Add support for modmap None (#291)

Unlike current xkbcommon, X11’s xkbcomp allows to remove entries in
the modifiers’ map using “modifier_map None { … }”.

“None” is translated to the special value “XkbNoModifier” defined in
“X11/extensions/XKB.h”. Then it relies on the fact that in "CopyModMapDef",
the following code:

    1U << entry->modifier

ends up being zero when “entry->modifier” is “XkbNoModifier” (i.e. 0xFF).
Indeed, it relies on the overflow behaviour of the left shift, which in
practice resolves to use only the 5 low bits of the shift amount, i.e.
0x1F here. Then the result of “1U << 0xFF” is cast to “char”, i.e. 0.

This is a good trick but too magical, so in libxkbcommon we will use
an explicit test against our new constant XKB_MOD_NONE.

1 files changed, 5 insertions, 0 deletions

diff --git a/meson.build b/meson.build
index 61c2f82..d55e8f3 100644
--- a/meson.build
+++ b/meson.build
@@ -675,6 +675,11 @@ test(
     env: test_env,
     suite: ['python-tests'],
 )
+test(
+    'modifiers',
+    executable('test-modifiers', 'test/modifiers.c', dependencies: test_dep),
+    env: test_env,
+)
 if get_option('enable-x11')
     test(
         'x11',