[Clang] Ensure a lambda DeclContext in BuildLambdaExpr (#176319)

zyn0217 · c-rhodes · commit 1c7f7d676c79 · 2026-01-20T15:41:35.000Z
Since 5f9630b, we only remove the LSI after the evaluation context is popped. The TreeTransform of immediate functions may call getCurLambda, which requires both the paired LSI and the lambda DeclContext. In TransformLambdaExpr, we already switched the context, but this is not the case when parsing a lambda expression. No release note, as this is a regression from 22. Fixes #176045 (cherry picked from commit be40637)
diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
@@ -2165,7 +2165,12 @@ ExprResult Sema::BuildLambdaExpr(SourceLocation StartLoc,
   // set as CurContext seems more faithful to the source.
   TemplateOrNonTemplateCallOperatorDecl->setLexicalDeclContext(Class);
 
-  PopExpressionEvaluationContext();
+  {
+    // TreeTransform of immediate functions may call getCurLambda, which
+    // requires both the paired LSI and the lambda DeclContext.
+    ContextRAII SavedContext(*this, CallOperator, /*NewThisContext=*/false);
+    PopExpressionEvaluationContext();
+  }
 
   sema::AnalysisBasedWarnings::Policy WP =
       AnalysisWarnings.getPolicyInEffectAt(EndLoc);
diff --git a/clang/test/SemaCXX/cxx2b-consteval-propagate.cpp b/clang/test/SemaCXX/cxx2b-consteval-propagate.cpp
@@ -629,6 +629,20 @@ void fn() {
 
 }
 
+namespace GH176045 {
+
+template <int NumArgs> struct MessageFormat {
+  template <int N> consteval MessageFormat(const char (&)[N]) {}
+};
+template <typename... Ts> void format(MessageFormat<sizeof...(Ts)>, Ts ...args);
+
+auto message = [] {
+   format("");
+   format("");
+};
+
+}
+
 
 namespace GH109096 {
 consteval void undefined();
