Don't emit diagnostics if MSStructAttr is implicit

Author: mstorsjo

clang/lib/Sema/SemaDeclAttr.cpp

@@ -6179,17 +6179,26 @@ static void handleMSStructAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
     S.Diag(First->getLocation(), diag::note_conflicting_attribute);
     return;
   }
+  if (const auto *Preexisting = D->getAttr<MSStructAttr>()) {
+    if (Preexisting->isImplicit())
+      D->dropAttr<MSStructAttr>();
+  }
 
   D->addAttr(::new (S.Context) MSStructAttr(S.Context, AL));
 }
 
 static void handleGCCStructAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   if (const auto *First = D->getAttr<MSStructAttr>()) {
-    S.Diag(AL.getLoc(), diag::err_attributes_are_not_compatible)
-        << AL << First << 0;
-    S.Diag(First->getLocation(), diag::note_conflicting_attribute);
-    return;
+    if (First->isImplicit()) {
+      D->dropAttr<MSStructAttr>();
+    } else {
+      S.Diag(AL.getLoc(), diag::err_attributes_are_not_compatible)
+          << AL << First << 0;
+      S.Diag(First->getLocation(), diag::note_conflicting_attribute);
+      return;
+    }
   }
+
   D->addAttr(::new (S.Context) GCCStructAttr(S.Context, AL));
 }
 

clang/test/Sema/gcc_ms_struct.c

@@ -8,3 +8,30 @@ __attribute__((gcc_struct))
 // expected-error@+1 {{'ms_struct' and 'gcc_struct' attributes are not compatible}}
 __attribute__((ms_struct))
 t1;
+
+struct {
+    int a;
+}
+// expected-note@+1 {{conflicting attribute is here}}
+__attribute__((ms_struct))
+// expected-error@+1 {{'gcc_struct' and 'ms_struct' attributes are not compatible}}
+__attribute__((gcc_struct))
+t2;
+
+#pragma ms_struct on
+struct {
+    int a;
+}
+// No diagnostic for an attribute, unambiguously overriding the pragma.
+__attribute__((gcc_struct))
+t3;
+
+struct {
+    int a;
+}
+// expected-note@+1 {{conflicting attribute is here}}
+__attribute__((ms_struct))
+// expected-error@+1 {{'gcc_struct' and 'ms_struct' attributes are not compatible}}
+__attribute__((gcc_struct))
+t4;
+#pragma ms_struct off