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

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -341,6 +341,7 @@ 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaLambda.cpp

@@ -1323,7 +1323,6 @@ 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;

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.

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 {