[clang] [Clang][Sema] Allow access to a public template alias declaration that refers to friend's private nested type (PR #83847)

[Qizhi Hu via cfe-commits]

https://github.com/jcsxky updated https://github.com/llvm/llvm-project/pull/83847

>From 32bcc78c7d563bda920b3b6150dc1149e1ca1df1 Mon Sep 17 00:00:00 2001
From: huqizhi <huqizhi at feysh.com>
Date: Mon, 4 Mar 2024 21:51:07 +0800
Subject: [PATCH] [Clang][Sema] Allow access to a public template alias
 declaration that refers to friend's private nested type

---
 clang/docs/ReleaseNotes.rst         |  2 ++
 clang/lib/Sema/SemaTemplate.cpp     | 10 +++++++---
 clang/test/SemaTemplate/PR25708.cpp | 20 ++++++++++++++++++++
 3 files changed, 29 insertions(+), 3 deletions(-)
 create mode 100644 clang/test/SemaTemplate/PR25708.cpp

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 88e552d5c46113..a49504b71ad18d 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -351,6 +351,8 @@ Bug Fixes to C++ Support
   when one of the function had more specialized templates.
   Fixes (`#82509 <https://github.com/llvm/llvm-project/issues/82509>`_)
   and (`#74494 <https://github.com/llvm/llvm-project/issues/74494>`_)
+- Allow access to a public template alias declaration that refers to friend's
+  private nested type. (#GH25708).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index d62095558d0ffb..d8c9a5c09944c4 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -4343,9 +4343,13 @@ QualType Sema::CheckTemplateIdType(TemplateName Name,
     if (Inst.isInvalid())
       return QualType();
 
-    CanonType = SubstType(Pattern->getUnderlyingType(),
-                          TemplateArgLists, AliasTemplate->getLocation(),
-                          AliasTemplate->getDeclName());
+    std::optional<ContextRAII> SavedContext;
+    if (!AliasTemplate->getDeclContext()->isFileContext())
+      SavedContext.emplace(*this, AliasTemplate->getDeclContext());
+
+    CanonType =
+        SubstType(Pattern->getUnderlyingType(), TemplateArgLists,
+                  AliasTemplate->getLocation(), AliasTemplate->getDeclName());
     if (CanonType.isNull()) {
       // If this was enable_if and we failed to find the nested type
       // within enable_if in a SFINAE context, dig out the specific
diff --git a/clang/test/SemaTemplate/PR25708.cpp b/clang/test/SemaTemplate/PR25708.cpp
new file mode 100644
index 00000000000000..6a214fc6b43bc1
--- /dev/null
+++ b/clang/test/SemaTemplate/PR25708.cpp
@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -std=c++11 -verify %s
+// expected-no-diagnostics
+
+struct FooAccessor
+{
+    template <typename T>
+    using Foo = typename T::Foo;
+};
+
+class Type
+{
+    friend struct FooAccessor;
+
+    using Foo = int;
+};
+
+int main()
+{
+    FooAccessor::Foo<Type> t;
+}