, *Ts1, *Ts2] = tuple[T, *Ts1, *Ts2]  # E: Can only use one TypeVarTuple in a type alias \
+                                                 # E: More than one variadic Unpack in a type is not allowed
+type TA3[*Ts1, T, *Ts2] = tuple[*Ts1, T, *Ts2]  # E: Can only use one TypeVarTuple in a type alias \
+                                                 # E: More than one variadic Unpack in a type is not allowed
+type TA4[*Ts] = tuple[*Ts]  # OK - single TypeVarTuple is fine
+[builtins fixtures/tuple.pyi]
+[typing fixtures/typing-full.pyi]
+
+
+[case testMultipleTypeVarTuplesOldStyle]
+# flags: --python-version 3.12
+from typing import Union
+from typing_extensions import TypeVarTuple, Unpack
+
+Ts1 = TypeVarTuple("Ts1")
+Ts2 = TypeVarTuple("Ts2")
+
+TA = Union[tuple[*Ts1], tuple[*Ts2]]  # E: Can only use one TypeVarTuple in a type alias
+[builtins fixtures/tuple.pyi]
+[typing fixtures/typing-full.pyi]