On Tue, Aug 29, 2023 at 04:42:33PM -0400, Jason Merrill wrote:
> On 8/28/23 19:24, Marek Polacek wrote:
> > On Fri, Aug 25, 2023 at 08:34:37PM -0400, Jason Merrill wrote:
> > > On 8/25/23 19:37, Marek Polacek wrote:
> > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > > >
> > > > -- >8 --
> > > >
> > > > 1) When saying that a conversion is erroneous because it would use
> > > > an explicit constructor, it might be nice to show where exactly
> > > > the explicit constructor is located. For example, with this patch:
> > > >
> > > > [...]
> > > > explicit.C:4:12: note: 'S::S(int)' declared here
> > > > 4 | explicit S(int) { }
> > > > | ^
> > > >
> > > > 2) When a conversion doesn't work out merely because the conversion
> > > > function necessary to do the conversion couldn't be used because
> > > > it was marked explicit, it would be useful to the user to say so,
> > > > rather than just saying "cannot convert". For example, with this patch:
> > > >
> > > > explicit.C:13:12: error: cannot convert 'S' to 'bool' in initialization
> > > > 13 | bool b = S{1};
> > > > | ^~~~
> > > > | |
> > > > | S
> > > > explicit.C:5:12: note: explicit conversion function was not considered
> > > > 5 | explicit operator bool() const { return true; }
> > > > | ^~~~~~~~
> > > >
> > > > gcc/cp/ChangeLog:
> > > >
> > > > * call.cc (convert_like_internal): Show where the conversion
> > > > function
> > > > was declared.
> > > > (maybe_show_nonconverting_candidate): New.
> > > > * cp-tree.h (maybe_show_nonconverting_candidate): Declare.
> > > > * typeck.cc (convert_for_assignment): Call it.
> > > >
> > > > gcc/testsuite/ChangeLog:
> > > >
> > > > * g++.dg/diagnostic/explicit.C: New test.
> > > > ---
> > > > gcc/cp/call.cc | 41
> > > > +++++++++++++++++++---
> > > > gcc/cp/cp-tree.h | 1 +
> > > > gcc/cp/typeck.cc | 5 +++
> > > > gcc/testsuite/g++.dg/diagnostic/explicit.C | 16 +++++++++
> > > > 4 files changed, 59 insertions(+), 4 deletions(-)
> > > > create mode 100644 gcc/testsuite/g++.dg/diagnostic/explicit.C
> > > >
> > > > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > > > index 23e458d3252..09ebcf6a115 100644
> > > > --- a/gcc/cp/call.cc
> > > > +++ b/gcc/cp/call.cc
> > > > @@ -8459,12 +8459,21 @@ convert_like_internal (conversion *convs, tree
> > > > expr, tree fn, int argnum,
> > > > if (pedwarn (loc, 0, "converting to %qT from
> > > > initializer list "
> > > > "would use explicit constructor %qD",
> > > > totype, convfn))
> > > > - inform (loc, "in C++11 and above a default
> > > > constructor "
> > > > - "can be explicit");
> > > > + {
> > > > + inform (loc, "in C++11 and above a default
> > > > constructor "
> > > > + "can be explicit");
> > > > + inform (DECL_SOURCE_LOCATION (convfn), "%qD
> > > > declared here",
> > > > + convfn);
> > >
> > > I'd swap these two informs.
> >
> > Done.
> > > > +++ b/gcc/testsuite/g++.dg/diagnostic/explicit.C
> > > > @@ -0,0 +1,16 @@
> > > > +// { dg-do compile { target c++11 } }
> > > > +
> > > > +struct S {
> > > > + explicit S(int) { }
> > > > + explicit operator bool() const { return true; } // { dg-message
> > > > "explicit conversion function was not considered" }
> > > > + explicit operator int() const { return 42; } // { dg-message
> > > > "explicit conversion function was not considered" }
> > > > +};
> > > > +
> > > > +void
> > > > +g ()
> > > > +{
> > > > + S s = {1}; // { dg-error "would use explicit constructor" }
> > > > + bool b = S{1}; // { dg-error "cannot convert .S. to .bool. in
> > > > initialization" }
> > > > + int i;
> > > > + i = S{2}; // { dg-error "cannot convert .S. to .int. in assignment" }
> > > > +}
> > >
> > > Let's also test other copy-initialization contexts: parameter passing,
> > > return, throw, aggregate member initialization.
> >
> > Done except for throw. To handle arg passing I moved the call to
> > maybe_show_nonconverting_candidate one line down. I guess a testcase
> > for throw would be
> >
> > struct T {
> > T() { } // #1
> > explicit T(const T&) { } // #2
> > };
> >
> > void
> > g ()
> > {
> > T t{};
> > throw t;
> > }
> >
> > but #2 isn't a viable candidate so this would take more effort to handle.
>
> True, copy-initialization is different when the types are the same.
>
> > We just say about #1 that "candidate expects 0 arguments, 1 provided".
> >
> > clang++ says
> >
> > e.C:3:12: note: explicit constructor is not a candidate
> > 3 | explicit T(const T&) { }
> > | ^
>
> That would be better; in add_candidates when we see an explicit constructor
> we could add it to bad_fns instead of ignoring it. But that doesn't need to
> be part of this patch.
I created https://gcc.gnu.org/PR111230 for that.
> > +void
> > +maybe_show_nonconverting_candidate (tree to, tree from, tree arg, int
> > flags)
> > +{
> > + if (!(flags & LOOKUP_ONLYCONVERTING))
> > + return;
> > +
> > + conversion_obstack_sentinel cos;
> > + conversion *c = implicit_conversion (to, from, arg, /*c_cast_p=*/false,
> > + flags & ~LOOKUP_ONLYCONVERTING, tf_none);
> > + if (c && !c->bad_p && c->user_conv_p)
> > + /* Ay, the conversion would have worked in copy-init context. */
>
> s/copy/direct/
>
> OK with that change.
Duh, of course. I'm pushing the patch with that fixed.
