3 files changed, 31 insertions, 1 deletions
diff --git a/clang/lib/AST/DeclCXX.cpp b/clang/lib/AST/DeclCXX.cpp
index 00ae7c9cda1..9bb6e3a013a 100644
--- a/clang/lib/AST/DeclCXX.cpp
+++ b/clang/lib/AST/DeclCXX.cpp
@@ -628,6 +628,24 @@ bool CXXRecordDecl::hasSubobjectAtOffsetZeroOfEmptyBaseType(
   return false;
 }
 
+bool CXXRecordDecl::lambdaIsDefaultConstructibleAndAssignable() const {
+  assert(isLambda() && "not a lambda");
+
+  // C++2a [expr.prim.lambda.capture]p11:
+  //   The closure type associated with a lambda-expression has no default
+  //   constructor if the lambda-expression has a lambda-capture and a
+  //   defaulted default constructor otherwise. It has a deleted copy
+  //   assignment operator if the lambda-expression has a lambda-capture and
+  //   defaulted copy and move assignment operators otherwise.
+  //
+  // C++17 [expr.prim.lambda]p21:
+  //   The closure type associated with a lambda-expression has no default
+  //   constructor and a deleted copy assignment operator.
+  if (getLambdaCaptureDefault() != LCD_None)
+    return false;
+  return getASTContext().getLangOpts().CPlusPlus2a;
+}
+
 void CXXRecordDecl::addedMember(Decl *D) {
   if (!D->isImplicit() &&
       !isa<FieldDecl>(D) &&
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index b81e5be56a2..a18b8dd5967 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -7088,7 +7088,8 @@ bool Sema::ShouldDeleteSpecialMember(CXXMethodDecl *MD, CXXSpecialMember CSM,
   //   The closure type associated with a lambda-expression has a
   //   deleted (8.4.3) default constructor and a deleted copy
   //   assignment operator.
-  if (RD->isLambda() &&
+  // C++2a adds back these operators if the lambda has no capture-default.
+  if (RD->isLambda() && !RD->lambdaIsDefaultConstructibleAndAssignable() &&
       (CSM == CXXDefaultConstructor || CSM == CXXCopyAssignment)) {
     if (Diagnose)
       Diag(RD->getLocation(), diag::note_lambda_decl);
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index e89e8a2e68f..26fb107688f 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -266,6 +266,17 @@ bool Sema::DiagnoseUseOfDecl(NamedDecl *D, ArrayRef<SourceLocation> Locs,
       return true;
   }
 
+  if (auto *MD = dyn_cast<CXXMethodDecl>(D)) {
+    // Lambdas are only default-constructible or assignable in C++2a onwards.
+    if (MD->getParent()->isLambda() &&
+        ((isa<CXXConstructorDecl>(MD) &&
+          cast<CXXConstructorDecl>(MD)->isDefaultConstructor()) ||
+         MD->isCopyAssignmentOperator() || MD->isMoveAssignmentOperator())) {
+      Diag(Loc, diag::warn_cxx17_compat_lambda_def_ctor_assign)
+        << !isa<CXXConstructorDecl>(MD);
+    }
+  }
+
   auto getReferencedObjCProp = [](const NamedDecl *D) ->
                                       const ObjCPropertyDecl * {
     if (const auto *MD = dyn_cast<ObjCMethodDecl>(D))