MDEV-19520 pushdown_from_having_into_where() not calling fix_fields() correctly

fix_fields() called in pushdown_from_having_into_where() assuming that the
item is immutable.
normalize_cond() extended to include (not f)=>(f == 0) to avoid the
issue where NOT(field) was being transformed to (field == 0) during
this fix_fields call.

s

mysql-test/main/having_cond_pushdown.result

@@ -4969,4 +4969,14 @@ SELECT a,b FROM t1 GROUP BY a,b HAVING a = (b IS NULL);
 a	b
 0	11
 DROP TABLE t1;
+#
+# MDEV-19520 pushdown_from_having_into_where() not calling fix_fields()
+#  correctly
+#
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (0),(1);
+SELECT a FROM t1 GROUP BY a HAVING NOT a;
+a
+0
+DROP TABLE t1;
 End of 10.4 tests

mysql-test/main/having_cond_pushdown.test

@@ -1485,4 +1485,14 @@ SELECT a,b FROM t1 GROUP BY a,b HAVING a = (b IS NULL);
 
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-19520 pushdown_from_having_into_where() not calling fix_fields()
+--echo #  correctly
+--echo #
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (0),(1);
+SELECT a FROM t1 GROUP BY a HAVING NOT a;
+DROP TABLE t1;
+
 --echo End of 10.4 tests

sql/sql_parse.cc

@@ -9111,6 +9111,7 @@ push_new_name_resolution_context(THD *thd,
 
 /**
   Fix condition which contains only field (f turns to  f <> 0 )
+    or only contains the function NOT field (not f turns to  f == 0)
 
   @param cond            The condition to fix
 
@@ -9126,6 +9127,22 @@ Item *normalize_cond(THD *thd, Item *cond)
     {
       cond= new (thd->mem_root) Item_func_ne(thd, cond, new (thd->mem_root) Item_int(thd, 0));
     }
+    else
+    {
+      if (type == Item::FUNC_ITEM)
+      {
+        Item_func *func_item= (Item_func *)cond;
+        if (func_item->functype() == Item_func::NOT_FUNC &&
+            func_item->argument_count() == 1)
+        {
+          Item *arg= func_item->arguments()[0];
+          if (arg->type() == Item::FIELD_ITEM ||
+              arg->type() == Item::REF_ITEM)
+            cond= new (thd->mem_root) Item_func_eq(thd, arg,
+                    new (thd->mem_root) Item_int(thd, 0));
+        }
+      }
+    }
   }
   return cond;
 }