Make index wait for pending indexerTy

Analysis/src/TypeFunction.cpp

@@ -2206,6 +2206,19 @@ TypeFunctionReductionResult<TypeId> indexFunctionImpl(
         return {std::nullopt, true, {}, {}};
 
     TypeId indexerTy = follow(typeParams.at(1));
+
+    // Whether the type is still pending.
+    // 
+    // TODO: A generalized sanity check function for all type functions?
+    // But it should be optional for customization
+    // for functions that specifically need type function
+    // This here fixes a stack overflow.
+    // https://github.com/luau-lang/luau/issues/1406
+    if (isPending(indexerTy, ctx->solver))
+    {
+        return {std::nullopt, false, {indexerTy}, {}};
+    }
+
     std::shared_ptr<const NormalizedType> indexerNormTy = ctx->normalizer->normalize(indexerTy);
 
     // if the indexer failed to normalize, we can't reduce, but know nothing about inhabitance.