[Clang] Fix the access checking for non-aggregates in default arguments

[zyn0217]

We check the accessibility of constructors when initializing a default argument whose type is not an aggregate.

Make sure the check is performed within the correct DeclContext. Otherwise, it will be delayed until the end of the declaration, at which point the context is mismatched.

Fixes #62444
Fixes #83608

[llvmbot]

@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)

Changes

We check the accessibility of constructors when initializing a default argument whose type is not an aggregate.

Make sure the check is performed within the correct DeclContext. Otherwise, it will be delayed until the end of the declaration, at which point the context is mismatched.

Fixes #83608

---

Full diff: https://github.com/llvm/llvm-project/pull/141207.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/lib/Sema/SemaTemplateInstantiate.cpp (+7-7)
• (modified) clang/test/SemaTemplate/default-arguments.cpp (+22)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 573ae97bff710..45c96af809505 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -767,6 +767,7 @@ Bug Fixes to C++ Support
 - Clang could incorrectly instantiate functions in discarded contexts (#GH140449)
 - Fix instantiation of default-initialized variable template specialization. (#GH140632) (#GH140622)
 - Clang modules now allow a module and its user to differ on TrivialAutoVarInit*
+- Fixed a C++20 access checking bug when initializing non-aggregates in default arguments (#GH83608)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index d028eea4f8f3e..cd20dfb4fe093 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -3340,13 +3340,13 @@ bool Sema::SubstDefaultArgument(
   }
 
   ExprResult Result;
+  // C++ [dcl.fct.default]p5:
+  //   The names in the [default argument] expression are bound, and
+  //   the semantic constraints are checked, at the point where the
+  //   default argument expression appears.
+  ContextRAII SavedContext(*this, FD);
   {
-    // C++ [dcl.fct.default]p5:
-    //   The names in the [default argument] expression are bound, and
-    //   the semantic constraints are checked, at the point where the
-    //   default argument expression appears.
-    ContextRAII SavedContext(*this, FD);
-    std::unique_ptr<LocalInstantiationScope> LIS;
+    std::optional<LocalInstantiationScope> LIS;
 
     if (ForCallExpr) {
       // When instantiating a default argument due to use in a call expression,
@@ -3354,7 +3354,7 @@ bool Sema::SubstDefaultArgument(
       // required to satisfy references from the default argument. For example:
       //   template<typename T> void f(T a, int = decltype(a)());
       //   void g() { f(0); }
-      LIS = std::make_unique<LocalInstantiationScope>(*this);
+      LIS.emplace(*this);
       FunctionDecl *PatternFD = FD->getTemplateInstantiationPattern(
           /*ForDefinition*/ false);
       if (addInstantiatedParametersToScope(FD, PatternFD, *LIS, TemplateArgs))
diff --git a/clang/test/SemaTemplate/default-arguments.cpp b/clang/test/SemaTemplate/default-arguments.cpp
index 5ea34c0254ec1..e9d781bfe5712 100644
--- a/clang/test/SemaTemplate/default-arguments.cpp
+++ b/clang/test/SemaTemplate/default-arguments.cpp
@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++98 %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
 template<typename T, int N = 2> struct X; // expected-note{{template is declared here}}
 
 X<int, 1> *x1;
@@ -282,4 +283,25 @@ static_assert(S<short *>().SizeOfT<char>() == sizeof(short *), "");
 
 } // namespace GH68490
 
+namespace GH83608 {
+
+class single;
+
+class check_constructible {
+  // This makes it a non-aggregate in C++20+.
+  check_constructible() = default;
+
+  friend class single;
+};
+
+struct single {
+  template <class T> single(T u, check_constructible = {}) {}
+};
+
+// We perform access checking when substituting into the default argument.
+// Make sure it runs within class single.
+single x(0);
+
+}
+
 #endif

[cor3ntin]

Thanks

[cor3ntin]

Does this fixes #62444 ?

[zyn0217]

Does this fixes #62444 ?

Yes 😎