has_function: Fix checking for builtins with includes

We were checking for builtins explicitly like this because the ordinary
checks don't work for builtins at all. We do exactly the same check as
Autoconf and it doesn't work with Autoconf either (Autoconf is broken!)

So now we check for it in two ways: if there's no #include in prefix, we
check if `__builtin_symbol` exists (has_function allows checking for
functions without providing includes). If there's a #include, we check
if `symbol` exists.

The old method was causing problems with some buggy toolchains such as
MSYS2 which define some builtins in the C library but don't expose them
via headers which meant that `__builtin_symbol` would be found even
though `symbol` is not available.

Doing this allows people to always get the correct answer as long as
they specify the includes that are required to find a function while
also not forcing people to always specify includes to find a function
which is cumbersome.

Closes #1083

1 files changed, 9 insertions, 1 deletions

diff --git a/test cases/common/43 has function/meson.build b/test cases/common/43 has function/meson.build
index e0d3344f6..323ed0093 100644
--- a/test cases/common/43 has function/meson.build
+++ b/test cases/common/43 has function/meson.build
@@ -1,5 +1,7 @@
 project('has function', 'c', 'cpp')
 
+host_system = host_machine.system()
+
 # This is used in the `test_compiler_check_flags_order` unit test
 unit_test_args = '-I/tmp'
 compilers = [meson.get_compiler('c'), meson.get_compiler('cpp')]
@@ -34,7 +36,7 @@ foreach cc : compilers
   # We can't check for the C library used here of course, but if it's not
   # implemented in glibc it's probably not implemented in any other 'slimmer'
   # C library variants either, so the check should be safe either way hopefully.
-  if host_machine.system() == 'linux' and cc.get_id() == 'gcc'
+  if host_system == 'linux' and cc.get_id() == 'gcc'
     assert (cc.has_function('poll', prefix : '#include <poll.h>',
                             args : unit_test_args),
             'couldn\'t detect "poll" when defined by a header')
@@ -42,6 +44,12 @@ foreach cc : compilers
     assert (not cc.has_function('lchmod', prefix : lchmod_prefix,
                                 args : unit_test_args),
             '"lchmod" check should have failed')
+    # Check that built-ins are found properly both with and without headers
+    assert(cc.has_function('alloca', args : unit_test_args),
+           'built-in alloca must be found on Linux')
+    assert(cc.has_function('alloca', prefix : '#include <alloca.h>',
+           args : unit_test_args),
+           'built-in alloca must be found on Linux with #include')
   endif
 
   # For some functions one needs to define _GNU_SOURCE before including the