From 43c03168ea90d21ffea519ff4c094b00bc579d4b Mon Sep 17 00:00:00 2001
From: nothing1649 <65153224+nothing1649@users.noreply.github.com>
Date: Mon, 16 Dec 2024 11:02:52 +1100
Subject: [PATCH] Analysis: Make typeof on a type userdata return "type"

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.
---
 Analysis/src/TypeFunctionRuntime.cpp |  7 +++++++
 tests/TypeFunction.user.test.cpp     | 20 ++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/Analysis/src/TypeFunctionRuntime.cpp b/Analysis/src/TypeFunctionRuntime.cpp
index 24c75a510..ffb4cf0e6 100644
--- a/Analysis/src/TypeFunctionRuntime.cpp
+++ b/Analysis/src/TypeFunctionRuntime.cpp
@@ -17,6 +17,7 @@ LUAU_DYNAMIC_FASTINT(LuauTypeFunctionSerdeIterationLimit)
 LUAU_FASTFLAGVARIABLE(LuauUserTypeFunPrintToError)
 LUAU_FASTFLAGVARIABLE(LuauUserTypeFunFixNoReadWrite)
 LUAU_FASTFLAGVARIABLE(LuauUserTypeFunThreadBuffer)
+LUAU_FASTFLAGVARIABLE(LuauUserTypeFunTypeofReturnsType)
 
 namespace Luau
 {
@@ -1504,6 +1505,12 @@ 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");
diff --git a/tests/TypeFunction.user.test.cpp b/tests/TypeFunction.user.test.cpp
index a5af44fba..e493e2c17 100644
--- a/tests/TypeFunction.user.test.cpp
+++ b/tests/TypeFunction.user.test.cpp
@@ -14,6 +14,7 @@ LUAU_FASTFLAG(LuauUserTypeFunExportedAndLocal)
 LUAU_FASTFLAG(LuauUserDefinedTypeFunParseExport)
 LUAU_FASTFLAG(LuauUserTypeFunThreadBuffer)
 LUAU_FASTFLAG(LuauUserTypeFunUpdateAllEnvs)
+LUAU_FASTFLAG(LuauUserTypeFunTypeofReturnsType)
 
 TEST_SUITE_BEGIN("UserDefinedTypeFunctionTests");
 
@@ -1393,4 +1394,23 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "print_to_error_plus_no_result")
     CHECK(toString(result.errors[3]) == R"(Type function instance t0<string> is uninhabited)");
 }
 
+TEST_CASE_FIXTURE(BuiltinsFixture, "typeof_type_userdata_returns_type")
+{
+    ScopedFastFlag solverV2{FFlag::LuauSolverV2, true};
+    ScopedFastFlag luauUserTypeFunPrintToError{FFlag::LuauUserTypeFunPrintToError, 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();