[Clang] Don't add top-level const qualifiers to captured function types (#118050)

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

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -761,6 +761,7 @@ 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)
 - Fixed a bug where bounds of partially expanded pack indexing expressions were checked too early. (#GH116105)

clang/lib/Sema/SemaExpr.cpp

@@ -18431,7 +18431,11 @@ 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++ [expr.prim.lambda]p10:
+    //   The type of such a data member is [...] an lvalue reference to the
+    //   referenced function type if the entity is a reference to a function.
+    //   [...]
+    if (Cap.isCopyCapture() && !DeclRefType->isFunctionType() &&
         !(isa<LambdaScopeInfo>(CSI) &&
           !cast<LambdaScopeInfo>(CSI)->lambdaCaptureShouldBeConst()) &&
         !(isa<CapturedRegionScopeInfo>(CSI) &&
@@ -18741,7 +18745,12 @@ 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++ [expr.prim.lambda]p10:
+    //   The type of such a data member is [...] an lvalue reference to the
+    //   referenced function type if the entity is a reference to a function.
+    //   [...]
+    if (Const && !CaptureType->isReferenceType() &&
+        !DeclRefType->isFunctionType())
       DeclRefType.addConst();
   }
 

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

@@ -335,3 +335,17 @@ 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