Merge remote-tracking branch 'origin/swift-5.1-branch' into stable

Author: swift-ci

lib/Sema/SemaDeclCXX.cpp

@@ -1301,6 +1301,10 @@ static DeclAccessPair findDecomposableBaseClass(Sema &S, SourceLocation Loc,
 static bool checkMemberDecomposition(Sema &S, ArrayRef<BindingDecl*> Bindings,
                                      ValueDecl *Src, QualType DecompType,
                                      const CXXRecordDecl *OrigRD) {
+  if (S.RequireCompleteType(Src->getLocation(), DecompType,
+                            diag::err_incomplete_type))
+    return true;
+
   CXXCastPath BasePath;
   DeclAccessPair BasePair =
       findDecomposableBaseClass(S, Src->getLocation(), OrigRD, BasePath);

test/SemaCXX/cxx1z-decomposition.cpp

@@ -81,4 +81,21 @@ struct PR37352 {
   void f() { static auto [a] = *this; } // expected-error {{cannot be declared 'static'}}
 };
 
+namespace instantiate_template {
+
+template <typename T1, typename T2>
+struct pair {
+  T1 a;
+  T2 b;
+};
+
+const pair<int, int> &f1();
+
+int f2() {
+  const auto &[a, b] = f1();
+  return a + b;
+}
+
+} // namespace instantiate_template
+
 // FIXME: by-value array copies