keyof - fix LUAU_ASSERT when there's only one key entry (#1388)

Fixes #1387

Was suggested by @alexmccord 

I changed ``singletons[0]`` to ``singletons.front()``, unsure if that
makes a huge difference, and then I added the rest of the things needed
for the return type.

Maybe it's also the ideal location since doing it before looping through
``keys`` won't add the string into the type arena.

I put comments next to it based on how I thought it would make sense.

 

``LUAU_ASSERT`` seems to trigger when there's only one entry being put
inside a UnionType. It's as if it was put there for quality.

Allow edits by maintainers is enabled.


I tested this with a quick Unit Test something like

```lua
local test: keyof<typeof({a="test"})>
```

Analysis/src/TypeFunction.cpp

@@ -2041,6 +2041,12 @@ TypeFunctionReductionResult<TypeId> keyofFunctionImpl(
     for (std::string key : keys)
         singletons.push_back(ctx->arena->addType(SingletonType{StringSingleton{key}}));
 
+    // If there's only one entry, we don't need a UnionType.
+    // We can take straight take it from the first entry
+    // because it was added into the type arena already.
+    if (singletons.size() == 1)
+        return {singletons.front(), false, {}, {}};
+
     return {ctx->arena->addType(UnionType{singletons}), false, {}, {}};
 }
 

tests/TypeFunction.test.cpp

@@ -388,6 +388,25 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "keyof_type_function_works_with_metatables")
     CHECK_EQ("\"w\" | \"x\" | \"y\" | \"z\"", toString(tpm->givenTp));
 }
 
+TEST_CASE_FIXTURE(BuiltinsFixture, "keyof_single_entry_no_uniontype")
+{
+    if (!FFlag::LuauSolverV2)
+        return;
+
+    CheckResult result = check(R"(
+        local tbl_A = { abc = "value" }
+        local tbl_B = { a1 = nil, ["a2"] = nil }
+
+        type keyof_A = keyof<typeof(tbl_A)>
+        type keyof_B = keyof<typeof(tbl_B)>
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK(toString(requireTypeAlias("keyof_A")) == "\"abc\"");
+    CHECK(toString(requireTypeAlias("keyof_B")) == "\"a1\" | \"a2\"");
+}
+
 TEST_CASE_FIXTURE(BuiltinsFixture, "keyof_type_function_errors_if_it_has_nontable_part")
 {
     if (!FFlag::LuauSolverV2)