feat(highlight): define the concept of altfont as a (c)term rendering attribute

runtime/doc/builtin.txt

@@ -8328,6 +8328,7 @@ synIDattr({synID}, {what} [, {mode}])			*synIDattr()*
 		"underdotted"	"1" if dotted underlined
 		"underdashed"	"1" if dashed underlined
 		"strikethrough"	"1" if struckthrough
+		"altfont"	"1" if alternative font
 		"nocombine"	"1" if nocombine
 
 		Returns an empty string on error.

runtime/doc/news.txt

@@ -149,6 +149,8 @@ The following new APIs or features were added.
   deterministic, and a `LUA_GEN_PRG` build parameter has been introduced to
   allow for a workaround for some remaining reproducibility problems.
 
+• |:highlight| now supports an additional attribute "altfont".
+
 ==============================================================================
 CHANGED FEATURES                                                 *news-changes*
 

runtime/doc/syntax.txt

@@ -4958,7 +4958,8 @@ the same syntax file on all UIs.
 					*bold* *underline* *undercurl*
 					*underdouble* *underdotted*
 					*underdashed* *inverse* *italic*
-					*standout* *nocombine* *strikethrough*
+					*standout* *strikethrough* *altfont*
+					*nocombine*
 cterm={attr-list}			*attr-list* *highlight-cterm* *E418*
 	attr-list is a comma-separated list (without spaces) of the
 	following items (in any order):
@@ -4973,6 +4974,7 @@ cterm={attr-list}			*attr-list* *highlight-cterm* *E418*
 		inverse		same as reverse
 		italic
 		standout
+		altfont
 		nocombine	override attributes instead of combining them
 		NONE		no attributes used (used to reset it)
 

runtime/doc/ui.txt

@@ -324,6 +324,7 @@ numerical highlight ids to the actual attributes.
 	`underdouble`:		double underlined text. The lines have `special` color.
 	`underdotted`:		underdotted text. The dots have `special` color.
 	`underdashed`:		underdashed text. The dashes have `special` color.
+	`altfont`:		alternative font.
 	`blend`:		Blend level (0-100). Could be used by UIs to
 				support blending floating windows to the
 				background or to signal a transparent cursor.

src/nvim/api/keysets.lua

@@ -114,6 +114,7 @@ return {
     "underdashed";
     "italic";
     "reverse";
+    "altfont";
     "nocombine";
     "default";
     "cterm";
@@ -140,6 +141,7 @@ return {
     "underdashed";
     "italic";
     "reverse";
+    "altfont";
     "nocombine";
   }};
   -- Autocmds

src/nvim/highlight.c

@@ -867,6 +867,10 @@ void hlattrs2dict(Dictionary *dict, HlAttrs ae, bool use_rgb)
     PUT_C(hl, "strikethrough", BOOLEAN_OBJ(true));
   }
 
+  if (mask & HL_ALTFONT) {
+    PUT_C(hl, "altfont", BOOLEAN_OBJ(true));
+  }
+
   if (mask & HL_NOCOMBINE) {
     PUT_C(hl, "nocombine", BOOLEAN_OBJ(true));
   }
@@ -932,6 +936,7 @@ HlAttrs dict2hlattrs(Dict(highlight) *dict, bool use_rgb, int *link_id, Error *e
   CHECK_FLAG(dict, mask, underdashed, , HL_UNDERDASHED);
   CHECK_FLAG(dict, mask, standout, , HL_STANDOUT);
   CHECK_FLAG(dict, mask, strikethrough, , HL_STRIKETHROUGH);
+  CHECK_FLAG(dict, mask, altfont, , HL_ALTFONT);
   if (use_rgb) {
     CHECK_FLAG(dict, mask, fg_indexed, , HL_FG_INDEXED);
     CHECK_FLAG(dict, mask, bg_indexed, , HL_BG_INDEXED);
@@ -1014,6 +1019,7 @@ HlAttrs dict2hlattrs(Dict(highlight) *dict, bool use_rgb, int *link_id, Error *e
     CHECK_FLAG(cterm, cterm_mask, undercurl, , HL_UNDERCURL);
     CHECK_FLAG(cterm, cterm_mask, standout, , HL_STANDOUT);
     CHECK_FLAG(cterm, cterm_mask, strikethrough, , HL_STRIKETHROUGH);
+    CHECK_FLAG(cterm, cterm_mask, altfont, , HL_ALTFONT);
     CHECK_FLAG(cterm, cterm_mask, nocombine, , HL_NOCOMBINE);
   } else if (dict->cterm.type == kObjectTypeArray && dict->cterm.data.array.size == 0) {
     // empty list from Lua API should clear all cterm attributes

src/nvim/highlight_defs.h

@@ -25,7 +25,8 @@ typedef enum {
   // 0x30 and 0x38 spare for underline styles
   HL_STANDOUT      = 0x0040,
   HL_STRIKETHROUGH = 0x0080,
-  // 0x0100-0x0200 spare
+  HL_ALTFONT       = 0x0100,
+  // 0x0200 spare
   HL_NOCOMBINE     = 0x0400,
   HL_BG_INDEXED    = 0x0800,
   HL_FG_INDEXED    = 0x1000,

src/nvim/highlight_group.c

@@ -67,11 +67,13 @@ Map(cstr_t, int) highlight_unames = MAP_INIT;
 static char *(hl_name_table[]) =
 { "bold", "standout", "underline",
   "undercurl", "underdouble", "underdotted", "underdashed",
-  "italic", "reverse", "inverse", "strikethrough", "nocombine", "NONE" };
+  "italic", "reverse", "inverse", "strikethrough", "altfont",
+  "nocombine", "NONE" };
 static int hl_attr_table[] =
 { HL_BOLD, HL_STANDOUT, HL_UNDERLINE,
   HL_UNDERCURL, HL_UNDERDOUBLE, HL_UNDERDOTTED, HL_UNDERDASHED,
-  HL_ITALIC, HL_INVERSE, HL_INVERSE, HL_STRIKETHROUGH, HL_NOCOMBINE, 0 };
+  HL_ITALIC, HL_INVERSE, HL_INVERSE, HL_STRIKETHROUGH, HL_ALTFONT,
+  HL_NOCOMBINE, 0 };
 
 /// Structure that stores information about a highlight group.
 /// The ID of a highlight group is also called group ID.  It is the index in
@@ -1595,7 +1597,12 @@ const char *highlight_has_attr(const int id, const int flag, const int modec)
     attr = hl_table[id - 1].sg_cterm;
   }
 
+  if (flag & HL_UNDERLINE_MASK) {
+    int ul = attr & HL_UNDERLINE_MASK;
+    return ul == flag ? "1" : NULL;
+  } else {
     return (attr & flag) ? "1" : NULL;
+  }
 }
 
 /// Return color name of the given highlight group

test/functional/api/highlight_spec.lua

@@ -33,6 +33,7 @@ describe('API: highlight',function()
     reverse = true,
     underline = true,
     strikethrough = true,
+    altfont = true,
     nocombine = true,
   }
   local expected_undercurl = {
@@ -61,7 +62,7 @@ describe('API: highlight',function()
     eq('Invalid highlight id: 30000', string.match(emsg, 'Invalid.*'))
 
     -- Test all highlight properties.
-    command('hi NewHighlight gui=underline,bold,italic,reverse,strikethrough,nocombine')
+    command('hi NewHighlight gui=underline,bold,italic,reverse,strikethrough,altfont,nocombine')
     eq(expected_rgb2, nvim("get_hl_by_id", hl_id, true))
 
     -- Test undercurl
@@ -215,10 +216,12 @@ describe("API: set highlight", function()
     reverse = true,
     underline = true,
     strikethrough = true,
+    altfont = true,
     cterm = {
       italic = true,
       reverse = true,
       strikethrough = true,
+      altfont = true,
       nocombine = true,
     }
   }
@@ -230,6 +233,7 @@ describe("API: set highlight", function()
     reverse = true,
     underline = true,
     strikethrough = true,
+    altfont = true,
   }
   local highlight3_result_cterm = {
     background = highlight_color.ctermbg,
@@ -237,6 +241,7 @@ describe("API: set highlight", function()
     italic = true,
     reverse = true,
     strikethrough = true,
+    altfont = true,
     nocombine = true,
   }
 
@@ -292,7 +297,7 @@ describe("API: set highlight", function()
       exec_capture('highlight Test_hl'))
 
     meths.set_hl(0, 'Test_hl2', highlight3_config)
-    eq('Test_hl2       xxx cterm=italic,reverse,strikethrough,nocombine ctermfg=8 ctermbg=15 gui=bold,underline,italic,reverse,strikethrough guifg=#ff0000 guibg=#0032aa',
+    eq('Test_hl2       xxx cterm=italic,reverse,strikethrough,altfont,nocombine ctermfg=8 ctermbg=15 gui=bold,underline,italic,reverse,strikethrough,altfont guifg=#ff0000 guibg=#0032aa',
       exec_capture('highlight Test_hl2'))
 
     -- Colors are stored with the name they are defined, but