Implement RFC, rename tag "class" to "extern" locked behind a flag, tenary operator

Analysis/src/TypeFunctionRuntime.cpp

@@ -21,6 +21,7 @@
 #include <vector>
 
 LUAU_DYNAMIC_FASTINT(LuauTypeFunctionSerdeIterationLimit)
+LUAU_FASTFLAGVARIABLE(LuauRenameClassToExtern)
 
 namespace Luau
 {
@@ -288,7 +289,7 @@ static std::string getTag(lua_State* L, TypeFunctionTypeId ty)
     else if (get<TypeFunctionFunctionType>(ty))
         return "function";
     else if (get<TypeFunctionExternType>(ty))
-        return "class";
+        return (FFlag::LuauRenameClassToExtern ? "extern" : "class");
     else if (get<TypeFunctionGenericType>(ty))
         return "generic";
 

tests/TypeFunction.user.test.cpp

@@ -13,6 +13,8 @@ LUAU_FASTFLAG(LuauEagerGeneralization4)
 LUAU_FASTFLAG(LuauTypeFunNoScopeMapRef)
 LUAU_FASTFLAG(LuauInstantiateResolvedTypeFunctions)
 
+LUAU_FASTFLAG(LuauRenameClassToExtern)
+
 TEST_SUITE_BEGIN("UserDefinedTypeFunctionTests");
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "udtf_nil_serialization_works")
@@ -2449,6 +2451,39 @@ end
     CHECK(toString(result.errors[0]) == R"(Redefinition of type 't0', previously defined at line 2)");
 }
 
+TEST_CASE_FIXTURE(BuiltinsFixture, "udtf_extern_tag")
+{
+    ScopedFastFlag sff[]{
+        {FFlag::LuauSolverV2, true},
+        {FFlag::LuauRenameClassToExtern, true}
+    };
+
+    loadDefinition(R"(
+        declare extern type CustomExternType with
+            function testFunc(self): number
+        end
+    )");
+
+    CheckResult result = check(R"(
+        --!strict
+        type function tyFunc(arg: type)
+            if (arg:is("extern")) then
+                return arg
+            end
+            -- this should never be returned
+            return types.boolean
+        end
+
+        type a = tyFunc<CustomExternType>
+    )");
+
+    auto test = requireTypeAlias("a");
+    // If it's not an ExternType it will fail.
+    LUAU_ASSERT( test->ty.get_if<ExternType>() );
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
 TEST_CASE_FIXTURE(BuiltinsFixture, "udtf_fuzz_environment_scope_crash")
 {
     ScopedFastFlag newSolver{FFlag::LuauSolverV2, true};