Pull in r314499 from upstream clang trunk (by Daniel Marjamäki):

[Sema] Suppress warnings for C's zero initializer

  Patch by S. Gilles!

  Differential Revision: https://reviews.llvm.org/D28148

Pull in r314838 from upstream clang trunk (by Richard Smith):

  Suppress -Wmissing-braces warning when aggregate-initializing a
  struct with a single field that is itself an aggregate.

  In C++, such initialization of std::array<T, N> types is guaranteed
  to work by the standard, is completely idiomatic, and the "suggested"
  alternative from Clang was technically invalid.

Together, these suppress unneeded "suggest braces around initialization
of subobject" warnings for C++11 initializer lists.

MFC after:	3 days

contrib/llvm/tools/clang/include/clang/AST/Expr.h

@@ -3986,6 +3986,10 @@ public:
   /// initializer)?
   bool isTransparent() const;
 
+  /// Is this the zero initializer {0} in a language which considers it
+  /// idiomatic?
+  bool isIdiomaticZeroInitializer(const LangOptions &LangOpts) const;
+
   SourceLocation getLBraceLoc() const { return LBraceLoc; }
   void setLBraceLoc(SourceLocation Loc) { LBraceLoc = Loc; }
   SourceLocation getRBraceLoc() const { return RBraceLoc; }
@@ -3995,6 +3999,9 @@ public:
   InitListExpr *getSemanticForm() const {
     return isSemanticForm() ? nullptr : AltForm.getPointer();
   }
+  bool isSyntacticForm() const {
+    return !AltForm.getInt() || !AltForm.getPointer();
+  }
   InitListExpr *getSyntacticForm() const {
     return isSemanticForm() ? AltForm.getPointer() : nullptr;
   }

contrib/llvm/tools/clang/lib/AST/Expr.cpp

@@ -1899,6 +1899,17 @@ bool InitListExpr::isTransparent() const {
          getInit(0)->getType().getCanonicalType();
 }
 
+bool InitListExpr::isIdiomaticZeroInitializer(const LangOptions &LangOpts) const {
+  assert(isSyntacticForm() && "only test syntactic form as zero initializer");
+
+  if (LangOpts.CPlusPlus || getNumInits() != 1) {
+    return false;
+  }
+
+  const IntegerLiteral *Lit = dyn_cast<IntegerLiteral>(getInit(0));
+  return Lit && Lit->getValue() == 0;
+}
+
 SourceLocation InitListExpr::getLocStart() const {
   if (InitListExpr *SyntacticForm = getSyntacticForm())
     return SyntacticForm->getLocStart();

contrib/llvm/tools/clang/lib/Sema/SemaInit.cpp

@@ -826,6 +826,34 @@ int InitListChecker::numStructUnionElements(QualType DeclType) {
   return InitializableMembers - structDecl->hasFlexibleArrayMember();
 }
 
+/// Determine whether Entity is an entity for which it is idiomatic to elide
+/// the braces in aggregate initialization.
+static bool isIdiomaticBraceElisionEntity(const InitializedEntity &Entity) {
+  // Recursive initialization of the one and only field within an aggregate
+  // class is considered idiomatic. This case arises in particular for
+  // initialization of std::array, where the C++ standard suggests the idiom of
+  //
+  //   std::array<T, N> arr = {1, 2, 3};
+  //
+  // (where std::array is an aggregate struct containing a single array field.
+
+  // FIXME: Should aggregate initialization of a struct with a single
+  // base class and no members also suppress the warning?
+  if (Entity.getKind() != InitializedEntity::EK_Member || !Entity.getParent())
+    return false;
+
+  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();
+}
+
 /// 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.
@@ -886,7 +914,9 @@ void InitListChecker::CheckImplicitInitList(const InitializedEntity &Entity,
     }
 
     // Complain about missing braces.
-    if (T->isArrayType() || T->isRecordType()) {
+    if ((T->isArrayType() || T->isRecordType()) &&
+        !ParentIList->isIdiomaticZeroInitializer(SemaRef.getLangOpts()) &&
+        !isIdiomaticBraceElisionEntity(Entity)) {
       SemaRef.Diag(StructuredSubobjectInitList->getLocStart(),
                    diag::warn_missing_braces)
           << StructuredSubobjectInitList->getSourceRange()
@@ -1833,7 +1863,9 @@ void InitListChecker::CheckStructUnionTypes(
   // worthwhile to skip over the rest of the initializer, though.
   RecordDecl *RD = DeclType->getAs<RecordType>()->getDecl();
   RecordDecl::field_iterator FieldEnd = RD->field_end();
-  bool CheckForMissingFields = true;
+  bool CheckForMissingFields =
+    !IList->isIdiomaticZeroInitializer(SemaRef.getLangOpts());
+
   while (Index < IList->getNumInits()) {
     Expr *Init = IList->getInit(Index);