[Clang] Improve error message for lambda captures that name a class member (#94865)

This introduces are more helpful error message when trying to 
explicitly capture a class member in a lambda.

Fixes #94764.

Author: CedricSwa

clang/docs/ReleaseNotes.rst

@@ -639,6 +639,9 @@ Improvements to Clang's diagnostics
 - Clang no longer emits a "declared here" note for a builtin function that has no declaration in source.
   Fixes #GH93369.
 
+- Clang now has an improved error message for captures that refer to a class member.
+  Fixes #GH94764.
+
 - Clang now diagnoses unsupported class declarations for ``std::initializer_list<E>`` when they are
   used rather than when they are needed for constant evaluation or when code is generated for them.
   The check is now stricter to prevent crashes for some unsupported declarations (Fixes #GH95495).

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -8204,6 +8204,8 @@ let CategoryName = "Lambda Issue" in {
     "'&' must precede a capture when the capture default is '='">;
   def err_capture_does_not_name_variable : Error<
     "%0 in capture list does not name a variable">;
+  def err_capture_class_member_does_not_name_variable : Error<
+    "class member %0 cannot appear in capture list as it is not a variable">;
   def err_capture_non_automatic_variable : Error<
     "%0 cannot be captured because it does not have automatic storage "
     "duration">;

clang/lib/Sema/SemaLambda.cpp

@@ -1246,7 +1246,11 @@ void Sema::ActOnLambdaExpressionAfterIntroducer(LambdaIntroducer &Intro,
 
       if (auto *BD = R.getAsSingle<BindingDecl>())
         Var = BD;
-      else
+      else if (auto *FD = R.getAsSingle<FieldDecl>()) {
+        Diag(C->Loc, diag::err_capture_class_member_does_not_name_variable)
+            << C->Id;
+        continue;
+      } else
         Var = R.getAsSingle<VarDecl>();
       if (Var && DiagnoseUseOfDecl(Var, C->Loc))
         continue;

clang/test/SemaCXX/lambda-expressions.cpp

@@ -609,6 +609,15 @@ namespace PR25627_dont_odr_use_local_consts {
   }
 }
 
+namespace PR94764 {
+  struct X {
+    int x;
+    void foo() {
+      [x](){}; // expected-error{{class member 'x' cannot appear in capture list as it is not a variable}}
+    }
+  };
+}
+
 namespace ConversionOperatorDoesNotHaveDeducedReturnType {
   auto x = [](int){};
   auto y = [](auto &v) -> void { v.n = 0; }; // cxx03-cxx11-error {{'auto' not allowed in lambda parameter}} cxx03-cxx11-note {{candidate function not viable}} cxx03-cxx11-note {{conversion candidate}}