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, 1 insertions, 0 deletions

diff --git a/src/xkbcomp/ast.h b/src/xkbcomp/ast.h
index 6c51ce4..8b0901f 100644
--- a/src/xkbcomp/ast.h
+++ b/src/xkbcomp/ast.h
@@ -303,6 +303,7 @@ typedef struct {
 typedef struct {
     ParseCommon common;
     enum merge_mode merge;
+    // NOTE: Can also be “None”, rather than a modifier name.
     xkb_atom_t modifier;
     ExprDef *keys;
 } ModMapDef;