Fix detection of __attribute__((may_alias)) to properly look through

type sugar.

We previously missed the attribute in a lot of cases in C++, because
there's often other type sugar there (eg, ElaboratedType).

clang/lib/CodeGen/CodeGenTBAA.cpp

@@ -78,17 +78,18 @@ llvm::MDNode *CodeGenTBAA::getChar() {
 
 static bool TypeHasMayAlias(QualType QTy) {
   // Tagged types have declarations, and therefore may have attributes.
-  if (const TagType *TTy = dyn_cast<TagType>(QTy))
-    return TTy->getDecl()->hasAttr<MayAliasAttr>();
+  if (auto *TD = QTy->getAsTagDecl())
+    if (TD->hasAttr<MayAliasAttr>())
+      return true;
 
-  // Typedef types have declarations, and therefore may have attributes.
-  if (const TypedefType *TTy = dyn_cast<TypedefType>(QTy)) {
-    if (TTy->getDecl()->hasAttr<MayAliasAttr>())
+  // Also look for may_alias as a declaration attribute on a typedef.
+  // FIXME: We should follow GCC and model may_alias as a type attribute
+  // rather than as a declaration attribute.
+  while (auto *TT = QTy->getAs<TypedefType>()) {
+    if (TT->getDecl()->hasAttr<MayAliasAttr>())
       return true;
-    // Also, their underlying types may have relevant attributes.
-    return TypeHasMayAlias(TTy->desugar());
+    QTy = TT->desugar();
   }
-
   return false;
 }
 

clang/test/CodeGenCXX/may_alias.cpp

@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 %s -triple %itanium_abi_triple -emit-llvm -O2 -disable-llvm-passes -o - | FileCheck %s
+// RUN: %clang_cc1 %s -triple %ms_abi_triple -emit-llvm -O2 -disable-llvm-passes -o - | FileCheck %s
+
+enum class __attribute__((may_alias)) E {};
+
+template<typename T> struct A {
+  using B __attribute__((may_alias)) = enum {};
+};
+
+template<typename T> using Alias = typename A<T>::B;
+
+// CHECK-LABEL: define {{.*}}foo
+// CHECK: load i{{[0-9]*}}, {{.*}}, !tbaa ![[MAY_ALIAS:[^ ,]*]]
+auto foo(E &r) { return r; }
+
+// CHECK-LABEL: define {{.*}}goo
+// CHECK: load i{{[0-9]*}}, {{.*}}, !tbaa ![[MAY_ALIAS]]
+auto goo(A<int>::B &r) { return r; }
+
+// CHECK-LABEL: define {{.*}}hoo
+// CHECK: load i{{[0-9]*}}, {{.*}}, !tbaa ![[MAY_ALIAS]]
+auto hoo(Alias<int> &r) { return r; }
+
+// CHECK: ![[CHAR:.*]] = !{!"omnipotent char", !{{.*}}, i64 0}
+// CHECK: ![[MAY_ALIAS]] = !{![[CHAR]], ![[CHAR]], i64 0}