Improve error message for ALTER COLUMN TYPE coercion failure.

Per recent discussion, the error message for this was actually a trifle
inaccurate, since it said "cannot be cast" which might be incorrect.
Adjust that wording, and add a HINT suggesting that a USING clause might
be needed.

src/backend/commands/tablecmds.c

@@ -7315,8 +7315,9 @@ ATPrepAlterColumnType(List **wqueue,
 		if (transform == NULL)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATATYPE_MISMATCH),
-					 errmsg("column \"%s\" cannot be cast to type %s",
-							colName, format_type_be(targettype))));
+					 errmsg("column \"%s\" cannot be cast automatically to type %s",
+							colName, format_type_be(targettype)),
+					 errhint("Specify a USING expression to perform the conversion.")));
 
 		/* Fix collations after all else */
 		assign_expr_collations(pstate, transform);
@@ -7482,7 +7483,7 @@ ATExecAlterColumnType(AlteredTableInfo *tab, Relation rel,
 		if (defaultexpr == NULL)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATATYPE_MISMATCH),
-				errmsg("default for column \"%s\" cannot be cast to type %s",
+				errmsg("default for column \"%s\" cannot be cast automatically to type %s",
 					   colName, format_type_be(targettype))));
 	}
 	else

src/test/regress/expected/alter_table.out

@@ -1673,7 +1673,8 @@ select f3,max(f1) from foo group by f3;
 
 -- Simple tests for alter table column type
 alter table foo alter f1 TYPE integer; -- fails
-ERROR:  column "f1" cannot be cast to type integer
+ERROR:  column "f1" cannot be cast automatically to type integer
+HINT:  Specify a USING expression to perform the conversion.
 alter table foo alter f1 TYPE varchar(10);
 create table anothertab (atcol1 serial8, atcol2 boolean,
 	constraint anothertab_chk check (atcol1 <= 3));
@@ -1688,7 +1689,8 @@ select * from anothertab;
 (2 rows)
 
 alter table anothertab alter column atcol1 type boolean; -- fails
-ERROR:  column "atcol1" cannot be cast to type boolean
+ERROR:  column "atcol1" cannot be cast automatically to type boolean
+HINT:  Specify a USING expression to perform the conversion.
 alter table anothertab alter column atcol1 type integer;
 select * from anothertab;
  atcol1 | atcol2 
@@ -1723,7 +1725,7 @@ select * from anothertab;
 
 alter table anothertab alter column atcol1 type boolean
         using case when atcol1 % 2 = 0 then true else false end; -- fails
-ERROR:  default for column "atcol1" cannot be cast to type boolean
+ERROR:  default for column "atcol1" cannot be cast automatically to type boolean
 alter table anothertab alter column atcol1 drop default;
 alter table anothertab alter column atcol1 type boolean
         using case when atcol1 % 2 = 0 then true else false end; -- fails