3 files changed, 34 insertions, 20 deletions

diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index 0e53ffa83fe..49e9126afce 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -8965,12 +8965,6 @@ bool clang::isBetterOverloadCandidate(
     // C++14 [over.match.best]p1 section 2 bullet 3.
   }
 
-  //    -- F1 is generated from a deduction-guide and F2 is not
-  auto *Guide1 = dyn_cast_or_null<CXXDeductionGuideDecl>(Cand1.Function);
-  auto *Guide2 = dyn_cast_or_null<CXXDeductionGuideDecl>(Cand2.Function);
-  if (Guide1 && Guide2 && Guide1->isImplicit() != Guide2->isImplicit())
-    return Guide2->isImplicit();
-
   //    -- F1 is a non-template function and F2 is a function template
   //       specialization, or, if not that,
   bool Cand1IsSpecialization = Cand1.Function &&
@@ -9015,6 +9009,23 @@ bool clang::isBetterOverloadCandidate(
     // Inherited from sibling base classes: still ambiguous.
   }
 
+  // Check C++17 tie-breakers for deduction guides.
+  {
+    auto *Guide1 = dyn_cast_or_null<CXXDeductionGuideDecl>(Cand1.Function);
+    auto *Guide2 = dyn_cast_or_null<CXXDeductionGuideDecl>(Cand2.Function);
+    if (Guide1 && Guide2) {
+      //  -- F1 is generated from a deduction-guide and F2 is not
+      if (Guide1->isImplicit() != Guide2->isImplicit())
+        return Guide2->isImplicit();
+
+      //  -- F1 is the copy deduction candidate(16.3.1.8) and F2 is not
+      if (Guide1->isCopyDeductionCandidate())
+        return true;
+    }
+  }
+  
+
+
   // FIXME: Work around a defect in the C++17 guaranteed copy elision wording,
   // as combined with the resolution to CWG issue 243.
   //
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 46cda5e9ac7..0930fdd653b 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -1837,7 +1837,6 @@ void Sema::DeclareImplicitDeductionGuides(TemplateDecl *Template,
   // for which some class template parameter without a default argument never
   // appears in a deduced context).
   bool AddedAny = false;
-  bool AddedCopyOrMove = false;
   for (NamedDecl *D : LookupConstructors(Transform.Primary)) {
     D = D->getUnderlyingDecl();
     if (D->isInvalidDecl() || D->isImplicit())
@@ -1854,20 +1853,22 @@ void Sema::DeclareImplicitDeductionGuides(TemplateDecl *Template,
 
     Transform.transformConstructor(FTD, CD);
     AddedAny = true;
-
-    AddedCopyOrMove |= CD->isCopyOrMoveConstructor();
   }
 
-  // Synthesize an X() -> X<...> guide if there were no declared constructors.
-  // FIXME: The standard doesn't say (how) to do this.
+  // C++17 [over.match.class.deduct]
+  //    --  If C is not defined or does not declare any constructors, an
+  //    additional function template derived as above from a hypothetical
+  //    constructor C().
   if (!AddedAny)
     Transform.buildSimpleDeductionGuide(None);
 
-  // Synthesize an X(X<...>) -> X<...> guide if there was no declared constructor
-  // resembling a copy or move constructor.
-  // FIXME: The standard doesn't say (how) to do this.
-  if (!AddedCopyOrMove)
-    Transform.buildSimpleDeductionGuide(Transform.DeducedType);
+  //    -- An additional function template derived as above from a hypothetical
+  //    constructor C(C), called the copy deduction candidate.
+  cast<CXXDeductionGuideDecl>(
+      cast<FunctionTemplateDecl>(
+          Transform.buildSimpleDeductionGuide(Transform.DeducedType))
+          ->getTemplatedDecl())
+      ->setIsCopyDeductionCandidate();
 }
 
 /// \brief Diagnose the presence of a default template argument on a
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 186a618387c..d93fbd7c830 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -1651,11 +1651,13 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(FunctionDecl *D,
   }
 
   FunctionDecl *Function;
-  if (auto *DGuide = dyn_cast<CXXDeductionGuideDecl>(D))
+  if (auto *DGuide = dyn_cast<CXXDeductionGuideDecl>(D)) {
     Function = CXXDeductionGuideDecl::Create(
-        SemaRef.Context, DC, D->getInnerLocStart(), DGuide->isExplicit(),
-        D->getNameInfo(), T, TInfo, D->getSourceRange().getEnd());
-  else {
+      SemaRef.Context, DC, D->getInnerLocStart(), DGuide->isExplicit(),
+      D->getNameInfo(), T, TInfo, D->getSourceRange().getEnd());
+    if (DGuide->isCopyDeductionCandidate())
+      cast<CXXDeductionGuideDecl>(Function)->setIsCopyDeductionCandidate();
+  } else {
     Function = FunctionDecl::Create(
         SemaRef.Context, DC, D->getInnerLocStart(), D->getNameInfo(), T, TInfo,
         D->getCanonicalDecl()->getStorageClass(), D->isInlineSpecified(),