MDEV-4283 Assertion `scale <= precision' fails in strings/decimal.c

  
with decimals=NOT_FIXED_DEC it is possible to have 'decimals' larger
than 'max_length', it's not an error for temporal functions.
  
But when Item_func_numhybrid converts the value to DECIMAL_RESULT,
it must limit 'decimals' to be a valid scale of a decimal number.

mysql-test/r/temporal_scale_4283.result

+create table t1 (a int);
+insert into t1 values (4),(8);
+select distinct 100 mod timestampadd( week, a, '2002-05-20' ) from t1;
+100 mod timestampadd( week, a, '2002-05-20' )
+100
+drop table t1;
+create table t1 (i int);
+insert into t1 values (2),(4);
+select distinct convert_tz( '2001-03-21', 'utc', 'met' ) mod i from t1;
+convert_tz( '2001-03-21', 'utc', 'met' ) mod i
+0
+drop table t1;

mysql-test/t/temporal_scale_4283.test

+#
+# MDEV-4283 Assertion `scale <= precision' fails in strings/decimal.c
+#
+create table t1 (a int);
+insert into t1 values (4),(8);
+select distinct 100 mod timestampadd( week, a, '2002-05-20' ) from t1;
+drop table t1;
+
+create table t1 (i int);
+insert into t1 values (2),(4);
+select distinct convert_tz( '2001-03-21', 'utc', 'met' ) mod i from t1;
+drop table t1;
+

sql/item_func.cc

@@ -742,13 +742,14 @@ void Item_num_op::find_num_type(void)
   {
     hybrid_type= DECIMAL_RESULT;
     result_precision();
+    fix_decimals();
   }
   else
   {
     DBUG_ASSERT(r0 == INT_RESULT && r1 == INT_RESULT);
-    decimals= 0;
     hybrid_type=INT_RESULT;
     result_precision();
+    decimals= 0;
   }
   DBUG_PRINT("info", ("Type: %s",
              (hybrid_type == REAL_RESULT ? "REAL_RESULT" :
@@ -1492,6 +1493,7 @@ void Item_func_div::fix_length_and_dec()
     break;
   case DECIMAL_RESULT:
     result_precision();
+    fix_decimals();
     break;
   case STRING_RESULT:
   case ROW_RESULT:

sql/item_func.h

@@ -348,6 +348,13 @@ class Item_func_numhybrid: public Item_func
   void fix_num_length_and_dec();
   virtual void find_num_type()= 0; /* To be called from fix_length_and_dec */
 
+  inline void fix_decimals()
+  {
+    DBUG_ASSERT(result_type() == DECIMAL_RESULT);
+    if (decimals == NOT_FIXED_DEC)
+      set_if_smaller(decimals, max_length - 1);
+  }
+
   double val_real();
   longlong val_int();
   my_decimal *val_decimal(my_decimal *);