[clang] Improve error message for invalid lambda captures (PR #94865)

[via cfe-commits]

================
@@ -1246,7 +1246,12 @@ void Sema::ActOnLambdaExpressionAfterIntroducer(LambdaIntroducer &Intro,
 
       if (auto *BD = R.getAsSingle<BindingDecl>())
         Var = BD;
-      else
+      else if (auto *FD = R.getAsSingle<FieldDecl>()) {
+        Var = R.getAsSingle<VarDecl>();
----------------
Sirraide wrote:

> Could you explain what kinds of tests or what to verify? I tested the scenario in the issue.

Well, we need to add tests that verify that this actually solves the issue (i.e. that we emit the diagnostic if someone tries to capture a class member); you can take a look at some of the files in `clang/test/SemaCXX` to see how that is usually done.

https://github.com/llvm/llvm-project/pull/94865