patch 8.2.1851: Vim9: "!" followed by space incorrectly used

Problem:    Vim9: "!" followed by space incorrectly used.
Solution:   Skip over trailing spaces. (closes #7131)

src/eval.c

@@ -3390,10 +3390,14 @@ eval7_leader(
 	f = rettv->vval.v_float;
     else
 #endif
+    {
+	while (VIM_ISWHITE(end_leader[-1]))
+	    --end_leader;
 	if (in_vim9script() && end_leader[-1] == '!')
 	    val = tv2bool(rettv);
 	else
 	    val = tv_get_number_chk(rettv, &error);
+    }
     if (error)
     {
 	clear_tv(rettv);

src/testdir/test_vim9_expr.vim

@@ -2292,6 +2292,22 @@ def Test_expr7_not()
       assert_equal(true, !!'asdf')
       assert_equal(true, !![2])
 
+      assert_equal(true, ! false)
+      assert_equal(true, !! true)
+      assert_equal(true, ! ! true)
+      assert_equal(true, !!! false)
+      assert_equal(true, ! ! ! false)
+
+      g:true = true
+      g:false = false
+      assert_equal(true, ! g:false)
+      assert_equal(true, !! g:true)
+      assert_equal(true, ! ! g:true)
+      assert_equal(true, !!! g:false)
+      assert_equal(true, ! ! ! g:false)
+      unlet g:true
+      unlet g:false
+
       assert_equal(true, !test_null_partial())
       assert_equal(false, !{-> 'yes'})
 
@@ -2314,8 +2330,7 @@ def Test_expr7_not()
       assert_equal(false, ![1, 2, 3]->reverse())
       assert_equal(true, ![]->reverse())
   END
-  CheckDefSuccess(lines)
+  CheckDefAndScriptSuccess(lines)
-  CheckScriptSuccess(['vim9script'] + lines)
 enddef
 
 func Test_expr7_fails()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1851,
 /**/
     1850,
 /**/

src/vim9compile.c

@@ -3041,7 +3041,7 @@ apply_leader(typval_T *rettv, int numeric_only, char_u *start, char_u **end)
 	    ++p;
 	    break;
 	}
-	else
+	else if (*p == '!')
 	{
 	    int v = tv2bool(rettv);
 
@@ -3178,12 +3178,13 @@ compile_leader(cctx_T *cctx, int numeric_only, char_u *start, char_u **end)
 	}
 	else
 	{
-	    int  invert = TRUE;
+	    int  invert = *p == '!';
 
-	    while (p > start && p[-1] == '!')
+	    while (p > start && (p[-1] == '!' || VIM_ISWHITE(p[-1])))
 	    {
+		if (p[-1] == '!')
+		    invert = !invert;
 		--p;
-		invert = !invert;
 	    }
 	    if (generate_2BOOL(cctx, invert) == FAIL)
 		return FAIL;