[Clang][Sema] Allow access to a public template alias declaration that refers to friend's private nested type

Author: huqizhi

clang/docs/ReleaseNotes.rst

@@ -259,6 +259,10 @@ Bug Fixes in This Version
   operator.
   Fixes (#GH83267).
 
+- Allow access to a public template alias declaration that refers to friend's
+  private nested type.
+  Fixes (#GH25708).
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -409,7 +413,7 @@ RISC-V Support
 CUDA/HIP Language Changes
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-- PTX is no longer included by default when compiling for CUDA. Using 
+- PTX is no longer included by default when compiling for CUDA. Using
   ``--cuda-include-ptx=all`` will return the old behavior.
 
 CUDA Support

clang/lib/Sema/SemaTemplate.cpp

@@ -4342,10 +4342,17 @@ QualType Sema::CheckTemplateIdType(TemplateName Name,
     InstantiatingTemplate Inst(*this, TemplateLoc, Template);
     if (Inst.isInvalid())
       return QualType();
+    if (!AliasTemplate->getDeclContext()->isFileContext()) {
+      ContextRAII SavedContext(*this, AliasTemplate->getDeclContext());
+      CanonType =
+          SubstType(Pattern->getUnderlyingType(), TemplateArgLists,
+                    AliasTemplate->getLocation(), AliasTemplate->getDeclName());
+    } else {
 
-    CanonType = SubstType(Pattern->getUnderlyingType(),
-                          TemplateArgLists, AliasTemplate->getLocation(),
-                          AliasTemplate->getDeclName());
+      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

clang/test/SemaTemplate/PR25708.cpp

@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -std=c++11 -verify %s
+// RUN: %clang_cc1 -std=c++14 -verify %s
+// RUN: %clang_cc1 -std=c++17 -verify %s
+// RUN: %clang_cc1 -std=c++20 -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;
+}