[C++17] Fix PR34970 - tweak overload resolution for class template deduction-guides in line with WG21's p0620r0.
In order to identify the copy deduction candidate, I considered two approaches:
- attempt to determine whether an implicit guide is a copy deduction candidate by checking certain properties of its subsituted parameter during overload-resolution.
- using one of the many bits (WillHaveBody) from FunctionDecl (that CXXDeductionGuideDecl inherits from) that are otherwise irrelevant for deduction guides
After some brittle gymnastics w the first strategy, I settled on the second, although to avoid confusion and to give that bit a better name, i turned it into a member of an anonymous union.
Given this identification 'bit', the tweak to overload resolution was a simple reordering of the deduction guide checks (in SemaOverload.cpp::isBetterOverloadCandidate), in-line with Jason Merrill's p0620r0 drafting which made it into the working paper. Concordant with that, I made sure the copy deduction candidate is always added.
References:
See https://bugs.llvm.org/show_bug.cgi?id=34970
See http://wg21.link/p0620r0
llvm-svn: 316292
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 46cda5e..0930fdd6 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -1837,7 +1837,6 @@
// 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 @@
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