From 4c3514fa53cc39db5de4dbbca9a54977159f24e8 Mon Sep 17 00:00:00 2001
From: Haojian Wu <hokein.wu@gmail.com>
Date: Thu, 18 Apr 2024 15:46:35 +0200
Subject: [PATCH] [clang] Fix a "!CodeSynthesisContexts.empty()" assertion
 failure when constructing aggregate deduction guides. (#89227)

We were missing to push an record to the instantiation stack in
`DeclareAggregateDeductionGuideForTypeAlias`. This patch fixes that.
---
 clang/lib/Sema/SemaTemplate.cpp              |  5 +++++
 clang/test/SemaCXX/cxx20-ctad-type-alias.cpp | 10 ++++++++++
 2 files changed, 15 insertions(+)

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index f4b6e1ceb6f023..d4976f9d0d11d8 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -3044,6 +3044,11 @@ FunctionTemplateDecl *DeclareAggregateDeductionGuideForTypeAlias(
     return nullptr;
 
   LocalInstantiationScope Scope(SemaRef);
+  Sema::InstantiatingTemplate BuildingDeductionGuides(
+      SemaRef, AliasTemplate->getLocation(), RHSDeductionGuide,
+      Sema::InstantiatingTemplate::BuildingDeductionGuidesTag{});
+  if (BuildingDeductionGuides.isInvalid())
+    return nullptr;
 
   // Build a new template parameter list for the synthesized aggregate deduction
   // guide by transforming the one from RHSDeductionGuide.
diff --git a/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp b/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
index b71cd46f884d63..6f04264a655ad5 100644
--- a/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
+++ b/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
@@ -279,3 +279,13 @@ Bar t = Foo<K<Container>>();
 
 Bar s = 1; // expected-error {{no viable constructor or deduction guide for deduction of template arguments of}}
 } // namespace test20
+
+namespace test21 {
+template <typename T, unsigned N>
+struct Array { const T member[N]; };
+template <unsigned N>
+using String = Array<char, N>;
+
+// Verify no crash on constructing the aggregate deduction guides.
+String s("hello");
+} // namespace test21