Analysis: Make typeof on a type userdata return "type" (#1568)

This change introduces a flag (`LuauUserTypeFunTypeofReturnsType`) that,
when enabled, sets `__type` on the type userdata's metatable to "type".
This behaviour was described in the user-defined type function RFC
(https://rfcs.luau.org/user-defined-type-functions.html), but seems to
have been missed; this change implements that behaviour.

Currently this does not change `typeof(t) == 'type'` emitting an unknown
type warning as I don't trust myself to implement it due to my general
lack of C++ knowledge; this can be worked on later.

Analysis/src/TypeFunctionRuntime.cpp

@@ -14,6 +14,7 @@
 #include <vector>
 
 LUAU_DYNAMIC_FASTINT(LuauTypeFunctionSerdeIterationLimit)
+LUAU_FASTFLAGVARIABLE(LuauUserTypeFunTypeofReturnsType)
 
 namespace Luau
 {
@@ -1564,6 +1565,12 @@ void registerTypeUserData(lua_State* L)
 
     // Create and register metatable for type userdata
     luaL_newmetatable(L, "type");
+
+    if (FFlag::LuauUserTypeFunTypeofReturnsType)
+    {
+        lua_pushstring(L, "type");
+        lua_setfield(L, -2, "__type");
+    }
 
     // Protect metatable from being changed
     lua_pushstring(L, "The metatable is locked");

tests/TypeFunction.user.test.cpp

@@ -9,6 +9,7 @@ using namespace Luau;
 
 LUAU_FASTFLAG(LuauSolverV2)
 LUAU_FASTFLAG(DebugLuauEqSatSimplification)
+LUAU_FASTFLAG(LuauUserTypeFunTypeofReturnsType)
 
 TEST_SUITE_BEGIN("UserDefinedTypeFunctionTests");
 
@@ -1866,4 +1867,22 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "udtf_eqsat_opaque")
     CHECK_EQ("t0<number & string>", toString(simplified->result)); // NOLINT(bugprone-unchecked-optional-access)
 }
 
+TEST_CASE_FIXTURE(BuiltinsFixture, "typeof_type_userdata_returns_type")
+{
+    ScopedFastFlag solverV2{FFlag::LuauSolverV2, true};
+    ScopedFastFlag luauUserTypeFunTypeofReturnsType{FFlag::LuauUserTypeFunTypeofReturnsType, true};
+
+    CheckResult result = check(R"(
+type function test(t)
+    print(typeof(t))
+    return t
+end
+
+local _:test<number>
+    )");
+
+    LUAU_REQUIRE_ERROR_COUNT(1, result);
+    CHECK(toString(result.errors[0]) == R"(type)");
+}
+
 TEST_SUITE_END();