fix(langmap): assert failure on mapping to char >= 256 (#37291)

Usually 'langmap' is used to map keyboard characters to ASCII motions or
mappings. It's not entirely clear what the purpose of mapping to Unicode
characters is, but since there is no error for mapping between two chars
both >= 256, only give a warning that this will not work properly when
mapping from a char < 256 to a char >= 256.

(cherry picked from commit 16c1334399)

src/nvim/mapping.c

@@ -2578,21 +2578,23 @@ const char *did_set_langmap(optset_T *args)
         p++;
       }
       int from = utf_ptr2char(p);
+      const char *const from_ptr = p;
       int to = NUL;
+      const char *to_ptr = "";
       if (p2 == NULL) {
         MB_PTR_ADV(p);
         if (p[0] != ',') {
           if (p[0] == '\\') {
             p++;
           }
-          to = utf_ptr2char(p);
+          to = utf_ptr2char(to_ptr = p);
         }
       } else {
         if (p2[0] != ',') {
           if (p2[0] == '\\') {
             p2++;
           }
-          to = utf_ptr2char(p2);
+          to = utf_ptr2char(to_ptr = p2);
         }
       }
       if (to == NUL) {
@@ -2605,7 +2607,10 @@ const char *did_set_langmap(optset_T *args)
       if (from >= 256) {
         langmap_set_entry(from, to);
       } else {
-        assert(to <= UCHAR_MAX);
+        if (to > UCHAR_MAX) {
+          swmsg(true, "'langmap': mapping from %.*s to %.*s will not work properly",
+                utf_ptr2len(from_ptr), from_ptr, utf_ptr2len(to_ptr), to_ptr);
+        }
         langmap_mapchar[from & 255] = (uint8_t)to;
       }
 

test/functional/editor/langmap_spec.lua

@@ -266,6 +266,18 @@ describe("'langmap'", function()
     command('nnoremap ï ix<esc>')
     testrecording('x', 'xhello', local_setup)
   end)
+  it('does not crash when mapping char < 256 to char >= 256', function()
+    -- Note: this warning should be removed when the pending tests above are fixed.
+    local wmsg = table.concat({
+      "'langmap': mapping from e to ε will not work properly",
+      "'langmap': mapping from ü to μ will not work properly",
+    }, '\n')
+    -- ä ë ï ö ü ÿ < 256, β γ ε μ >= 256
+    command('set langmap=iw,wi,äë,ëä,ïx,xï,βγ,γβ,eε,εe,üμ,μü,ÿy,yÿ')
+    eq(wmsg, n.exec_capture('messages'))
+    command('messages clear | set langmap=iwäëïxβγeεüμÿy;wiëäxïγβεeμüyÿ')
+    eq(wmsg, n.exec_capture('messages'))
+  end)
   -- This test is to ensure the behaviour doesn't change from what's already
   -- around. I (hardenedapple) personally think this behaviour should be
   -- changed.