AC_HEADER_STDBOOL rejects valid C99 implementation

[Bruno Haible]

Hi,

The macro AC_HEADER_STDBOOL (in gnulib: in stdbool.m4, in autoconf-2.59c:
in headers.m4) rejects valid 'bool' implementations.

Seen with Sun C 5.9 ("c99 -Xa") for Linux/x86. The config.log contained:

configure:30747: checking for stdbool.h that conforms to C99
configure:30806: /opt/sun/compilers-20060314/bin/c99 -Xa -c -xO5 
-I/packages/inst-sunpro/include conftest.c >&5
"conftest.c", line 126: integral constant expression expected
"conftest.c", line 126: zero or negative subscript
"conftest.c", line 127: warning: improper pointer/integer combination: op "="
"conftest.c", line 128: integral constant expression expected
"conftest.c", line 128: zero or negative subscript
"conftest.c", line 144: warning: statement not reached
c99: acomp failed for conftest.c

The lines of the gnulib macro that this compiler rejects are:

          char d[(bool) -0.5 == true ? 1 : -1];

          char f[(_Bool) -0.0 == false ? 1 : -1];

Let's take a close look:

============================= boolcast.c =============================
#include <stdbool.h>

char ac[(int) 3.999 == 3 ? 1 : -1];
char am[((int) 3.999 == 3) * 2 - 1];

char dc[(bool) -0.5 == true ? 1 : -1];
char dm[((bool) -0.5 == true) * 2 - 1];

char fc[(_Bool) -0.0 == false ? 1 : -1];
char fm[((_Bool) -0.0 == false) * 2 - 1];
======================================================================

$ c99 -Xa -c boolcast.c
"boolcast.c", line 6: integral constant expression expected
"boolcast.c", line 6: zero or negative subscript
"boolcast.c", line 7: warning: can not declare variably modified type at file 
scope
"boolcast.c", line 9: integral constant expression expected
"boolcast.c", line 9: zero or negative subscript
"boolcast.c", line 10: warning: can not declare variably modified type at file 
scope

Why is the compiler right in signalling these errors?

ISO C 99, section 6.6 paragraph 6, defines the term "integer constant
expression":

    An _integer constant expression_ shall have integer type and shall
    only have operands that are integer constants, enumeration constants,
    character constants, sizeof expressions whose results are integer
    constants, and floating constants that are the immediate operands of
    casts. Cast operators in an integer constant expression shall only
    convert arithmetic types to integer types, except as part of an operand
    to the sizeof operator.

ISO C 99, section 6.2.5 paragraph 17, defines the term "integer types":

    The type char, the signed and unsigned integer types, and the enumerated
    types are collectively called integer types.

So, _Bool and bool are _not_ integer types.
=> The expressions (bool) -0.5 and (_Bool) -0.0 are not casts converting
   to integer types.
=> Expressions containing (bool) -0.5 or (_Bool) -0.0 are not integer
   constant expressions.

I suggest to remove these tests, because
  1) the value of (bool) -0.5 or (_Bool) -0.0 cannot be tested in an
     AC_TRY_COMPILE and therefore cannot be cross-compiled.
  2) gnulib's stdbool.h substitute cannot achieve (bool) -0.5 == true.


Patch for gnulib:

2006-05-07  Bruno Haible  <address@hidden>

        * stdbool.m4 (AC_HEADER_STDBOOL): Remove tests that convert a
        floating-point constant to bool; these are not integer constant
        expressions.

*** stdbool.m4.bak      2006-01-26 14:32:38.000000000 +0100
--- stdbool.m4  2006-05-08 00:11:56.000000000 +0200
***************
*** 64,72 ****
          char a[true == 1 ? 1 : -1];
          char b[false == 0 ? 1 : -1];
          char c[__bool_true_false_are_defined == 1 ? 1 : -1];
-         char d[(bool) -0.5 == true ? 1 : -1];
          bool e = &s;
-         char f[(_Bool) -0.0 == false ? 1 : -1];
          char g[true];
          char h[sizeof (_Bool)];
          char i[sizeof s.t];
--- 64,70 ----
***************
*** 104,110 ****
          *pq |= q;
          *pq |= ! q;
          /* Refer to every declared value, to avoid compiler optimizations.  */
!         return (!a + !b + !c + !d + !e + !f + !g + !h + !i + !!j + !k + !!l
                  + !m + !n + !o + !p + !q + !pq);
        ],
        [ac_cv_header_stdbool_h=yes],
--- 102,108 ----
          *pq |= q;
          *pq |= ! q;
          /* Refer to every declared value, to avoid compiler optimizations.  */
!         return (!a + !b + !c + !e + !g + !h + !i + !!j + !k + !!l
                  + !m + !n + !o + !p + !q + !pq);
        ],
        [ac_cv_header_stdbool_h=yes],

Patch for autoconf:

2006-05-07  Bruno Haible  <address@hidden>

        * lib/autoconf/headers.m4 (AC_HEADER_STDBOOL): Remove tests that
        convert a floating-point constant to bool; these are not integer
        constant expressions.

*** autoconf-2.59c/lib/autoconf/headers.m4.bak  2006-04-10 20:27:24.000000000 
+0200
--- autoconf-2.59c/lib/autoconf/headers.m4      2006-05-08 00:16:40.000000000 
+0200
***************
*** 582,590 ****
        char a[true == 1 ? 1 : -1];
        char b[false == 0 ? 1 : -1];
        char c[__bool_true_false_are_defined == 1 ? 1 : -1];
-       char d[(bool) -0.5 == true ? 1 : -1];
        bool e = &s;
-       char f[(_Bool) -0.0 == false ? 1 : -1];
        char g[true];
        char h[sizeof (_Bool)];
        char i[sizeof s.t];
--- 582,588 ----
***************
*** 622,628 ****
        *pq |= q;
        *pq |= ! q;
        /* Refer to every declared value, to avoid compiler optimizations.  */
!       return (!a + !b + !c + !d + !e + !f + !g + !h + !i + !!j + !k + !!l
                + !m + !n + !o + !p + !q + !pq);
        ]])],
        [ac_cv_header_stdbool_h=yes],
--- 620,626 ----
        *pq |= q;
        *pq |= ! q;
        /* Refer to every declared value, to avoid compiler optimizations.  */
!       return (!a + !b + !c + !e + !g + !h + !i + !!j + !k + !!l
                + !m + !n + !o + !p + !q + !pq);
        ]])],
        [ac_cv_header_stdbool_h=yes],