[clang] [Clang] Implement diagnostics for why is_empty is false (PR #145044)

[Aaron Ballman via cfe-commits]

================
@@ -2285,6 +2286,68 @@ static void DiagnoseNonTriviallyCopyableReason(Sema &SemaRef,
   SemaRef.Diag(D->getLocation(), diag::note_defined_here) << D;
 }
 
+static void DiagnoseIsEmptyReason(Sema &S, SourceLocation Loc,
+                                  const CXXRecordDecl *D) {
+  // Non-static data members (ignore zero-width bit‐fields).
+  for (auto *Field : D->fields()) {
+    if (Field->isBitField() && Field->getBitWidthValue() == 0)
+      continue;
+    S.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::NonEmptyMember << Field
+        << Field->getType() << Field->getSourceRange();
+  }
+
+  // Virtual functions.
+  for (auto *M : D->methods()) {
+    if (M->isVirtual()) {
+      S.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::VirtualFunction << M->getDeclName()
+          << M->getSourceRange();
+      break;
+    }
+  }
+
+  // Virtual bases and non-empty bases.
+  for (auto &B : D->bases()) {
+    auto *BR = B.getType()->getAsCXXRecordDecl();
----------------
AaronBallman wrote:

```suggestion
  for (const auto &B : D->bases()) {
    const CXXRecordDecl *BR = B.getType()->getAsCXXRecordDecl();
```

https://github.com/llvm/llvm-project/pull/145044