Pushed to trunk.
On Fri, 1 Mar 2024 at 15:09, Jonathan Wakely <jwak...@redhat.com> wrote:
> Does the text of these new diagnostics look good?
>
> There are of course other ways for a type to be not-formattable (e.g.
> the formatter::format member doesn't return the right type or has some
> other kind of incorrect signature, or the formatter::parse member isn't
> constexpr) but we can't predict/detect them all reliably. This just
> attempts to give a user-friendly explanation for a couple of common
> mistakes. It should not have any false positives, because the
> basic_format_arg constructor requires __formattable_with<_Tp, _Context>
> so if either of these assertions fails, constructing __arg will fail
> too. The static_assert only adds a more readable error for a
> compilation that's going to fail anyway.
>
> Tested x86_64-linux.
>
> -- >8 --
>
> This adds two new static_assert messages to the internals of
> std::make_format_args to give better diagnostics for invalid format
> args. Rather than just getting an error saying that basic_format_arg
> cannot be constructed, we get more specific errors for the cases where
> std::formatter isn't specialized for the type at all, and where it's
> specialized but only meets the BasicFormatter requirements and so can
> only format non-const arguments.
>
> Also add a test for the existing static_assert when constructing a
> format_string for non-formattable args.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/format (_Arg_store::_S_make_elt): Add two
> static_assert checks to give more user-friendly error messages.
> * testsuite/lib/prune.exp (libstdc++-dg-prune): Prune another
> form of "in requirements with" note.
> * testsuite/std/format/arguments/args_neg.cc: Check for
> user-friendly diagnostics for non-formattable types.
> * testsuite/std/format/string_neg.cc: Likewise.
> ---
> libstdc++-v3/include/std/format | 13 +++++++
> libstdc++-v3/testsuite/lib/prune.exp | 1 +
> .../std/format/arguments/args_neg.cc | 34 ++++++++++++++++++-
> .../testsuite/std/format/string_neg.cc | 4 +++
> 4 files changed, 51 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/std/format
> b/libstdc++-v3/include/std/format
> index ee189f9086c..1e839e88db4 100644
> --- a/libstdc++-v3/include/std/format
> +++ b/libstdc++-v3/include/std/format
> @@ -3704,6 +3704,19 @@ namespace __format
> static _Element_t
> _S_make_elt(_Tp& __v)
> {
> + using _Tq = remove_const_t<_Tp>;
> + using _CharT = typename _Context::char_type;
> + static_assert(is_default_constructible_v<formatter<_Tq, _CharT>>,
> + "std::formatter must be specialized for the type "
> + "of each format arg");
> + using __format::__formattable_with;
> + if constexpr (is_const_v<_Tp>)
> + if constexpr (!__formattable_with<_Tp, _Context>)
> + if constexpr (__formattable_with<_Tq, _Context>)
> + static_assert(__formattable_with<_Tp, _Context>,
> + "format arg must be non-const because its "
> + "std::formatter specialization has a "
> + "non-const reference parameter");
> basic_format_arg<_Context> __arg(__v);
> if constexpr (_S_values_only)
> return __arg._M_val;
> diff --git a/libstdc++-v3/testsuite/lib/prune.exp
> b/libstdc++-v3/testsuite/lib/prune.exp
> index 24a15ccad22..071dcf34c1e 100644
> --- a/libstdc++-v3/testsuite/lib/prune.exp
> +++ b/libstdc++-v3/testsuite/lib/prune.exp
> @@ -54,6 +54,7 @@ proc libstdc++-dg-prune { system text } {
> regsub -all "(^|\n)\[^\n\]*: . skipping \[0-9\]* instantiation
> contexts \[^\n\]*" $text "" text
> regsub -all "(^|\n)\[^\n\]*: in .constexpr. expansion \[^\n\]*"
> $text "" text
> regsub -all "(^|\n)\[^\n\]*: in requirements .with\[^\n\]*" $text
> "" text
> + regsub -all "(^|\n)\[^\n\]*: in requirements with\[^\n\]*" $text ""
> text
> regsub -all "(^|\n) inlined from \[^\n\]*" $text "" text
> # Why doesn't GCC need these to strip header context?
> regsub -all "(^|\n)In file included from \[^\n\]*" $text "" text
> diff --git a/libstdc++-v3/testsuite/std/format/arguments/args_neg.cc
> b/libstdc++-v3/testsuite/std/format/arguments/args_neg.cc
> index 16ac3040146..ded56fe63ab 100644
> --- a/libstdc++-v3/testsuite/std/format/arguments/args_neg.cc
> +++ b/libstdc++-v3/testsuite/std/format/arguments/args_neg.cc
> @@ -6,7 +6,39 @@
>
> std::string rval() { return "path/etic/experience"; }
>
> -void f()
> +void test_rval()
> {
> (void)std::make_format_args(rval()); // { dg-error "cannot bind
> non-const lvalue reference" }
> }
> +
> +void test_missing_specialization()
> +{
> + struct X { };
> + X x;
> + (void)std::make_format_args(x); // { dg-error "here" }
> +// { dg-error "std::formatter must be specialized" "" { target *-*-* } 0 }
> +}
> +
> +struct Y { };
> +template<> class std::formatter<Y> {
> +public:
> + constexpr typename format_parse_context::iterator
> + parse(format_parse_context& c)
> + { return c.begin(); }
> +
> + template<class C>
> + typename C::iterator format(Y&, C&) const;
> +};
> +
> +void test(std::formatter<Y>& f, std::format_parse_context& pc) {
> + f.parse(pc);
> +}
> +
> +void test_const_arg()
> +{
> + const Y y;
> + (void)std::make_format_args(y); // { dg-error "here" }
> +// { dg-error "format arg must be non-const" "" { target *-*-* } 0 }
> +}
> +
> +// { dg-prune-output "no matching function for call to
> .*::basic_format_arg<" }
> diff --git a/libstdc++-v3/testsuite/std/format/string_neg.cc
> b/libstdc++-v3/testsuite/std/format/string_neg.cc
> index 69bcc736cff..d5963145b18 100644
> --- a/libstdc++-v3/testsuite/std/format/string_neg.cc
> +++ b/libstdc++-v3/testsuite/std/format/string_neg.cc
> @@ -4,3 +4,7 @@
>
> auto s = std::format(" {9} "); // { dg-error "call to consteval function"
> }
> // { dg-error "invalid.arg.id" "" { target *-*-* } 0 }
> +
> +struct X { };
> +std::format_string<X> str(""); // dg-error "here" }
> +// { dg-error "std::formatter must be specialized" "" { target *-*-* } 0 }
> --
> 2.43.2
>
>
