Improve error message and hint for ALTER COLUMN TYPE can't-cast failure.

We already tried to improve this once, but the "improved" text was rather
off-target if you had provided a USING clause.  Also, it seems helpful
to provide the exact text of a suggested USING clause, so users can just
copy-and-paste it when needed.  Per complaint from Keith Rarick and a
suggestion from Merlin Moncure.

Back-patch to 9.2 where the current wording was adopted.

src/backend/commands/tablecmds.c

@@ -7888,11 +7888,26 @@ ATPrepAlterColumnType(List **wqueue,
 										  COERCE_IMPLICIT_CAST,
 										  -1);
 		if (transform == NULL)
-			ereport(ERROR,
+		{
-					(errcode(ERRCODE_DATATYPE_MISMATCH),
+			/* error text depends on whether USING was specified or not */
-			  errmsg("column \"%s\" cannot be cast automatically to type %s",
+			if (def->cooked_default != NULL)
-					 colName, format_type_be(targettype)),
+				ereport(ERROR,
-					 errhint("Specify a USING expression to perform the conversion.")));
+						(errcode(ERRCODE_DATATYPE_MISMATCH),
+						 errmsg("result of USING clause for column \"%s\""
+								" cannot be cast automatically to type %s",
+								colName, format_type_be(targettype)),
+						 errhint("You might need to add an explicit cast.")));
+			else
+				ereport(ERROR,
+						(errcode(ERRCODE_DATATYPE_MISMATCH),
+						 errmsg("column \"%s\" cannot be cast automatically to type %s",
+								colName, format_type_be(targettype)),
+				/* translator: USING is SQL, don't translate it */
+					   errhint("You might need to specify \"USING %s::%s\".",
+							   quote_identifier(colName),
+							   format_type_with_typemod(targettype,
+														targettypmod))));
+		}
 
 		/* Fix collations after all else */
 		assign_expr_collations(pstate, transform);

src/test/regress/expected/alter_table.out

@@ -1665,7 +1665,7 @@ 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 automatically to type integer
-HINT:  Specify a USING expression to perform the conversion.
+HINT:  You might need to specify "USING f1::integer".
 alter table foo alter f1 TYPE varchar(10);
 create table anothertab (atcol1 serial8, atcol2 boolean,
 	constraint anothertab_chk check (atcol1 <= 3));
@@ -1680,7 +1680,10 @@ select * from anothertab;
 
 alter table anothertab alter column atcol1 type boolean; -- fails
 ERROR:  column "atcol1" cannot be cast automatically to type boolean
-HINT:  Specify a USING expression to perform the conversion.
+HINT:  You might need to specify "USING atcol1::boolean".
+alter table anothertab alter column atcol1 type boolean using atcol1::int; -- fails
+ERROR:  result of USING clause for column "atcol1" cannot be cast automatically to type boolean
+HINT:  You might need to add an explicit cast.
 alter table anothertab alter column atcol1 type integer;
 select * from anothertab;
  atcol1 | atcol2 

src/test/regress/sql/alter_table.sql

@@ -1175,6 +1175,7 @@ insert into anothertab (atcol1, atcol2) values (default, false);
 select * from anothertab;
 
 alter table anothertab alter column atcol1 type boolean; -- fails
+alter table anothertab alter column atcol1 type boolean using atcol1::int; -- fails
 alter table anothertab alter column atcol1 type integer;
 
 select * from anothertab;