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libstdc++: Reject basic_format_parse_context::check_dynamic_spec<>(n)

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LWG 4142 (approved in Wrocław, November 2024) made it ill-formed to call
basic_format_parse_context::check_dynamic_spec with an empty template
argument list.

This adds a static_assert to enforce that, and adjusts the tests.

libstdc++-v3/ChangeLog:

	* include/std/format
	(basic_format_parse_context::check_dynamic_spec): Require a
	non-empty parameter pack, as per LWG 4142.
	* testsuite/std/format/parse_ctx.cc: Remove call of
	check_dynamic_spec with empty template argument list.
	* testsuite/std/format/parse_ctx_neg.cc: Add dg-error to call of
	check_dynamic_spec with empty template argument list.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
This commit is contained in:
Jonathan Wakely 2025-03-08 11:58:49 +00:00 • committed by Jonathan Wakely
parent bb83e83cad
commit a21847acb8
No known key found for this signature in database
3 changed files with 9 additions and 3 deletions
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3
libstdc++-v3/include/std/format
View file

@ -4338,6 +4338,9 @@ namespace __format
constexpr void
basic_format_parse_context<_CharT>::check_dynamic_spec(size_t __id) noexcept
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 4142. check_dynamic_spec should require at least one type
static_assert(sizeof...(_Ts) >= 1);
// This call enforces the Mandates: condition that _Ts contains valid
// types and each type appears at most once. It could be a static_assert
// but this way failures give better diagnostics, due to calling the

1
libstdc++-v3/testsuite/std/format/parse_ctx.cc
View file

@ -491,7 +491,6 @@ test_dynamic_type_check()
std::format_parse_context pc("{1}.{2}");
// None of these calls should do anything at runtime, only during consteval:
pc.check_dynamic_spec<>(0);
pc.check_dynamic_spec<int, const char*>(0);
pc.check_dynamic_spec_integral(0);
pc.check_dynamic_spec_string(0);

8
libstdc++-v3/testsuite/std/format/parse_ctx_neg.cc
View file

@ -12,8 +12,9 @@ test_invalid()
pc.check_dynamic_spec<bool, char, int, unsigned, long long,
unsigned long long, float, double, long double,
const char*, std::string_view, const void*>(0);
// For some reason, an empty pack of types is valid:
pc.check_dynamic_spec<>(0);
// LWG 4142. check_dynamic_spec should require at least one type
pc.check_dynamic_spec<>(0); // { dg-error "here" }
pc.check_dynamic_spec<void>(0); // { dg-error "here" }
// const void* is allowed, but void* is not
@ -25,6 +26,7 @@ test_invalid()
pc.check_dynamic_spec<char8_t>(0); // { dg-error "here" }
// std::string_view is allowed, but std::string is not
pc.check_dynamic_spec<std::string>(0); // { dg-error "here" }
// The types in the pack must be unique.
pc.check_dynamic_spec<int, bool, int>(0); // { dg-error "here" }
std::wformat_parse_context wpc(L"");
@ -38,3 +40,5 @@ test_invalid()
// Each failure above will point to a call to this non-constexpr function:
// { dg-error "__invalid_dynamic_spec" "" { target *-*-* } 0 }
// Except the check_dynamic_spec<>(0) one for LWG 4142 which matches this:
// { dg-error "static assertion failed" "" { target *-*-* } 0 }