EricWF added inline comments.
================
Comment at: include/clang/Basic/DiagnosticSemaKinds.td:2129
+def warn_class_template_argument_deduction_no_user_defined_guides : Warning<
+ "using class template argument deduction for %0 that has no user-defined
deduction guides" >,
+ InGroup<ImplicitCTADUsage>, DefaultIgnore;
----------------
rsmith wrote:
> gromer wrote:
> > EricWF wrote:
> > > rsmith wrote:
> > > > gromer wrote:
> > > > > I'd prefer a phrasing more along the lines of "using class template
> > > > > argument deduction for %0 that might not intentionally support it".
> > > > > That gives us more room to do things like add an attribute later if
> > > > > necessary, and it helps the user understand why this warning
> > > > > indicates a potential problem.
> > > > I like that approach; something like "using class template argument
> > > > deduction for %0 that might not intend to support it" -- or perhaps
> > > > more directly "%0 might not intend to support class template argument
> > > > deduction" -- along with a note describing how to syntactically
> > > > suppress the warning (w"add a deduction guide to suppress this warning"
> > > > or "use the [[clang::ctad]] attribute to suppress this warning" or
> > > > whatever we decide is best).
> > > This sounds like a reasonable change to me. Done.
> > >
> > > I'm not sure an attribute is needed at this point; AFAIK there is no case
> > > where a user defined guide can't be added. Even if it's just a dummy
> > > guide to suppress the warning. For example:
> > >
> > > ```
> > >
> > > struct allow_ctad_t {
> > > allow_ctad_t() = delete;
> > > };
> > >
> > > template <class T>
> > > struct TestSuppression {
> > > TestSuppression(int, T) {}
> > > };
> > > TestSuppression(allow_ctad_t) -> TestSuppression<void>;
> > > ```
> > >
> > > Also, before we add an attribute, we want to be sure that it's not
> > > harmful to allowing users to suppress the warning without actually
> > > addressing the root problem (tha implicit CTAD results are often
> > > surprising). I would like to see real world examples of types that don't
> > > need user-defined guides to work.
> > >
> > > I'm not sure an attribute is needed at this point
> >
> > I agree; I just want to keep the option open in case we decide we need one
> > later.
> `TestSuppression(allow_ctad_t) -> TestSuppression<void>;` doesn't always work:
>
> ```
> struct X { X(); };
> template<typename T = X> struct TestSuppression {
> TestSuppression(T);
> };
> TestSuppression(allow_ctad_t) -> TestSuppression<void>;
> TestSuppression x({}); // ok without deduction guide, ill-formed with
> ```
>
> I think this should work reliably (note that the incomplete parameter type
> makes the deduction guide always non-viable):
>
> ```TestSuppression(struct AllowCTAD) -> TestSuppression<void>;```
>
> ... and maybe we should suggest that in our note, perhaps with a fix-it hint
> pointing immediately after the class template definition?
I'm a bit skeptical of a diagnostic pointing at white space at the end of
classes. If we could produce a actionable fixit, then maybe, but what do we put
on the right hand side of `->`?
================
Comment at: include/clang/Basic/DiagnosticSemaKinds.td:2132
+def note_suppress_ctad_maybe_unsupported : Note<
+ "add a user-defined deduction guide to suppress this warning">;
+
----------------
rsmith wrote:
> Drop "user-defined" here? (There's no other kind of deduction guide.)
lol. Are you saying asking the user to define a user-defined entity is
redundant?
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D56731/new/
https://reviews.llvm.org/D56731
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