Fix prefix/suffix support

Author: jhance

mypy/subtypes.py

@@ -351,8 +351,8 @@ def check_mixed(
                     unpack_index = right.type.type_var_tuple_prefix
                     assert unpack_index is not None
                     type_params = zip(
-                        left_prefix + right_prefix,
-                        left_suffix + right_suffix,
+                        left_prefix + right_suffix,
+                        right_prefix + right_suffix,
                         right.type.defn.type_vars[:unpack_index] +
                         right.type.defn.type_vars[unpack_index+1:]
                     )

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

@@ -131,14 +131,15 @@ from typing_extensions import TypeVarTuple, Unpack
 
 Ts = TypeVarTuple("Ts")
 T = TypeVar("T")
+S = TypeVar("S")
 
-class Variadic(Generic[T, Ts]):
+class Variadic(Generic[T, Ts, S]):
     pass
 
-def foo(t: Variadic[int, Unpack[Ts]]) -> Tuple[int, Unpack[Ts]]:
+def foo(t: Variadic[int, Unpack[Ts], object]) -> Tuple[int, Unpack[Ts]]:
     ...
 
-v: Variadic[int, str, bool]
+v: Variadic[int, str, bool, object]
 reveal_type(foo(v))  # N: Revealed type is "Tuple[builtins.int, builtins.str, builtins.bool]"
 
 [builtins fixtures/tuple.pyi]