MDEV-20285 Wrong result on INSERT..SELECT when converting from SIGNED to UNSIGNED

mysql-test/main/type_int.result

@@ -236,5 +236,18 @@ DROP FUNCTION sint32;
 DROP FUNCTION uint64;
 DROP FUNCTION sint64;
 #
+# MDEV-20285 Wrong result on INSERT..SELECT when converting from SIGNED to UNSIGNED
+#
+CREATE TABLE t1 (a TINYINT UNSIGNED);
+CREATE TABLE t2 (a TINYINT);
+INSERT INTO t1 VALUES (255);
+INSERT IGNORE INTO t2 SELECT a FROM t1;
+Warnings:
+Warning	1264	Out of range value for column 'a' at row 1
+SELECT * FROM t2;
+a
+127
+DROP TABLE t1, t2;
+#
 # End of 10.3 tests
 #

mysql-test/main/type_int.test

@@ -176,6 +176,17 @@ DROP FUNCTION sint32;
 DROP FUNCTION uint64;
 DROP FUNCTION sint64;
 
+--echo #
+--echo # MDEV-20285 Wrong result on INSERT..SELECT when converting from SIGNED to UNSIGNED
+--echo #
+
+CREATE TABLE t1 (a TINYINT UNSIGNED);
+CREATE TABLE t2 (a TINYINT);
+INSERT INTO t1 VALUES (255);
+INSERT IGNORE INTO t2 SELECT a FROM t1;
+SELECT * FROM t2;
+DROP TABLE t1, t2;
+
 --echo #
 --echo # End of 10.3 tests
 --echo #

mysql-test/main/warnings.result

@@ -115,6 +115,7 @@ insert ignore into t2 select b,c from t1;
 Warnings:
 Warning	1265	Data truncated for column 'b' at row 1
 Warning	1265	Data truncated for column 'b' at row 2
+Warning	1264	Out of range value for column 'a' at row 3
 Warning	1265	Data truncated for column 'b' at row 3
 Warning	1048	Column 'a' cannot be null
 Warning	1265	Data truncated for column 'b' at row 4

sql/field.h

@@ -1708,13 +1708,6 @@ class Field_num :public Field {
   {
     return to->store(val_int(), MY_TEST(flags & UNSIGNED_FLAG));
   }
-  bool memcpy_field_possible(const Field *from) const
-  {
-    return real_type() == from->real_type() &&
-           pack_length() == from->pack_length() &&
-           !((flags & UNSIGNED_FLAG) && !(from->flags & UNSIGNED_FLAG)) &&
-           decimals() == from->decimals();
-  }
   uint is_equal(Create_field *new_field);
   uint row_pack_length() const { return pack_length(); }
   uint32 pack_length_from_metadata(uint field_metadata) {
@@ -1892,7 +1885,10 @@ class Field_real :public Field_num {
       e.g. a DOUBLE(53,10) into a DOUBLE(10,10).
       But it should be OK the other way around.
     */
-    return Field_num::memcpy_field_possible(from) &&
+    return real_type() == from->real_type() &&
+           pack_length() == from->pack_length() &&
+           is_unsigned() <= from->is_unsigned() &&
+           decimals() == from->decimals() &&
            field_length >= from->field_length;
   }
   int store_decimal(const my_decimal *);
@@ -1983,7 +1979,10 @@ class Field_new_decimal :public Field_num {
   }
   bool memcpy_field_possible(const Field *from) const
   {
-    return Field_num::memcpy_field_possible(from) &&
+    return real_type() == from->real_type() &&
+           pack_length() == from->pack_length() &&
+           is_unsigned() <= from->is_unsigned() &&
+           decimals() == from->decimals() &&
            field_length == from->field_length;
   }
   int  reset(void);
@@ -2041,6 +2040,12 @@ class Field_int :public Field_num
     :Field_num(ptr_arg, len_arg, null_ptr_arg, null_bit_arg,
                unireg_check_arg, field_name_arg, 0, zero_arg, unsigned_arg)
     {}
+  bool memcpy_field_possible(const Field *from) const
+  {
+    return real_type() == from->real_type() &&
+           pack_length() == from->pack_length() &&
+           is_unsigned() == from->is_unsigned();
+  }
   int store_decimal(const my_decimal *);
   my_decimal *val_decimal(my_decimal *);
   bool val_bool() { return val_int() != 0; }