[libc++][format] Allows width arg-id with value 0.

[Mordante]

Summary

Implements:

• LWG3721 Allow an arg-id with a value of zero for width in std-format-spec

[vitaut]

One check less, yay!

libcxx/test/std/utilities/format/format.functions/format_tests.h

242

As before, I'd recommend getting rid of template operator()<...> in favor of something more readable like a normal function call.

[Mordante]

Thanks for the review!

libcxx/test/std/utilities/format/format.functions/format_tests.h

242

Agreed, that's on my todo list. But I want to look at that when there are no patches in review that would result in merge conflicts. (The wouldn't be hard to solve, but I prefer avoiding the hassle.)

[Mordante]

In D130649#3683539, @vitaut wrote:

One check less, yay!

I'm actually surprised how fast this issue went through LWG :-)

[vitaut]

I'm actually surprised how fast this issue went through LWG :-)

Move Fast and Break ABI

libcxx/docs/Status/Cxx2bIssues.csv

 "`3708 <https://wg21.link/LWG3708>`__","``take_while_view::sentinel``'s conversion constructor should move","July 2022","","","|ranges|"
 "`3709 <https://wg21.link/LWG3709>`__","LWG-3703 was underly ambitious","July 2022","",""
 "`3710 <https://wg21.link/LWG3710>`__","The ``end`` of ``chunk_view`` for input ranges can be ``const``","July 2022","","","|ranges|"
 "`3711 <https://wg21.link/LWG3711>`__","Missing preconditions for slide_view constructor","July 2022","","","|ranges|"
 "`3712 <https://wg21.link/LWG3712>`__","``chunk_view`` and ``slide_view`` should not be ``default_initializable``","July 2022","","","|ranges|"
 "`3713 <https://wg21.link/LWG3713>`__","Sorted with respect to comparator (only)","July 2022","",""
 "`3715 <https://wg21.link/LWG3715>`__","``view_interface::empty`` is overconstrained","July 2022","",""
 "`3719 <https://wg21.link/LWG3719>`__","Directory iterators should be usable with default sentinel","July 2022","",""
-"`3721 <https://wg21.link/LWG3721>`__","Allow an ``arg-id`` with a value of zero for ``width`` in ``std-format-spec``","July 2022","","","|format|"
+"`3721 <https://wg21.link/LWG3721>`__","Allow an ``arg-id`` with a value of zero for ``width`` in ``std-format-spec``","July 2022","|Complete|","16.0","|format|"
 "`3724 <https://wg21.link/LWG3724>`__","``decay-copy`` should be constrained","July 2022","|Complete|","14.0"
 "","","","",""
 "`3645 <https://wg21.link/LWG3645>`__","``resize_and_overwrite`` is overspecified to call its callback with lvalues","Not voted in","|Complete|","14.0",""
 "`3631 <https://wg21.link/LWG3631>`__","``basic_format_arg(T&&)`` should use ``remove_cvref_t<T>`` throughout","Not voted in","|Complete|","15.0",""
 "","","","",""

libcxx/include/__format/parser_std_format_spec.h

_LIBCPP_HIDE_FROM_ABI constexpr void __parse_type(const _CharT*& __begin) {
     ++__begin;
   }
 
   _LIBCPP_HIDE_FROM_ABI
   int32_t __get_width(auto& __ctx) const {
     if (!__width_as_arg_)
       return __width_;
 
-    int32_t __result = __format_spec::__substitute_arg_id(__ctx.arg(__width_));
-    if (__result == 0)
-      __throw_format_error("A format-spec width field replacement should have a positive value");
-    return __result;
+    return __format_spec::__substitute_arg_id(__ctx.arg(__width_));
   }
 
   _LIBCPP_HIDE_FROM_ABI
   int32_t __get_precision(auto& __ctx) const {
     if (!__precision_as_arg_)
       return __precision_;
 
     return __format_spec::__substitute_arg_id(__ctx.arg(__precision_));

libcxx/test/std/utilities/format/format.functions/format_tests.h

void format_test_string(const W& world, const U& universe, TestFunction check, ExceptionTest check_exception) {
 
   // *** alternate form ***
   check_exception("The format-spec should consume the input or end with a '}'", SV("hello {:#}"), world);
 
   // *** zero-padding ***
   check_exception("A format-spec width field shouldn't have a leading zero", SV("hello {:0}"), world);
 
   // *** width ***
+  // Width 0 allowed, but not useful for string arguments.
+  check.template operator()<"hello {:{}}">(SV("hello world"), world, 0);
+
 #ifdef _LIBCPP_VERSION
   // This limit isn't specified in the Standard.
   static_assert(std::__format::__number_max == 2'147'483'647, "Update the assert and the test.");
   check_exception("The numeric value of the format-spec is too large", SV("{:2147483648}"), world);
   check_exception("The numeric value of the format-spec is too large", SV("{:5000000000}"), world);
   check_exception("The numeric value of the format-spec is too large", SV("{:10000000000}"), world);
 #endif
 
-  check_exception("A format-spec width field replacement should have a positive value", SV("hello {:{}}"), world, 0);
   check_exception("A format-spec arg-id replacement shouldn't have a negative value", SV("hello {:{}}"), world, -1);
   check_exception("A format-spec arg-id replacement exceeds the maximum supported value", SV("hello {:{}}"), world,
                   unsigned(-1));
   check_exception("Argument index out of bounds", SV("hello {:{}}"), world);
   check_exception("A format-spec arg-id replacement argument isn't an integral type", SV("hello {:{}}"), world,
                   universe);
   check_exception("Using manual argument numbering in automatic argument numbering mode", SV("hello {:{0}}"), world, 1);
   check_exception("Using automatic argument numbering in manual argument numbering mode", SV("hello {0:{}}"), world, 1);