[Clang] correct error message when assigning to const reference captured in lambda (#105647)

Fixes #98772

Author: nfrmtk

clang/docs/ReleaseNotes.rst

@@ -246,6 +246,8 @@ Improvements to Clang's diagnostics
   potential misaligned members get processed before they can get discarded.
   (#GH144729)
 
+- Clang now emits dignostic with correct message in case of assigning to const reference captured in lambda. (#GH105647)
+
 - Fixed false positive in ``-Wmissing-noreturn`` diagnostic when it was requiring the usage of
   ``[[noreturn]]`` on lambdas before C++23 (#GH154493).
 

clang/lib/Sema/SemaExpr.cpp

@@ -13620,6 +13620,8 @@ static NonConstCaptureKind isReferenceToNonConstCapture(Sema &S, Expr *E) {
   VarDecl *Var = dyn_cast<VarDecl>(Value);
   if (!Var)
     return NCCK_None;
+  if (Var->getType()->isReferenceType())
+    return NCCK_None;
 
   assert(Var->hasLocalStorage() && "capture added 'const' to non-local?");
 

clang/test/SemaCXX/lambda-expressions.cpp

@@ -194,6 +194,11 @@ namespace ModifyingCapture {
     [=] {
       n = 1; // expected-error {{cannot assign to a variable captured by copy in a non-mutable lambda}}
     };
+    const int cn = 0;
+    // cxx03-cxx11-warning@+1 {{initialized lambda captures are a C++14 extension}}
+    [&cnr = cn]{ // expected-note {{variable 'cnr' declared const here}}
+      cnr = 1; // expected-error {{cannot assign to variable 'cnr' with const-qualified type 'const int &'}}
+    };
   }
 }