Re: [PATCH v2] New mechanism for declaring const-covariant string functions.

[Zack Weinberg <zackw at panix dot com>]

On Tue, Nov 22, 2016 at 1:06 AM, Paul Eggert <eggert@cs.ucla.edu> wrote:
> Zack Weinberg wrote:
>>
>> This patch absorbs the entire mess into a pair of macros,
>> __CONST_COV_PROTO and __CONST_COV_BUILTIN, which are far less
>> error-prone to use, and which make the public header files much nicer
>> to read, too.
>
> Question: if I get the types wrong with a C or C++ program, what do the
> resulting diagnostics look like?

The diagnostics are unfortunately not as easy to read.  For the same
program you posted, I get this with gcc 6:

strchr-wrong-types.c: In function ‘foo’:
strchr-wrong-types.c:5:17: error: passing argument 1 of ‘strchr’ from
incompatible pointer type [-Werror=incompatible-pointer-types]
  return strchr (x, p);
                 ^
In file included from ../include/bits/const-covariance.h:1:0,
                 from ../string/string.h:39,
                 from ../include/string.h:56,
                 from strchr-wrong-types.c:1:
../string/string.h:181:22: note: expected ‘const char *’ but argument
is of type ‘int *’
 __CONST_COV_BUILTIN (strchr, __attribute_pure__ __nonnull ((1)),
                      ^
../string/bits/const-covariance.h:90:17: note: in definition of macro
‘__CONST_COV_PROTO’
    extern rtype name __CC_XPROTO(const rtype, __VA_ARGS__) __THROW attrs
                 ^~~~
../string/string.h:181:1: note: in expansion of macro ‘__CONST_COV_BUILTIN’
 __CONST_COV_BUILTIN (strchr, __attribute_pure__ __nonnull ((1)),
 ^~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

... and this with g++ 6 ...

strchr-wrong-types.c: In function ‘char* foo(int*, int)’:
strchr-wrong-types.c:5:21: error: no matching function for call to
‘strchr(int*&, int&)’
  return strchr (x, p);
                     ^
In file included from ../include/bits/const-covariance.h:1:0,
                 from ../string/string.h:39,
                 from ../include/string.h:56,
                 from strchr-wrong-types.c:1:
../string/string.h:181:22: note: candidate: char* strchr(char*, int)
 __CONST_COV_BUILTIN (strchr, __attribute_pure__ __nonnull ((1)),
                      ^
../string/bits/const-covariance.h:119:5: note: in definition of macro
‘__CONST_COV_BUILTIN’
     name __CC_XPROTO (rtype, __VA_ARGS__) __THROW   \
     ^~~~
../string/string.h:181:22: note:   no known conversion for argument 1
from ‘int*’ to ‘char*’
 __CONST_COV_BUILTIN (strchr, __attribute_pure__ __nonnull ((1)),
                      ^
../string/bits/const-covariance.h:119:5: note: in definition of macro
‘__CONST_COV_BUILTIN’
     name __CC_XPROTO (rtype, __VA_ARGS__) __THROW   \
     ^~~~
../string/string.h:181:22: note: candidate: const char* strchr(const char*, int)
 __CONST_COV_BUILTIN (strchr, __attribute_pure__ __nonnull ((1)),
                      ^
../string/bits/const-covariance.h:122:5: note: in definition of macro
‘__CONST_COV_BUILTIN’
     name __CC_XPROTO (const rtype, __VA_ARGS__) __THROW   \
     ^~~~
../string/string.h:181:22: note:   no known conversion for argument 1
from ‘int*’ to ‘const char*’
 __CONST_COV_BUILTIN (strchr, __attribute_pure__ __nonnull ((1)),
                      ^
../string/bits/const-covariance.h:122:5: note: in definition of macro
‘__CONST_COV_BUILTIN’
     name __CC_XPROTO (const rtype, __VA_ARGS__) __THROW   \
     ^~~~
cc1plus: all warnings being treated as errors


clang 3.9 doesn't flood the screen with a bunch of macro expansions,
but what you do get is still a bit confusing:

strchr-wrong-types.c:5:17: error: incompatible pointer types passing 'int *' to
      parameter of type 'const char *' [-Werror,-Wincompatible-pointer-types]
        return strchr (x, p);
                       ^
../string/string.h:182:28: note: passing argument to parameter '__s' here
                   char *, __s, int, __c);
                           ^

or in C++ mode

strchr-wrong-types.c:5:9: error: no matching function for call to 'strchr'
        return strchr (x, p);
               ^~~~~~
../string/string.h:181:22: note: candidate function not viable: no known
      conversion from 'int *' to 'const char *' for 1st argument
__CONST_COV_BUILTIN (strchr, __attribute_pure__ __nonnull ((1)),
                     ^
../string/bits/const-covariance.h:92:54: note: expanded from macro
      '__CONST_COV_BUILTIN'
# define __CONST_COV_BUILTIN(...) __CONST_COV_PROTO (__VA_ARGS__)
                                                     ^
../string/bits/const-covariance.h:90:17: note: expanded from macro
      '__CONST_COV_PROTO'
   extern rtype name __CC_XPROTO(const rtype, __VA_ARGS__) __THROW attrs
                ^

(Note: if testing against an uninstalled build tree you will need to
pass both -D_ISOMAC and -D__NO_STRING_INLINES to reproduce these
errors.)