fix(highlight): use winhl=Foo:Bar even when Bar is empty

fixes #22906

src/nvim/api/options.c

@@ -53,7 +53,7 @@ static int validate_option_value_args(Dict(option) *opts, int *scope, int *opt_t
   }
 
   if (HAS_KEY(opts->win)) {
-    VALIDATE_T("win", kObjectTypeInteger, opts->win.type, {
+    VALIDATE_T_HANDLE("win", kObjectTypeWindow, opts->win.type, {
       return FAIL;
     });
 
@@ -65,7 +65,7 @@ static int validate_option_value_args(Dict(option) *opts, int *scope, int *opt_t
   }
 
   if (HAS_KEY(opts->buf)) {
-    VALIDATE_T("buf", kObjectTypeInteger, opts->buf.type, {
+    VALIDATE_T_HANDLE("buf", kObjectTypeBuffer, opts->buf.type, {
       return FAIL;
     });
 

src/nvim/api/private/validate.h

@@ -67,6 +67,15 @@
     } \
   } while (0)
 
+/// Checks that actual_t is either the correct handle type or a type erased handle (integer)
+#define VALIDATE_T_HANDLE(name, expected_t, actual_t, code) \
+  do { \
+    if (expected_t != actual_t && kObjectTypeInteger != actual_t) { \
+      api_err_exp(err, name, api_typename(expected_t), api_typename(actual_t)); \
+      code; \
+    } \
+  } while (0)
+
 #define VALIDATE_RANGE(cond, name, code) \
   do { \
     if (!(cond)) { \

src/nvim/highlight.c

@@ -303,7 +303,7 @@ int hl_get_ui_attr(int ns_id, int idx, int final_id, bool optional)
   bool available = false;
 
   if (final_id > 0) {
-    int syn_attr = syn_ns_id2attr(ns_id, final_id, optional);
+    int syn_attr = syn_ns_id2attr(ns_id, final_id, &optional);
     if (syn_attr > 0) {
       attrs = syn_attr2entry(syn_attr);
       available = true;

src/nvim/highlight_group.c

@@ -1988,18 +1988,23 @@ static int syn_add_group(const char *name, size_t len)
 /// @see syn_attr2entry
 int syn_id2attr(int hl_id)
 {
-  return syn_ns_id2attr(-1, hl_id, false);
+  bool optional = false;
+  return syn_ns_id2attr(-1, hl_id, &optional);
 }
 
-int syn_ns_id2attr(int ns_id, int hl_id, bool optional)
+int syn_ns_id2attr(int ns_id, int hl_id, bool *optional)
+  FUNC_ATTR_NONNULL_ALL
 {
-  hl_id = syn_ns_get_final_id(&ns_id, hl_id);
+  if (syn_ns_get_final_id(&ns_id, &hl_id)) {
+    // If the namespace explicitly defines a group to be empty, it is not optional
+    *optional = false;
+  }
   HlGroup *sgp = &hl_table[hl_id - 1];  // index is ID minus one
 
   int attr = ns_get_hl(&ns_id, hl_id, false, sgp->sg_set);
 
   // if a highlight group is optional, don't use the global value
-  if (attr >= 0 || (optional && ns_id > 0)) {
+  if (attr >= 0 || (*optional && ns_id > 0)) {
     return attr;
   }
   return sgp->sg_attr;
@@ -2008,16 +2013,20 @@ int syn_ns_id2attr(int ns_id, int hl_id, bool optional)
 /// Translate a group ID to the final group ID (following links).
 int syn_get_final_id(int hl_id)
 {
-  int id = curwin->w_ns_hl_active;
+  int ns_id = curwin->w_ns_hl_active;
-  return syn_ns_get_final_id(&id, hl_id);
+  syn_ns_get_final_id(&ns_id, &hl_id);
+  return hl_id;
 }
 
-int syn_ns_get_final_id(int *ns_id, int hl_id)
+bool syn_ns_get_final_id(int *ns_id, int *hl_idp)
 {
   int count;
+  int hl_id = *hl_idp;
+  bool used = false;
 
   if (hl_id > highlight_ga.ga_len || hl_id < 1) {
-    return 0;  // Can be called from eval!!
+    *hl_idp = 0;
+    return false;  // Can be called from eval!!
   }
 
   // Follow links until there is no more.
@@ -2030,8 +2039,10 @@ int syn_ns_get_final_id(int *ns_id, int hl_id)
     // syn_id2attr time
     int check = ns_get_hl(ns_id, hl_id, true, sgp->sg_set);
     if (check == 0) {
-      return hl_id;  // how dare! it broke the link!
+      *hl_idp = hl_id;
+      return true;  // how dare! it broke the link!
     } else if (check > 0) {
+      used = true;
       hl_id = check;
       continue;
     }
@@ -2045,7 +2056,8 @@ int syn_ns_get_final_id(int *ns_id, int hl_id)
     }
   }
 
-  return hl_id;
+  *hl_idp = hl_id;
+  return used;
 }
 
 /// Refresh the color attributes of all highlight groups.
@@ -2128,7 +2140,8 @@ void highlight_changed(void)
       abort();
     }
     int ns_id = -1;
-    int final_id = syn_ns_get_final_id(&ns_id, id);
+    int final_id = id;
+    syn_ns_get_final_id(&ns_id, &final_id);
     if (hlf == HLF_SNC) {
       id_SNC = final_id;
     } else if (hlf == HLF_S) {

test/functional/ui/float_spec.lua

@@ -4935,6 +4935,53 @@ describe('float window', function()
       end)
     end)
 
+    it("can use Normal as background", function()
+      local buf = meths.create_buf(false,false)
+      meths.buf_set_lines(buf,0,-1,true,{"here", "float"})
+      local win = meths.open_win(buf, false, {relative='editor', width=20, height=2, row=2, col=5})
+      meths.set_option_value('winhl', 'Normal:Normal', {win=win})
+
+      if multigrid then
+        screen:expect{grid=[[
+        ## grid 1
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [3:----------------------------------------]|
+        ## grid 2
+          ^                                        |
+          {0:~                                       }|
+          {0:~                                       }|
+          {0:~                                       }|
+          {0:~                                       }|
+          {0:~                                       }|
+        ## grid 3
+                                                  |
+        ## grid 5
+          here                |
+          float               |
+        ]], float_pos={
+          [5] = {{id = 1002}, "NW", 1, 2, 5, true, 50};
+        }, win_viewport={
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
+        }}
+      else
+        screen:expect{grid=[[
+          ^                                        |
+          {0:~                                       }|
+          {0:~    }here                {0:               }|
+          {0:~    }float               {0:               }|
+          {0:~                                       }|
+          {0:~                                       }|
+                                                  |
+        ]]}
+      end
+    end)
+
     describe("handles :wincmd", function()
       local win
       local expected_pos

test/functional/ui/highlight_spec.lua

@@ -2432,6 +2432,23 @@ describe("'winhighlight' highlight", function()
                                               |
     ]]}
   end)
+
+  it('can link to empty highlight group', function()
+    command 'hi NormalNC guibg=Red' -- czerwone time
+    command 'set winhl=NormalNC:Normal'
+    command 'split'
+
+    screen:expect{grid=[[
+      ^                    |
+      {0:~                   }|
+      {0:~                   }|
+      {3:[No Name]           }|
+                          |
+      {0:~                   }|
+      {4:[No Name]           }|
+                          |
+    ]]}
+  end)
 end)
 
 describe('highlight namespaces', function()