[clang] [Clang] Fix handling of placeholder variables name in init captures (PR #107055)

[via cfe-commits]

https://github.com/cor3ntin created https://github.com/llvm/llvm-project/pull/107055

We were incorrectly not deduplicating results when looking up `_` which, for a lambda init capture, would result in an ambiguous lookup.

The same bug caused some diagnostic notes to be emitted twice.

Fixes #107024

>From 18c11cbf5a3768f03234df2ec2bdd79d5e971f55 Mon Sep 17 00:00:00 2001
From: Corentin Jabot <corentinjabot at gmail.com>
Date: Tue, 3 Sep 2024 09:52:55 +0200
Subject: [PATCH] [Clang] Fix handling of placeholder variables name in init
 captures

We were incorrectly not deduplicating results when
looking up `_` which, for a lambda init capture, would result
in an ambiguous lookup.

The same bug caused some diagnostic notes to be emitted twice.

Fixes #107024
---
 clang/docs/ReleaseNotes.rst                   | 1 +
 clang/lib/Sema/SemaLookup.cpp                 | 2 +-
 clang/test/SemaCXX/cxx2c-placeholder-vars.cpp | 6 ++++--
 3 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index fc940db4813948..af4388eada8e5a 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -339,6 +339,7 @@ Bug Fixes to C++ Support
 - Template parameter names are considered in the name lookup of out-of-line class template
   specialization right before its declaration context. (#GH64082)
 - Fixed a constraint comparison bug for friend declarations. (#GH78101)
+- Fix handling of ``_`` as the name of a lambda's init capture variable. (#GH107024)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
index 7a6a64529f52ec..d3d4bf27ae7283 100644
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -570,7 +570,7 @@ void LookupResult::resolveKind() {
 
     // For non-type declarations, check for a prior lookup result naming this
     // canonical declaration.
-    if (!D->isPlaceholderVar(getSema().getLangOpts()) && !ExistingI) {
+    if (!ExistingI) {
       auto UniqueResult = Unique.insert(std::make_pair(D, I));
       if (!UniqueResult.second) {
         // We've seen this entity before.
diff --git a/clang/test/SemaCXX/cxx2c-placeholder-vars.cpp b/clang/test/SemaCXX/cxx2c-placeholder-vars.cpp
index 5cf66b48784e91..29ca3b5ef3df72 100644
--- a/clang/test/SemaCXX/cxx2c-placeholder-vars.cpp
+++ b/clang/test/SemaCXX/cxx2c-placeholder-vars.cpp
@@ -50,14 +50,16 @@ void f() {
 
 void lambda() {
     (void)[_ = 0, _ = 1] { // expected-warning {{placeholder variables are incompatible with C++ standards before C++2c}} \
-                           // expected-note 4{{placeholder declared here}}
+                           // expected-note 2{{placeholder declared here}}
         (void)_++; // expected-error {{ambiguous reference to placeholder '_', which is defined multiple times}}
     };
 
     {
         int _ = 12;
-        (void)[_ = 0]{}; // no warning (different scope)
+        (void)[_ = 0]{ return _;}; // no warning (different scope)
     }
+
+    auto GH107024 = [_ = 42]() { return _; }();
 }
 
 namespace global_var {