[Clang] Don't add top-level const qualifiers to captured function types

[zyn0217]

This aligns with the logic in TreeTransform::RebuildQualifiedType() where we refrain from adding const qualifiers to function types. Previously, we seemed to overlook this edge case when copy-capturing a variable that is of function type within a const-qualified lambda.

This issue also reveals other related problems as in incorrect type printout and a suspicious implementation in DeduceTemplateArguments. I decide to leave them in follow-up work.

Fixes #84961

[llvmbot]

@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)

Changes

This aligns with the logic in TreeTransform::RebuildQualifiedType() where we refrain from adding const qualifiers to function types. Previously, we seemed to overlook this edge case when copy-capturing a variable that is of function type within a const-qualified lambda.

This issue also reveals other related problems as in incorrect type printout and a suspicious implementation in DeduceTemplateArguments. I decide to leave them in follow-up work.

Fixes #84961

---

Full diff: https://github.com/llvm/llvm-project/pull/118050.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/lib/Sema/SemaExpr.cpp (+9-2)
• (modified) clang/test/SemaCXX/lambda-capture-type-deduction.cpp (+14)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8bd06fadfdc984..553856f3060bc1 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -712,6 +712,7 @@ Bug Fixes to C++ Support
 - Name independent data members were not correctly initialized from default member initializers. (#GH114069)
 - Fixed expression transformation for ``[[assume(...)]]``, allowing using pack indexing expressions within the
   assumption if they also occur inside of a dependent lambda. (#GH114787)
+- Lambdas now capture function types without considering top-level const qualifiers. (#GH84961)
 - Clang now uses valid deduced type locations when diagnosing functions with trailing return type
   missing placeholder return type. (#GH78694)
 
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 6c7472ce92703b..04b713a91dedfa 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -18479,7 +18479,10 @@ static bool isVariableAlreadyCapturedInScopeInfo(CapturingScopeInfo *CSI,
     // are mutable in the sense that user can change their value - they are
     // private instances of the captured declarations.
     const Capture &Cap = CSI->getCapture(Var);
-    if (Cap.isCopyCapture() &&
+    // C++ [dcl.fct]p7:
+    //   [When] adding cv-qualifications on top of the function type [...] the
+    //   cv-qualifiers are ignored.
+    if (Cap.isCopyCapture() && !DeclRefType->isFunctionType() &&
         !(isa<LambdaScopeInfo>(CSI) &&
           !cast<LambdaScopeInfo>(CSI)->lambdaCaptureShouldBeConst()) &&
         !(isa<CapturedRegionScopeInfo>(CSI) &&
@@ -18789,7 +18792,11 @@ static bool captureInLambda(LambdaScopeInfo *LSI, ValueDecl *Var,
     //   parameter-declaration-clause is not followed by mutable.
     DeclRefType = CaptureType.getNonReferenceType();
     bool Const = LSI->lambdaCaptureShouldBeConst();
-    if (Const && !CaptureType->isReferenceType())
+    // C++ [dcl.fct]p7:
+    //   [When] adding cv-qualifications on top of the function type [...] the
+    //   cv-qualifiers are ignored.
+    if (Const && !CaptureType->isReferenceType() &&
+        !DeclRefType->isFunctionType())
       DeclRefType.addConst();
   }
 
diff --git a/clang/test/SemaCXX/lambda-capture-type-deduction.cpp b/clang/test/SemaCXX/lambda-capture-type-deduction.cpp
index a86f3018989927..ba7ab34f943be1 100644
--- a/clang/test/SemaCXX/lambda-capture-type-deduction.cpp
+++ b/clang/test/SemaCXX/lambda-capture-type-deduction.cpp
@@ -319,3 +319,17 @@ constexpr void foo() {
 }
 
 } // namespace GH47400
+
+namespace GH84961 {
+
+template <typename T> void g(const T &t) {}
+
+template <typename T> void f(const T &t) {
+  [t] { g(t); }();
+}
+
+void h() {
+  f(h);
+}
+
+} // namespace GH84961