MDEV-9334 ALTER from DECIMAL to BIGINT UNSIGNED returns a wrong result

When altering from DECIMAL to *INT UNIGNED or to BIT, go through val_decimal(),
to avoid truncation to the biggest possible signed integer
(0x7FFFFFFFFFFFFFFF / 9223372036854775807).

mysql-test/r/type_bit.result

@@ -830,3 +830,22 @@ def					COALESCE(val, 1)	246	2	1	Y	32896	0	63
 COALESCE(val, 1)
 0
 DROP TABLE t1;
+#
+# End of 10.1 tests
+#
+#
+# Start of 10.2 tests
+#
+#
+# MDEV-9334 ALTER from DECIMAL to BIGINT UNSIGNED returns a wrong result
+#
+CREATE TABLE t1 (a DECIMAL(30,0));
+INSERT INTO t1 VALUES (CAST(0xFFFFFFFFFFFFFFFF AS UNSIGNED));
+ALTER TABLE t1 MODIFY a BIT(64);
+SELECT a+0 FROM t1;
+a+0
+18446744073709551615
+DROP TABLE IF EXISTS t1;
+#
+# End of 10.2 tests
+#

mysql-test/r/type_int.result

@@ -91,5 +91,18 @@ a
 10
 DROP TABLE t1;
 #
+# MDEV-9334 ALTER from DECIMAL to BIGINT UNSIGNED returns a wrong result
+#
+CREATE TABLE t1 (a DECIMAL(30,0));
+INSERT INTO t1 VALUES (CAST(0xFFFFFFFFFFFFFFFF AS UNSIGNED));
+SELECT * FROM t1;
+a
+18446744073709551615
+ALTER TABLE t1 MODIFY a BIGINT UNSIGNED;
+SELECT * FROM t1;
+a
+18446744073709551615
+DROP TABLE t1;
+#
 # End of 10.2 tests
 #

mysql-test/t/type_bit.test

@@ -458,3 +458,28 @@ DROP TABLE t2;
 SELECT COALESCE(val, 1) FROM t1;
 --disable_metadata
 DROP TABLE t1;
+
+--echo #
+--echo # End of 10.1 tests
+--echo #
+
+
+--echo #
+--echo # Start of 10.2 tests
+--echo #
+
+--echo #
+--echo # MDEV-9334 ALTER from DECIMAL to BIGINT UNSIGNED returns a wrong result
+--echo #
+
+CREATE TABLE t1 (a DECIMAL(30,0));
+INSERT INTO t1 VALUES (CAST(0xFFFFFFFFFFFFFFFF AS UNSIGNED));
+ALTER TABLE t1 MODIFY a BIT(64);
+SELECT a+0 FROM t1;
+DROP TABLE IF EXISTS t1;
+
+
+--echo #
+--echo # End of 10.2 tests
+--echo #
+

mysql-test/t/type_int.test

@@ -73,6 +73,18 @@ ALTER TABLE t1 MODIFY a INT;
 SELECT * FROM t1;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-9334 ALTER from DECIMAL to BIGINT UNSIGNED returns a wrong result
+--echo #
+
+CREATE TABLE t1 (a DECIMAL(30,0));
+INSERT INTO t1 VALUES (CAST(0xFFFFFFFFFFFFFFFF AS UNSIGNED));
+SELECT * FROM t1;
+ALTER TABLE t1 MODIFY a BIGINT UNSIGNED;
+SELECT * FROM t1;
+DROP TABLE t1;
+
+
 --echo #
 --echo # End of 10.2 tests
 --echo #

sql/field.h

@@ -1636,6 +1636,8 @@ class Field_num :public Field {
   bool eq_def(const Field *field) const;
   Copy_func *get_copy_func(const Field *from) const
   {
+    if (unsigned_flag && from->cmp_type() == DECIMAL_RESULT)
+      return do_field_decimal;
     return do_field_int;
   }
   int save_in_field(Field *to)
@@ -3661,6 +3663,8 @@ class Field_bit :public Field {
   }
   Copy_func *get_copy_func(const Field *from) const
   {
+    if (from->cmp_type() == DECIMAL_RESULT)
+      return do_field_decimal;
     return do_field_int;
   }
   int save_in_field(Field *to) { return to->store(val_int(), true); }