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

mypy/semanal.py

@@ -5641,6 +5641,15 @@ 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 +2237,13 @@ 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]