Use correct globalScope in on demand type checker (#923)

Fixes #922 

Unsure if this needs to be put behind another FFlag, since the on demand
type checker fflag currently exists

Analysis/src/Frontend.cpp

@@ -38,6 +38,7 @@ LUAU_FASTFLAGVARIABLE(DebugLuauLogSolverToJson, false)
 LUAU_FASTFLAG(LuauRequirePathTrueModuleName)
 LUAU_FASTFLAGVARIABLE(DebugLuauReadWriteProperties, false)
 LUAU_FASTFLAGVARIABLE(LuauSplitFrontendProcessing, false)
+LUAU_FASTFLAGVARIABLE(LuauTypeCheckerUseCorrectScope, false)
 
 namespace Luau
 {
@@ -1397,7 +1398,9 @@ ModulePtr Frontend::check(const SourceModule& sourceModule, Mode mode, std::vect
     }
     else
     {
-        TypeChecker typeChecker(globals.globalScope, forAutocomplete ? &moduleResolverForAutocomplete : &moduleResolver, builtinTypes, &iceHandler);
+        TypeChecker typeChecker(FFlag::LuauTypeCheckerUseCorrectScope ? (forAutocomplete ? globalsForAutocomplete.globalScope : globals.globalScope)
+                                                                      : globals.globalScope,
+            forAutocomplete ? &moduleResolverForAutocomplete : &moduleResolver, builtinTypes, &iceHandler);
 
         if (prepareModuleScope)
         {

tests/Autocomplete.test.cpp

@@ -3503,4 +3503,26 @@ local a: T@1
     CHECK_EQ(ac.context, AutocompleteContext::Type);
 }
 
+TEST_CASE_FIXTURE(ACFixture, "frontend_use_correct_global_scope")
+{
+    ScopedFastFlag sff("LuauTypeCheckerUseCorrectScope", true);
+
+    loadDefinition(R"(
+        declare class Instance
+            Name: string
+        end
+    )");
+
+    CheckResult result = check(R"(
+        local a: unknown = nil
+        if typeof(a) == "Instance" then
+            local b = a.@1
+        end
+    )");
+    auto ac = autocomplete('1');
+
+    CHECK_EQ(1, ac.entryMap.size());
+    CHECK(ac.entryMap.count("Name"));
+}
+
 TEST_SUITE_END();