Add the ability to get the ExternType's name within type functions

Analysis/src/TypeFunctionRuntime.cpp

@@ -21,6 +21,7 @@
 #include <vector>
 
 LUAU_DYNAMIC_FASTINT(LuauTypeFunctionSerdeIterationLimit)
+LUAU_FASTFLAGVARIABLE(DebugLuauTypeFunExternNameMethod)
 
 namespace Luau
 {
@@ -1312,6 +1313,24 @@ static int getWriteParent(lua_State* L)
     return 1;
 }
 
+// Luau: `self:externname() -> string?`
+// Returns the name of a class or 'nil' if there's no name.
+static int getExternTypeName(lua_State* L) {
+    TypeFunctionTypeId self = getTypeUserData(L, 1);
+    auto tfEx = get<TypeFunctionExternType>(self);
+    if (!tfEx)
+        luaL_error(L, "type.externname: expected self to be an extern type, but got %s instead", getTag(L, self).c_str());
+
+    if (auto exTy = get<ExternType>(tfEx->externTy))
+    {
+        lua_pushstring(L, exTy->name.c_str());
+    }
+    else
+        lua_pushnil(L);
+
+    return 1;
+}
+
 // Luau: `self:name() -> string?`
 // Returns the name of the generic or 'nil' if the generic is unnamed
 static int getGenericName(lua_State* L)
@@ -1747,6 +1766,7 @@ void registerTypeUserData(lua_State* L)
         {"name", getGenericName},
         {"ispack", getGenericIsPack},
 
+        {(FFlag::DebugLuauTypeFunExternNameMethod) ? "externname" : nullptr, (FFlag::DebugLuauTypeFunExternNameMethod) ? getExternTypeName : nullptr},
         {nullptr, nullptr}
     };
 

tests/TypeFunction.user.test.cpp

@@ -13,6 +13,7 @@ LUAU_FASTFLAG(LuauEagerGeneralization4)
 LUAU_FASTFLAG(LuauTrackFreeInteriorTypePacks)
 LUAU_FASTFLAG(LuauResetConditionalContextProperly)
 LUAU_FASTFLAG(LuauTypeFunNoScopeMapRef)
+LUAU_FASTFLAG(DebugLuauTypeFunExternNameMethod)
 
 TEST_SUITE_BEGIN("UserDefinedTypeFunctionTests");
 
@@ -2452,6 +2453,35 @@ end
     CHECK(toString(result.errors[0]) == R"(Redefinition of type 't0', previously defined at line 2)");
 }
 
+TEST_CASE_FIXTURE(BuiltinsFixture, "udtf_externtype_externname_api")
+{
+    ScopedFastFlag sff[]{
+        {FFlag::LuauSolverV2, true},
+        {FFlag::DebugLuauTypeFunExternNameMethod, true}
+    };
+
+    loadDefinition(R"(
+        declare class CustomClass
+            function testFunc(self): number
+        end
+    )");
+
+    CheckResult result = check(R"(
+        type function pass(arg, compare)
+            if (arg:is("class")) then
+                assert(arg:externname() == compare:value())
+            end
+
+            return types.unknown
+        end
+
+        type a = pass<CustomClass, "CustomClass">
+        type b = pass<vector, "vector">
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
 TEST_CASE_FIXTURE(BuiltinsFixture, "udtf_fuzz_environment_scope_crash")
 {
     ScopedFastFlag newSolver{FFlag::LuauSolverV2, true};