[Bug c++/69035] New: [constexpr] Using bultins sometimes make the expression non-constant

["wielkiegie at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69035

            Bug ID: 69035
           Summary: [constexpr] Using bultins sometimes make the
                    expression non-constant
           Product: gcc
           Version: 5.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wielkiegie at gmail dot com
  Target Milestone: ---

Hello,

I have a problem when using builtins (like __builtin_clz()) inside constexpr
functions.

Example:

constexpr int foo(int x) { return __builtin_clz(x); }

Using foo(<constant>) as a constant-expression results in the following error:

char tab[foo(1)];
// error: size of array âtabâ is not an integral constant-expression

However, if I add something to the foo(1) (and this something is not zero) it
starts to work:

char tab[foo(1)+1];
// no error.

Using __builtin_clz() directly also works:

char tab[__builtin_clz(1)];
// no error.

As does adding 1 inside foo():

constexpr int foo2(int x) { return __builtin_clz(x) + 1; }
char tab[foo(1)];
// no error.

The clang compiler does not produce these errors and works with every example
provided here.