On 8/30/22 17:37, Jakub Jelinek wrote:
On Tue, Aug 30, 2022 at 11:18:20PM +0200, Jakub Jelinek via Gcc-patches wrote:
On Tue, Aug 30, 2022 at 09:10:37PM +0000, Joseph Myers wrote:
I'm seeing build failures of glibc for powerpc64, as illustrated by the
following C code:
#if 0
\NARG
#endif
(the actual sysdeps/powerpc/powerpc64/sysdep.h code is inside #ifdef
__ASSEMBLER__).
This shows some problems with this feature - and with delimited escape
sequences - as it affects C. It's fine to accept it as an extension
inside string and character literals, because \N or \u{...} would be
invalid in the absence of the feature (i.e. the syntax for such literals
fails to match, meaning that the rule about undefined behavior for a
single ' or " as a pp-token applies). But outside string and character
literals, the usual lexing rules apply, the \ is a pp-token on its own and
the code is valid at the preprocessing level, and with expansion of macros
appearing before or after the \ (e.g. u defined as a macro in the \u{...}
case) it may be valid code at the language level as well. I don't know
what older C++ versions say about this, but for C this means e.g.
#define z(x) 0
#define a z(
int x = a\NARG);
needs to be accepted as expanding to "int x = 0;", not interpreted as
using the \N feature in an identifier and produce an error.
Thanks, will look at it tomorrow.
If
#define z(x) 0
#define a z(
int x = a\NARG);
is valid in C and C++ <= 20 then
#define z(x) 0
#define a z(
int x = a\N{LATIN SMALL LETTER A WITH ACUTE});
is too and shall preprocess to int x = 0; too.
Which would likely mean that we want to only handle it in identifiers if
in C++23 and not actually treat it as an extension except in literals.
Jason, your thoughts about that?
The problem in glibc is with \N that is not followed by {; it certainly
seems that we need to not treat that as an ill-formed named universal
char escape outside of a literal. I think for an actual \N{...}
sequence, we should treat it as an extension unless in strict
conformance mode, kind of like we do with the ext_numeric_numerals flag.
Jason
