Massage polymorphism check for standard_layout

to deconflict it and make it coherent with existing tablegen diagnostics.

Author: snarang181

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -1797,7 +1797,6 @@ def note_unsatisfied_trait_reason
            "%MixedAccess{has mixed access specifiers}|"
            "%MixedAccessField{field %1 has a different access specifier than field %2}|"
            "%MultipleDataBase{has multiple base classes with data members}|"
-           "%VirtualFunction{has a virtual function}|"
            "%NonStandardLayoutMember{has a non-standard-layout member %1 of type %2}|"
            "%IndirectBaseWithFields{has an indirect base %1 with data members}|"
            "%DeletedDtr{has a %select{deleted|user-provided}1 destructor}|"

clang/lib/Sema/SemaTypeTraits.cpp

@@ -2453,15 +2453,26 @@ static void DiagnoseNonStandardLayoutReason(Sema &SemaRef, SourceLocation Loc,
         << diag::TraitNotSatisfiedReason::MultipleDataBase;
   }
   if (D->isPolymorphic()) {
-    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
-        << diag::TraitNotSatisfiedReason::VirtualFunction;
-
-    for (const CXXMethodDecl *Method : D->methods()) {
-      if (Method->isVirtual()) {
-        SemaRef.Diag(Method->getLocation(), diag::note_defined_here) << Method;
+    // Find the best location to point “defined here” at.
+    const CXXMethodDecl *VirtualMD = nullptr;
+    // First, look for a virtual method.
+    for (const auto *M : D->methods()) {
+      if (M->isVirtual()) {
+        VirtualMD = M;
         break;
       }
     }
+    if (VirtualMD) {
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::VirtualFunction << VirtualMD;
+      SemaRef.Diag(VirtualMD->getLocation(), diag::note_defined_here)
+          << VirtualMD;
+    } else {
+      // If no virtual method, point to the record declaration itself.
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::VirtualFunction << D;
+      SemaRef.Diag(D->getLocation(), diag::note_defined_here) << D;
+    }
   }
   for (const FieldDecl *Field : D->fields()) {
     if (!Field->getType()->isStandardLayoutType()) {

clang/test/SemaCXX/type-traits-unsatisfied-diags.cpp

@@ -642,7 +642,7 @@ struct WithVirtual { // #sl-Virtual
 static_assert(__is_standard_layout(WithVirtual));
 // expected-error@-1 {{static assertion failed due to requirement '__is_standard_layout(standard_layout_tests::WithVirtual)'}} \
 // expected-note@-1 {{'WithVirtual' is not standard-layout}} \
-// expected-note@-1 {{because it has a virtual function}} \
+// expected-note@-1 {{because it has a virtual function 'foo'}} \
 // expected-note@#sl-Virtual-Foo {{'foo' defined here}} \
 // expected-note@#sl-Virtual {{'WithVirtual' defined here}}
 
@@ -667,7 +667,7 @@ static_assert(__is_standard_layout(VB));
 // expected-note@-1 {{'VB' is not standard-layout}} \
 // expected-note@-1 {{because it has a virtual base 'VirtualBase'}} \
 // expected-note@-1 {{because it has a non-standard-layout base 'VirtualBase'}} \
-// expected-note@-1 {{because it has a virtual function}}
+// expected-note@-1 {{because it has a virtual function '~VB'}} \
 // expected-note@#sl-VB {{'VB' defined here}}
 // expected-note@#sl-VB {{'~VB' defined here}}