[clang] [Clang] Fix incorrect mutability of decltype((x)) in lambda return types (PR #180690)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Samrudh Nelli (SamrudhNelli)

<details>
<summary>Changes</summary>

Previously, the parser did not update the lambda's mutability state before parsing the trailing return type. This caused decltype((x)) to incorrectly deduce a non-const reference even in a non-mutable lambda.
Fixes #<!-- -->180460 

---
Full diff: https://github.com/llvm/llvm-project/pull/180690.diff


3 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+2) 
- (modified) clang/lib/Parse/ParseExprCXX.cpp (+3) 
- (modified) clang/test/SemaCXX/lambda-expressions.cpp (+13) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index a1bb1bd2467b7..f7a1a97cc1caf 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -262,6 +262,8 @@ Bug Fixes to C++ Support
 - Fixed a crash when instantiating ``requires`` expressions involving substitution failures in C++ concepts. (#GH176402)
 - Fixed a crash when a default argument is passed to an explicit object parameter. (#GH176639)
 - Fixed a crash when diagnosing an invalid static member function with an explicit object parameter (#GH177741)
+- Fixed a bug where captured variables in non-mutable lambdas were incorrectly treated as mutable 
+  when used inside decltype in the return type. (#GH180460)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Parse/ParseExprCXX.cpp b/clang/lib/Parse/ParseExprCXX.cpp
index 842b52375eb14..0fb050cce329c 100644
--- a/clang/lib/Parse/ParseExprCXX.cpp
+++ b/clang/lib/Parse/ParseExprCXX.cpp
@@ -1383,6 +1383,9 @@ ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
     addStaticToLambdaDeclSpecifier(*this, StaticLoc, DS);
     addConstexprToLambdaDeclSpecifier(*this, ConstexprLoc, DS);
     addConstevalToLambdaDeclSpecifier(*this, ConstevalLoc, DS);
+    if (clang::sema::LambdaScopeInfo *LSI = Actions.getCurLambda()) {
+      LSI->Mutable = MutableLoc.isValid();
+    }
   }
 
   Actions.ActOnLambdaClosureParameters(getCurScope(), ParamInfo);
diff --git a/clang/test/SemaCXX/lambda-expressions.cpp b/clang/test/SemaCXX/lambda-expressions.cpp
index f9d7cfcbbd18d..ee6ae81107082 100644
--- a/clang/test/SemaCXX/lambda-expressions.cpp
+++ b/clang/test/SemaCXX/lambda-expressions.cpp
@@ -799,3 +799,16 @@ template auto t::operator()<int>(int a) const; // expected-note {{in instantiati
 
 }
 #endif
+
+namespace GH180460 {
+// Trailing return type incorrectly treated 'x' as mutable.
+void test_lambda_return_type() {
+  float x = 0.0f;
+
+  [=]() -> decltype((x)) {
+    decltype(x) y1 = x;
+    decltype((x)) y2 = y1;       // expected-note{{binding reference variable 'y2' here}}
+    return y2;                   // expected-warning{{reference to stack memory associated with local variable 'y1' returned}}
+  };
+}
+}
\ No newline at end of file

``````````

</details>


https://github.com/llvm/llvm-project/pull/180690