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

[cor3ntin]

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

[llvmbot]

@llvm/pr-subscribers-clang

Author: cor3ntin (cor3ntin)

Changes

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

---

Full diff: https://github.com/llvm/llvm-project/pull/107214.diff

4 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+2)
• (modified) clang/lib/Sema/SemaLambda.cpp (-1)
• (modified) clang/lib/Sema/SemaLookup.cpp (+1-1)
• (modified) clang/test/SemaCXX/cxx2c-placeholder-vars.cpp (+4-2)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 5eed9827343dd5..5acc44ed177a7d 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -1121,6 +1121,8 @@ Bug Fixes to C++ Support
   Fixes (#GH85992).
 - Fixed a crash-on-invalid bug involving extraneous template parameter with concept substitution. (#GH73885)
 - Fixed assertion failure by skipping the analysis of an invalid field declaration. (#GH99868)
+- 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/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index 601077e9f3334d..809b94bb7412b9 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -1318,7 +1318,6 @@ void Sema::ActOnLambdaExpressionAfterIntroducer(LambdaIntroducer &Intro,
 
     if (C->Init.isUsable()) {
       addInitCapture(LSI, cast<VarDecl>(Var), C->Kind == LCK_ByRef);
-      PushOnScopeChains(Var, CurScope, false);
     } else {
       TryCaptureKind Kind = C->Kind == LCK_ByRef ? TryCapture_ExplicitByRef
                                                  : TryCapture_ExplicitByVal;
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 {

[tru]

This has a conflict in the release notes file.

[cor3ntin]

@tru thanks for noticing, fixed!

[tru]

Can you rebase this so there is just one commit with the changes and no merge commits. Otherwise I have to merge it manually.

[cor3ntin]

@tru done

[github-actions]

@cor3ntin (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix. When you are done, please add the release:note label to this PR.