patch 8.2.3124: Vim9: no error for white space between option and "=9"

Problem:    Vim9: no error for white space between option and "=9".
Solution:   Check for extraneous white space. (issue #8408)

src/option.c

@@ -1358,7 +1358,22 @@ do_set(
 	    // remember character after option name
 	    afterchar = arg[len];
 
-	    if (!in_vim9script())
+	    if (in_vim9script())
+	    {
+		char_u *p = skipwhite(arg + len);
+
+		// disallow white space before =val, +=val, -=val, ^=val
+		if (p > arg + len && (p[0] == '='
+			|| (vim_strchr((char_u *)"+-^", p[0]) != NULL
+							      && p[1] == '=')))
+		{
+		    errmsg = e_no_white_space_allowed_between_option_and;
+		    arg = p;
+		    startarg = p;
+		    goto skip;
+		}
+	    }
+	    else
 		// skip white space, allow ":set ai  ?", ":set hlsearch  !"
 		while (VIM_ISWHITE(arg[len]))
 		    ++len;

src/testdir/test_vim9_script.vim

@@ -4075,6 +4075,45 @@ def Test_mapping_line_number()
   delfunc g:FuncA
 enddef
 
+def Test_option_set()
+  # legacy script allows for white space
+  var lines =<< trim END
+      set foldlevel  =11
+      call assert_equal(11, &foldlevel)
+  END
+  CheckScriptSuccess(lines)
+
+  set foldlevel
+  set foldlevel=12
+  assert_equal(12, &foldlevel)
+  set foldlevel+=2
+  assert_equal(14, &foldlevel)
+  set foldlevel-=3
+  assert_equal(11, &foldlevel)
+
+  lines =<< trim END
+      set foldlevel =1
+  END
+  CheckDefExecAndScriptFailure(lines, 'E1205: No white space allowed between option and: =1')
+
+  lines =<< trim END
+      set foldlevel +=1
+  END
+  CheckDefExecAndScriptFailure(lines, 'E1205: No white space allowed between option and: +=1')
+
+  lines =<< trim END
+      set foldlevel ^=1
+  END
+  CheckDefExecAndScriptFailure(lines, 'E1205: No white space allowed between option and: ^=1')
+
+  lines =<< trim END
+      set foldlevel -=1
+  END
+  CheckDefExecAndScriptFailure(lines, 'E1205: No white space allowed between option and: -=1')
+
+  set foldlevel&
+enddef
+
 def Test_option_modifier()
   # legacy script allows for white space
   var lines =<< trim END

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3124,
 /**/
     3123,
 /**/