[Clang] Recover GLTemplateParameterList for generic lambdas in RebuildLambdaScopeInfo

clang/docs/ReleaseNotes.rst

@@ -714,6 +714,8 @@ Bug Fixes to C++ Support
   assumption if they also occur inside of a dependent lambda. (#GH114787)
 - Clang now uses valid deduced type locations when diagnosing functions with trailing return type
   missing placeholder return type. (#GH78694)
+- Fixed an incorrect lambda scope of generic lambdas that caused Clang to crash when computing potential lambda
+  captures at the end of a full expression. (#GH115931)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDecl.cpp

@@ -15504,10 +15504,25 @@ LambdaScopeInfo *Sema::RebuildLambdaScopeInfo(CXXMethodDecl *CallOperator) {
   LSI->CallOperator = CallOperator;
   LSI->Lambda = LambdaClass;
   LSI->ReturnType = CallOperator->getReturnType();
-  // This function in calls in situation where the context of the call operator
-  // is not entered, so we set AfterParameterList to false, so that
+  // When this function is called in situation where the context of the call
+  // operator is not entered, we set AfterParameterList to false, so that
   // `tryCaptureVariable` finds explicit captures in the appropriate context.
-  LSI->AfterParameterList = false;
+  //
+  // There is also at least a situation as in FinishTemplateArgumentDeduction(),
+  // where we would set the CurContext to the lambda operator before
+  // substituting into it. In this case the flag needs to be true such that
+  // tryCaptureVariable can correctly handle potential captures thereof.
+  LSI->AfterParameterList = CurContext == CallOperator;
+  // GLTemplateParameterList is necessary for getCurGenericLambda() which is
+  // used at the point of dealing with potential captures.
+  //
+  // We don't use LambdaClass->isGenericLambda() because this value doesn't
+  // flip for instantiated generic lambdas, where no FunctionTemplateDecls are
+  // associated. (Technically, we could recover that list from their
+  // instantiation patterns, but for now, the GLTemplateParameterList seems
+  // unnecessary in these cases.)
+  if (FunctionTemplateDecl *FTD = CallOperator->getDescribedFunctionTemplate())
+    LSI->GLTemplateParameterList = FTD->getTemplateParameters();
   const LambdaCaptureDefault LCD = LambdaClass->getLambdaCaptureDefault();
 
   if (LCD == LCD_None)

clang/test/SemaCXX/lambda-capture-type-deduction.cpp

@@ -298,6 +298,22 @@ void __trans_tmp_1() {
 
 }
 
+namespace GH115931 {
+
+struct Range {};
+
+template <Range>
+struct LengthPercentage {};
+
+void reflectSum() {
+  Range resultR;
+  [&] (auto) -> LengthPercentage<resultR> { 
+    return {};
+  }(0);
+}
+
+} // namespace GH115931
+
 namespace GH47400 {
 
 struct Foo {};