[clang] [Clang] Add captures to the instantiation scope for noexcept specifiers (PR #97166)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 updated https://github.com/llvm/llvm-project/pull/97166

>From 91a28dcde9dc569b955df92db91c858c550f6ad3 Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Sat, 29 Jun 2024 21:32:01 +0800
Subject: [PATCH] [Clang] Add captures to the instantiation scope for noexcept
 specifiers

---
 clang/docs/ReleaseNotes.rst                   |  1 +
 .../lib/Sema/SemaTemplateInstantiateDecl.cpp  |  6 +++++
 clang/test/SemaTemplate/generic-lambda.cpp    | 23 +++++++++++++++++++
 3 files changed, 30 insertions(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index da967fcdda8089..8c5bd989c60cea 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -935,6 +935,7 @@ Bug Fixes to C++ Support
 - Fix an assertion failure caused by parsing a lambda used as a default argument for the value of a
   forward-declared class. (#GH93512).
 - Fixed a bug in access checking inside return-type-requirement of compound requirements. (#GH93788).
+- Fixed a bug where references to lambda capture inside a ``noexcept`` specifier were not correctly instantiated. (#GH95735).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 0681520764d9a0..d33db60d784487 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -4719,6 +4719,12 @@ void Sema::InstantiateExceptionSpec(SourceLocation PointOfInstantiation,
     return;
   }
 
+  // The noexcept specification could reference any lambda captures. Ensure
+  // those are added to the LocalInstantiationScope.
+  LambdaScopeForCallOperatorInstantiationRAII PushLambdaCaptures(
+      *this, Decl, TemplateArgs, Scope,
+      /*ShouldAddDeclsFromParentScope=*/false);
+
   SubstExceptionSpec(Decl, Template->getType()->castAs<FunctionProtoType>(),
                      TemplateArgs);
 }
diff --git a/clang/test/SemaTemplate/generic-lambda.cpp b/clang/test/SemaTemplate/generic-lambda.cpp
index fb5fa09ebcc1fd..804eeaa29d6a1d 100644
--- a/clang/test/SemaTemplate/generic-lambda.cpp
+++ b/clang/test/SemaTemplate/generic-lambda.cpp
@@ -60,3 +60,26 @@ template<class T1> C1<X<X<T1>>> auto t3() {
 template C1<X<X<int>>> auto t3<int>();
 static_assert(is_same<decltype(t3<int>()), X<X<X<int>>>>);
 #endif
+
+namespace GH95735 {
+
+int g(int fn) {
+  return [f = fn](auto tpl) noexcept(noexcept(f)) { return f; }(0);
+}
+
+int foo(auto... fn) {
+  // FIXME: This one hits the assertion "if the exception specification is dependent,
+  // then the noexcept expression should be value-dependent" in the constructor of
+  // FunctionProtoType.
+  // One possible solution is to update Sema::canThrow() to consider expressions
+  // (e.g. DeclRefExpr/FunctionParmPackExpr) involving unexpanded parameters as Dependent.
+  // This would effectively add an extra value-dependent flag to the noexcept expression.
+  // However, I'm afraid that would also cause ABI breakage.
+  // [...f = fn](auto tpl) noexcept(noexcept(f)) { return 0; }(0);
+  [...f = fn](auto tpl) noexcept(noexcept(g(fn...))) { return 0; }(0);
+  return [...f = fn](auto tpl) noexcept(noexcept(g(f...))) { return 0; }(0);
+}
+
+int v = foo(42);
+
+} // namespace GH95735