Re: [C++ Patch] PR 80955 (Macros expanded in definition of user-defined literals)

[Mukesh Kapoor <mukesh dot kapoor at oracle dot com>]

On 10/25/2017 6:44 PM, Mukesh Kapoor wrote:
> On 10/25/2017 4:20 AM, Nathan Sidwell wrote:
> > On 10/25/2017 12:03 AM, Mukesh Kapoor wrote:
> >
> > > Thanks for pointing this out. Checking in the front end will be 
> > > difficult because the front end gets tokens after macro expansion. I 
> > > think the difficulty of fixing this bug comes because of the 
> > > requirement to maintain backward compatibility with the option 
> > > -Wliteral-suffix for -std=c++11.
> >
> > IIUC the warning's intent is to catch cases of:
> > printf ("some format"PRIx64 ..., ...);
> > where there's no space between the string literals and the PRIx64 
> > macro.  I suspect it's very common for there to be a following 
> > string-literal, so perhaps the preprocessor could detect:
> >
> > <string-literal>NON-FN-MACRO<maybe-space><string-literal>
> >
> > and warn on that sequence?
>
> Yes, this can be done easily and this is also the usage mentioned in 
> the man page. I made this change in the compiler, bootstrapped it and 
> ran the tests. The following two tests fail after the fix:
>
> g++.dg/cpp0x/Wliteral-suffix.C
> g++.dg/cpp0x/warn_cxx0x4.C
>
> Both tests have code similar to the following (from Wliteral-suffix.C):
>
> #define BAR "bar"
> #define PLUS_ONE + 1
>
>   char c = '3'PLUS_ONE;   // { dg-warning "invalid suffix on literal" }
>   char s[] = "foo"BAR;    // { dg-warning "invalid suffix on literal" }
>
> Other compilers don't accept this code. Maybe I should just modify 
> these tests to have error messages instead of warnings and submit my 
> revised fix?

Actually, according to the man page for -Wliteral-suffix, only macro 
names that don't start with an underscore should be considered when 
issuing a warning:

       -Wliteral-suffix (C++ and Objective-C++ only)
           Warn when a string or character literal is followed by a 
ud-suffix
           which does not begin with an underscore...

So the fix is simply to check if the macro name in is_macro() starts 
with an underscore. The function is_macro() is called only at three 
places. At two places it's used to check for the warning related to 
-Wliteral-suffix and the check for underscore should be made for these 
two cases; at one place it is used to check for the warning related to 
-Wc++11-compat and there is no need to check for underscore for this case.

The fix is simply to pass a bool flag as an additional argument to 
is_macro() to decide whether the macro name starts with an underscore or 
not. I have tested the attached patch on x86_64-linux. Thanks.

Mukesh

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