vim-patch:8.2.1703: ":highlight clear" does not restore default link

Problem:    ":highlight clear" does not restore default link.
Solution:   Remember the default link and restore it. (Antony Scriven,
            closes vim/vim#6970, closes vim/vim#4405)
213da551de

runtime/doc/syntax.txt

@@ -4689,6 +4689,7 @@ in their own color.
 			highlighting for groups added by the user!
 			Uses the current value of 'background' to decide which
 			default colors to use.
+			If there was a default link, restore it. |:hi-link|
 
 :hi[ghlight] clear {group-name}
 :hi[ghlight] {group-name} NONE

src/nvim/syntax.c

@@ -59,6 +59,7 @@ struct hl_group {
   bool sg_cleared;              ///< "hi clear" was used
   int sg_attr;                  ///< Screen attr @see ATTR_ENTRY
   int sg_link;                  ///< link to this highlight group ID
+  int sg_deflink;               ///< default link; restored in highlight_clear()
   int sg_set;                   ///< combination of flags in \ref SG_SET
   sctx_T sg_script_ctx;         ///< script in which the group was last set
   // for terminal UIs
@@ -6601,6 +6602,7 @@ void do_highlight(const char *line, const bool forceit, const bool init)
     const char *to_end;
     int from_id;
     int to_id;
+    struct hl_group *hlgroup = NULL;
 
     from_end = (const char *)skiptowhite((const char_u *)from_start);
     to_start = (const char *)skipwhite((const char_u *)from_end);
@@ -6627,7 +6629,14 @@ void do_highlight(const char *line, const bool forceit, const bool init)
                               (int)(to_end - to_start));
     }
 
-    if (from_id > 0 && (!init || HL_TABLE()[from_id - 1].sg_set == 0)) {
+    if (from_id > 0) {
+      hlgroup = &HL_TABLE()[from_id - 1];
+      if (dodefault && (forceit || hlgroup->sg_deflink == 0)) {
+        hlgroup->sg_deflink = to_id;
+      }
+    }
+
+    if (from_id > 0 && (!init || hlgroup->sg_set == 0)) {
       // Don't allow a link when there already is some highlighting
       // for the group, unless '!' is used
       if (to_id > 0 && !forceit && !init
@@ -6635,17 +6644,16 @@ void do_highlight(const char *line, const bool forceit, const bool init)
         if (sourcing_name == NULL && !dodefault) {
           EMSG(_("E414: group has settings, highlight link ignored"));
         }
-      } else if (HL_TABLE()[from_id - 1].sg_link != to_id
+      } else if (hlgroup->sg_link != to_id
-                 || HL_TABLE()[from_id - 1].sg_script_ctx.sc_sid
+                 || hlgroup->sg_script_ctx.sc_sid != current_sctx.sc_sid
-                 != current_sctx.sc_sid
+                 || hlgroup->sg_cleared) {
-                 || HL_TABLE()[from_id - 1].sg_cleared) {
         if (!init) {
-          HL_TABLE()[from_id - 1].sg_set |= SG_LINK;
+          hlgroup->sg_set |= SG_LINK;
         }
-        HL_TABLE()[from_id - 1].sg_link = to_id;
+        hlgroup->sg_link = to_id;
-        HL_TABLE()[from_id - 1].sg_script_ctx = current_sctx;
+        hlgroup->sg_script_ctx = current_sctx;
-        HL_TABLE()[from_id - 1].sg_script_ctx.sc_lnum += sourcing_lnum;
+        hlgroup->sg_script_ctx.sc_lnum += sourcing_lnum;
-        HL_TABLE()[from_id - 1].sg_cleared = false;
+        hlgroup->sg_cleared = false;
         redraw_all_later(SOME_VALID);
 
         // Only call highlight changed() once after multiple changes
@@ -7106,6 +7114,8 @@ static void highlight_clear(int idx)
   XFREE_CLEAR(HL_TABLE()[idx].sg_rgb_bg_name);
   XFREE_CLEAR(HL_TABLE()[idx].sg_rgb_sp_name);
   HL_TABLE()[idx].sg_blend = -1;
+  // Restore any default link.
+  HL_TABLE()[idx].sg_link = HL_TABLE()[idx].sg_deflink;
   // Clear the script ID only when there is no link, since that is not
   // cleared.
   if (HL_TABLE()[idx].sg_link == 0) {

src/nvim/testdir/test_highlight.vim

@@ -623,10 +623,47 @@ func Test_xxlast_highlight_RGB_color()
   hi clear
 endfunc
 
-" Test default highlighting is restored
+func Test_highlight_clear_restores_links()
-func Test_highlight_restore_defaults()
+  let aaa_id = hlID('aaa')
-  hi! link TestLink Identifier
+  call assert_equal(aaa_id, 0)
-  hi! TestHi ctermbg=red
+
+  " create default link aaa --> bbb
+  hi def link aaa bbb
+  let id_aaa = hlID('aaa')
+  let hl_aaa_bbb = HighlightArgs('aaa')
+
+  " try to redefine default link aaa --> ccc; check aaa --> bbb
+  hi def link aaa ccc
+  call assert_equal(HighlightArgs('aaa'), hl_aaa_bbb)
+
+  " clear aaa; check aaa --> bbb
+  hi clear aaa
+  call assert_equal(HighlightArgs('aaa'), hl_aaa_bbb)
+
+  " link aaa --> ccc; clear aaa; check aaa --> bbb
+  hi link aaa ccc
+  let id_ccc = hlID('ccc')
+  call assert_equal(synIDtrans(id_aaa), id_ccc)
+  hi clear aaa
+  call assert_equal(HighlightArgs('aaa'), hl_aaa_bbb)
+
+  " forcibly set default link aaa --> ddd
+  hi! def link aaa ddd
+  let id_ddd = hlID('ddd')
+  let hl_aaa_ddd = HighlightArgs('aaa')
+  call assert_equal(synIDtrans(id_aaa), id_ddd)
+
+  " link aaa --> eee; clear aaa; check aaa --> ddd
+  hi link aaa eee
+  let eee_id = hlID('eee')
+  call assert_equal(synIDtrans(id_aaa), eee_id)
+  hi clear aaa
+  call assert_equal(HighlightArgs('aaa'), hl_aaa_ddd)
+endfunc
+
+func Test_highlight_default_colorscheme_restores_links()
+  hi link TestLink Identifier
+  hi TestHi ctermbg=red
 
   let hlTestLinkPre = HighlightArgs('TestLink')
   let hlTestHiPre = HighlightArgs('TestHi')
@@ -637,19 +674,16 @@ func Test_highlight_restore_defaults()
     syntax reset
   endif
   let g:colors_name = 'test'
-  hi! link TestLink ErrorMsg
+  hi link TestLink ErrorMsg
-  hi! TestHi ctermbg=green
+  hi TestHi ctermbg=green
 
   " Restore default highlighting
   colorscheme default
-  syntax on
   " 'default' should work no matter if highlight group was cleared
   hi def link TestLink Identifier
   hi def TestHi ctermbg=red
-
   let hlTestLinkPost = HighlightArgs('TestLink')
   let hlTestHiPost = HighlightArgs('TestHi')
-
   call assert_equal(hlTestLinkPre, hlTestLinkPost)
   call assert_equal(hlTestHiPre, hlTestHiPost)
   hi clear