ALTER COLUMN SET DATA TYPE does not change the generation expression's collation

hi.

ATPrepAlterColumnType forbids us to ALTER COLUMN SET DATA TYPE USING (expr)
for generated columns.
however we can still change the generated column type from non-text to text
or text type from one collation to another collation.

In ATExecAlterColumnType, we also need to set the generation
expression collation?

We can do this by adding exprSetCollation:

--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -14115,6 +14115,7 @@ ATExecAlterColumnType(AlteredTableInfo *tab,
Relation rel,
errmsg("default for
column \"%s\" cannot be cast automatically to type %s",

colName, format_type_be(targettype))));
}
+ exprSetCollation(defaultexpr, targetcollid);

---------------------
CREATE TABLE x1(a int,
b int GENERATED ALWAYS AS (a * 2) stored,
c text GENERATED ALWAYS AS ('1') stored );
ALTER TABLE x1 alter column b set data type text collate "C";
ALTER TABLE x1 alter column c set data type text collate "C";

SELECT pg_get_expr(d.adbin, d.adrelid) AS default_value, d.adbin
FROM pg_catalog.pg_attribute a
JOIN pg_catalog.pg_attrdef d ON (a.attrelid, a.attnum) = (d.adrelid, d.adnum)
AND a.attrelid = 'x1'::regclass
AND a.attname in ('b', 'c');
by adding exprSetCollation, the output is

default_value | (a * 2)
adbin | {COERCEVIAIO :arg {OPEXPR :opno 514 :opfuncid 141
:opresulttype 23 :opretset false :opcollid 0 :inputcollid 0 :args
({VAR :varno 1 :varattno 1 :vartype 23 :vartypmod -1 :varcollid 0
:varnullingrels (b) :varlevelsup 0 :varreturningtype 0 :varnosyn 1
:varattnosyn 1 :location -1} {CONST :consttype 23 :consttypmod -1
:constcollid 0 :constlen 4 :constbyval true :constisnull false
:location -1 :constvalue 4 [ 2 0 0 0 0 0 0 0 ]}) :location -1}
:resulttype 25 :resultcollid 950 :coerceformat 2 :location -1}
-[ RECORD 2 ]-+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
default_value | '1'::text COLLATE "C"
adbin | {CONST :consttype 25 :consttypmod -1 :constcollid 950
:constlen -1 :constbyval false :constisnull false :location -1
:constvalue 5 [ 20 0 0 0 49 ]}

master behavior:

default_value | (a * 2)
adbin | {COERCEVIAIO :arg {OPEXPR :opno 514 :opfuncid 141
:opresulttype 23 :opretset false :opcollid 0 :inputcollid 0 :args
({VAR :varno 1 :varattno 1 :vartype 23 :vartypmod -1 :varcollid 0
:varnullingrels (b) :varlevelsup 0 :varreturningtype 0 :varnosyn 1
:varattnosyn 1 :location -1} {CONST :consttype 23 :consttypmod -1
:constcollid 0 :constlen 4 :constbyval true :constisnull false
:location -1 :constvalue 4 [ 2 0 0 0 0 0 0 0 ]}) :location -1}
:resulttype 25 :resultcollid 0 :coerceformat 2 :location -1}
-[ RECORD 2 ]-+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
default_value | '1'::text
adbin | {CONST :consttype 25 :consttypmod -1 :constcollid 100
:constlen -1 :constbyval false :constisnull false :location -1
:constvalue 5 [ 20 0 0 0 49 ]}