[clang] [Clang] Recover GLTemplateParameterList for generic lambdas in RebuildLambdaScopeInfo (PR #118176)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)

<details>
<summary>Changes</summary>

The NTTP argument appearing inside a trailing return type of a generic lambda would have to check for potential lambda captures, where the function needs GLTemplateParameterList of the current LSI to tell whether the lambda is generic.

The lambda scope in this context is rebuilt by the LambdaScopeForCallOperatorInstantiationRAII when substituting the lambda operator during template argument deduction. Thus, I think the template parameter list should be preserved in the rebuilding process, as it seems otherwise innocuous to me.

Fixes #<!-- -->115931

---
Full diff: https://github.com/llvm/llvm-project/pull/118176.diff


3 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+2) 
- (modified) clang/lib/Sema/SemaDecl.cpp (+18-3) 
- (modified) clang/test/SemaCXX/lambda-capture-type-deduction.cpp (+16) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8bd06fadfdc984..e8e8a8cff76e45 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/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
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 74b0e5ad23bd48..75389d6ba7bfd8 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/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)
diff --git a/clang/test/SemaCXX/lambda-capture-type-deduction.cpp b/clang/test/SemaCXX/lambda-capture-type-deduction.cpp
index a86f3018989927..234cb6806f041a 100644
--- a/clang/test/SemaCXX/lambda-capture-type-deduction.cpp
+++ b/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 {};

``````````

</details>


https://github.com/llvm/llvm-project/pull/118176