Add -Wctad-maybe-unsupported to diagnose CTAD on types with no user defined deduction guides.

Summary:
Some style guides want to allow using CTAD only on types that "opt-in"; i.e. on types that are designed to support it and not just types that *happen* to work with it.

This patch implements the `-Wctad-maybe-unsupported` warning, which is off by default, which warns when CTAD is used on a type that does not define any deduction guides.

The following pattern can be used to suppress the warning in cases where the type intentionally doesn't define any deduction guides:

```
struct allow_ctad_t;

template <class T>
struct TestSuppression {
  TestSuppression(T) {}
};
TestSuppression(allow_ctad_t)->TestSuppression<void>; // guides with incomplete parameter types are never considered.
```

Reviewers: rsmith, james.dennett, gromer

Reviewed By: rsmith

Subscribers: jdennett, Quuxplusone, lebedev.ri, cfe-commits

Differential Revision: https://reviews.llvm.org/D56731

llvm-svn: 351484

1 files changed, 18 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index 10c0c6b..b5a0a88 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -9264,9 +9264,14 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
   OverloadCandidateSet Candidates(Kind.getLocation(),
                                   OverloadCandidateSet::CSK_Normal);
   OverloadCandidateSet::iterator Best;
+
+  bool HasAnyDeductionGuide = false;
+
   auto tryToResolveOverload =
       [&](bool OnlyListConstructors) -> OverloadingResult {
     Candidates.clear(OverloadCandidateSet::CSK_Normal);
+    HasAnyDeductionGuide = false;
+
     for (auto I = Guides.begin(), E = Guides.end(); I != E; ++I) {
       NamedDecl *D = (*I)->getUnderlyingDecl();
       if (D->isInvalidDecl())
@@ -9278,6 +9283,9 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
       if (!GD)
         continue;
 
+      if (!GD->isImplicit())
+        HasAnyDeductionGuide = true;
+
       // C++ [over.match.ctor]p1: (non-list copy-initialization from non-class)
       //   For copy-initialization, the candidate functions are all the
       //   converting constructors (12.3.1) of that class.
@@ -9430,5 +9438,15 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
   Diag(TSInfo->getTypeLoc().getBeginLoc(),
        diag::warn_cxx14_compat_class_template_argument_deduction)
       << TSInfo->getTypeLoc().getSourceRange() << 1 << DeducedType;
+
+  // Warn if CTAD was used on a type that does not have any user-defined
+  // deduction guides.
+  if (!HasAnyDeductionGuide) {
+    Diag(TSInfo->getTypeLoc().getBeginLoc(),
+         diag::warn_ctad_maybe_unsupported)
+        << TemplateName;
+    Diag(Template->getLocation(), diag::note_suppress_ctad_maybe_unsupported);
+  }
+
   return DeducedType;
 }