Remove warning "suggest braces" for aggregate initialization of an empty class with an aggregate base class.

I recently ran into issues with aggregates and inheritance, I'm using
it for creating a type-safe library where most of the types are build
over "tagged" std::array. After bit of cleaning and enabling -Wall
-Wextra -pedantic I noticed clang only in my pipeline gives me warning.
After a bit of focusing on it I found it's not helpful, and contemplate
disabling the warning all together. After a discussion with other
library authors I found it's bothering more people and decided to fix
it.

Removes this warning:

template<typename T, int N> struct StdArray {
    T contents[N];
  };

template<typename T, int N> struct AggregateAndEmpty : StdArray<T,N> { };

AggregateAndEmpty<int, 3> p = {1, 2, 3}; // <-- warning here about omitted braces

clang/lib/Sema/SemaInit.cpp

@@ -1007,23 +1007,35 @@ static bool isIdiomaticBraceElisionEntity(const InitializedEntity &Entity) {
   //
   // (where std::array is an aggregate struct containing a single array field.
 
-  // FIXME: Should aggregate initialization of a struct with a single
+  if (!Entity.getParent())
-  // base class and no members also suppress the warning?
-  if (Entity.getKind() != InitializedEntity::EK_Member || !Entity.getParent())
     return false;
 
+  // Allows elide brace initialization for aggregates with empty base.
+  if (Entity.getKind() == InitializedEntity::EK_Base) {
     auto *ParentRD =
         Entity.getParent()->getType()->castAs<RecordType>()->getDecl();
-  if (CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(ParentRD))
+    CXXRecordDecl *CXXRD = cast<CXXRecordDecl>(ParentRD);
-    if (CXXRD->getNumBases())
+    return CXXRD->getNumBases() == 1 && CXXRD->field_empty();
-      return false;
+  }
 
+  // Allow brace elision if the only subobject is a field.
+  if (Entity.getKind() == InitializedEntity::EK_Member) {
+    auto *ParentRD =
+        Entity.getParent()->getType()->castAs<RecordType>()->getDecl();
+    if (CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(ParentRD)) {
+      if (CXXRD->getNumBases()) {
+        return false;
+      }
+    }
     auto FieldIt = ParentRD->field_begin();
     assert(FieldIt != ParentRD->field_end() &&
            "no fields but have initializer for member?");
     return ++FieldIt == ParentRD->field_end();
   }
 
+  return false;
+}
+
 /// Check whether the range of the initializer \p ParentIList from element
 /// \p Index onwards can be used to initialize an object of type \p T. Update
 /// \p Index to indicate how many elements of the list were consumed.

clang/test/SemaCXX/aggregate-initialization.cpp

@@ -172,11 +172,20 @@ namespace IdiomaticStdArrayInitDoesNotWarn {
   };
   ArrayAndBaseClass<int, 3> z = {1, 2, 3}; // expected-warning {{suggest braces}}
 
-  // It's not clear whether we should be warning in this case. If this
+  // This pattern is used for tagged aggregates and must not warn
-  // pattern becomes idiomatic, it would be reasonable to suppress the
-  // warning here too.
   template<typename T, int N> struct JustABaseClass : StdArray<T, N> {};
-  JustABaseClass<int, 3> w = {1, 2, 3}; // expected-warning {{suggest braces}}
+  JustABaseClass<int, 3> w = {1, 2, 3};
+  // but this should be also ok
+  JustABaseClass<int, 3> v = {{1, 2, 3}};
+
+  template <typename T, int N> struct OnionBaseClass : JustABaseClass<T, N> {};
+  OnionBaseClass<int, 3> u = {1, 2, 3};
+  OnionBaseClass<int, 3> t = {{{1, 2, 3}}};
+
+  struct EmptyBase {};
+
+  template <typename T, int N> struct AggregateAndEmpty : StdArray<T, N>, EmptyBase {};
+  AggregateAndEmpty<int, 3> p = {1, 2, 3}; // expected-warning {{suggest braces}}
 #endif
 
 #pragma clang diagnostic pop