MDEV-11586 UNION of FLOAT type results in erroneous precision

Fixing the asymmetry in the array field_types_merge_rules[][]
which caused data loss when mixing FLOAT + BIGINT in UNIONs
or hybrid functions:

1. FLOAT  + INT    = DOUBLE
2. FLOAT  + BIGINT = FLOAT
3. INT    + FLOAT  = DOUBLE
4. BIGINT + FLOAT  = DOUBLE

Now FLOAT + BIGINT (as in #2) also produces DOUBLE, like the cases #1,#3,#4 do.

mysql-test/r/type_float.result

@@ -772,5 +772,31 @@ t1	CREATE TABLE `t1` (
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table if exists t1;
 #
+# MDEV-11586 UNION of FLOAT type results in erroneous precision
+#
+CREATE TABLE t1 (f FLOAT);
+INSERT INTO t1 VALUES (1.1);
+SELECT f FROM t1 UNION SELECT 1;
+f
+1.100000023841858
+1
+SELECT 1 UNION SELECT f FROM t1;
+1
+1
+1.100000023841858
+SELECT f FROM t1 UNION SELECT 2147483647;
+f
+1.100000023841858
+2147483647
+SELECT 2147483647 UNION SELECT f FROM t1;
+2147483647
+2147483647
+1.100000023841858
+SELECT CASE WHEN 0 THEN (SELECT f FROM t1) ELSE 2147483647 END AS c1,
+CASE WHEN 1 THEN 2147483647 ELSE (SELECT f FROM t1) END AS c2;
+c1	c2
+2147483647	2147483647
+DROP TABLE t1;
+#
 # End of 10.2 tests
 #

mysql-test/t/type_float.test

@@ -533,6 +533,21 @@ show create table t1;
 
 drop table if exists t1;
 
+--echo #
+--echo # MDEV-11586 UNION of FLOAT type results in erroneous precision
+--echo #
+
+CREATE TABLE t1 (f FLOAT);
+INSERT INTO t1 VALUES (1.1);
+SELECT f FROM t1 UNION SELECT 1;
+SELECT 1 UNION SELECT f FROM t1;
+SELECT f FROM t1 UNION SELECT 2147483647;
+SELECT 2147483647 UNION SELECT f FROM t1;
+SELECT CASE WHEN 0 THEN (SELECT f FROM t1) ELSE 2147483647 END AS c1,
+       CASE WHEN 1 THEN 2147483647 ELSE (SELECT f FROM t1) END AS c2;
+DROP TABLE t1;
+
+
 --echo #
 --echo # End of 10.2 tests
 --echo #

sql/field.cc

@@ -231,7 +231,7 @@ static enum_field_types field_types_merge_rules [FIELDTYPE_NUM][FIELDTYPE_NUM]=
   //MYSQL_TYPE_NULL         MYSQL_TYPE_TIMESTAMP
     MYSQL_TYPE_FLOAT,       MYSQL_TYPE_VARCHAR,
   //MYSQL_TYPE_LONGLONG     MYSQL_TYPE_INT24
-    MYSQL_TYPE_FLOAT,       MYSQL_TYPE_FLOAT,
+    MYSQL_TYPE_DOUBLE,      MYSQL_TYPE_FLOAT,
   //MYSQL_TYPE_DATE         MYSQL_TYPE_TIME
     MYSQL_TYPE_VARCHAR,     MYSQL_TYPE_VARCHAR,
   //MYSQL_TYPE_DATETIME     MYSQL_TYPE_YEAR