patch 8.2.2173: Vim9: get internal error when assigning to undefined variable

Problem:    Vim9: get internal error when assigning to undefined variable.
Solution:   Add error message. (closes #7475)

src/errors.h

@@ -321,3 +321,5 @@ EXTERN char e_command_not_followed_by_white_space_str[]
 	INIT(= N_("E1144: Command is not followed by white space: %s"));
 EXTERN char e_missing_heredoc_end_marker_str[]
 	INIT(= N_("E1145: Missing heredoc end marker: %s"));
+EXTERN char e_command_not_recognized_str[]
+	INIT(= N_("E1146: Command not recognized: %s"));

src/testdir/test_vim9_cmd.vim

@@ -710,5 +710,16 @@ def Test_ambiguous_user_cmd()
   CheckScriptFailure(lines, 'E464:')
 enddef
 
+def Test_command_not_recognized()
+  var lines =<< trim END
+    d.key = 'asdf'
+  END
+  CheckDefFailure(lines, 'E1146:', 1)
+
+  lines =<< trim END
+    d['key'] = 'asdf'
+  END
+  CheckDefFailure(lines, 'E1146:', 1)
+enddef
 
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2173,
 /**/
     2172,
 /**/

src/vim9compile.c

@@ -7681,8 +7681,9 @@ compile_def_function(ufunc_T *ufunc, int set_return_type, cctx_T *outer_cctx)
 	    // Expression or function call.
 	    if (ea.cmdidx != CMD_eval)
 	    {
-		// CMD_var cannot happen, compile_assignment() above is used
-		iemsg("Command from find_ex_command() not handled");
+		// CMD_var cannot happen, compile_assignment() above would be
+		// used.  Most likely an assignment to a non-existing variable.
+		semsg(_(e_command_not_recognized_str), ea.cmd);
 		goto erret;
 	    }
 	}

src/vim9execute.c

@@ -606,8 +606,7 @@ call_ufunc(ufunc_T *ufunc, int argcount, ectx_T *ectx, isn_T *iptr)
 	return FAIL;
     if (ufunc->uf_def_status == UF_COMPILED)
     {
-	int error = check_user_func_argcount(ufunc, argcount);
-
+	error = check_user_func_argcount(ufunc, argcount);
 	if (error != FCERR_UNKNOWN)
 	{
 	    if (error == FCERR_TOOMANY)