Bug #13975: "same string" + 0 has 2 different results

  The 'decimals' member of Item_func was being improperly initialized,
  which resulted in improper results when handling large numeric values.

mysql-test/r/func_str.result

@@ -1046,4 +1046,10 @@ cast(ltrim('  20.06 ') as decimal(19,2))
 select cast(rtrim(ltrim('  20.06 ')) as decimal(19,2));
 cast(rtrim(ltrim('  20.06 ')) as decimal(19,2))
 20.06
+select conv("18383815659218730760",10,10) + 0;
+conv("18383815659218730760",10,10) + 0
+1.8383815659219e+19
+select "18383815659218730760" + 0;
+"18383815659218730760" + 0
+1.8383815659219e+19
 End of 5.0 tests

mysql-test/t/func_str.test

@@ -698,4 +698,10 @@ select cast(rtrim('  20.06 ') as decimal(19,2));
 select cast(ltrim('  20.06 ') as decimal(19,2));
 select cast(rtrim(ltrim('  20.06 ')) as decimal(19,2));
 
+#
+# Bug #13975: "same string" + 0 has 2 different results
+#
+select conv("18383815659218730760",10,10) + 0;
+select "18383815659218730760" + 0;
+
 --echo End of 5.0 tests

sql/item_strfunc.h

@@ -542,7 +542,7 @@ class Item_func_conv :public Item_str_func
   void fix_length_and_dec()
   {
     collation.set(default_charset());
-    decimals=0; max_length=64;
+    max_length= 64;
   }
 };