[clang] [Clang] Add diagnostic for why std::is_abstract is false (PR #156199)

[Shafik Yaghmour via cfe-commits]

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@@ -2774,6 +2775,66 @@ static void DiagnoseNonAggregateReason(Sema &SemaRef, SourceLocation Loc,
     DiagnoseNonAggregateReason(SemaRef, Loc, D);
 }
 
+static void DiagnoseNonAbstractReason(Sema &SemaRef, SourceLocation Loc,
+                                      const CXXRecordDecl *D) {
+  // If this type has any abstract base classes, their respective virtual
+  // functions must have been overridden.
+  for (const CXXBaseSpecifier &B : D->bases()) {
+    assert(B.getType()->getAsCXXRecordDecl() && "invalid base?");
+    if (B.getType()->getAsCXXRecordDecl()->isAbstract()) {
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::OverridesAllPureVirtual
+          << B.getType() << B.getSourceRange();
+    }
+  }
+}
+
+static void DiagnoseNonAbstractReason(Sema &SemaRef, SourceLocation Loc,
+                                      QualType T) {
+  SemaRef.Diag(Loc, diag::note_unsatisfied_trait)
+      << T << diag::TraitName::Abstract;
+
+  if (T->isReferenceType()) {
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::Ref;
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::NotStructOrClass;
+    return;
+  }
+
+  if (T->isUnionType()) {
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shafik wrote:

This case does not look covered in the tests. Always make sure you cover all the diagnostics and various branches.

https://github.com/llvm/llvm-project/pull/156199