Here are my v15-0001 (code/test) review comments which are "nits".

Basically, these are more likely to be cosmetic rather than functional, but that doesn't mean I think they are unimportant.

======
src/backend/commands/subscriptioncmds.c 

CreateSubscription:

N1.
+	/* Update the Conflict Resolvers in pg_subscription_conflict */
+	SetSubConflictResolvers(subid, conflict_resolvers);

nit - no need for uppercase /Conflict Resolvers/conflict resolvers/
nit - missing period in comments

~

AlterSubscription:

N2.
+				conflict_resolvers = ParseAndGetSubConflictResolvers(
+																	 pstate,
+																	 stmt->resolvers,
+																	 false);

N2a.
The comment is inconsistent with other call to this same function. Either they should both say "validate" or they both should not.

~

N2b.
nit - rearrange this for less harsh indenting.

~~~

N3.
DropSubscription:

+	/* Remove any associated conflict resolvers */
+	RemoveSubConflictResolvers(subid);
+

nit - add period to comment.

======
src/backend/parser/gram.y 

N4.
+		;
+conflict_type:
+			Sconst								{ $$ = $1; }

nit - add whiespace line above label.  
                  
======
src/backend/replication/logical/conflict.c 

N5.
+ConflictType
+ValidateConflictType(const char *conflict_type)
+{

nit - lets call all the names, with suffix '_name'. So here would be /conflict_type/conflict_type_name/

~~~

ValidateConflictTypeAndResolver:

N6.
+ConflictType
+ValidateConflictTypeAndResolver(const char *conflict_type,
+								const char *conflict_resolver)

nit - lets call all the names, with suffix '_name'. So here would be:
/conflict_type/conflict_type_name/
/conflict_resolver/conflict_resolver_name/

~

N7.
+	if (!valid)
+	{
+		ereport(ERROR,
+				errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				errmsg("%s is not a valid conflict resolver", conflict_resolver));
+	}

nit - Unnecessary { }.

~~~

ParseAndGetSubConflictResolvers:

N7.
+	/* Loop through the user provided resolvers */
+	foreach_ptr(DefElem, defel, stmtresolvers)
+	{
+		char	   *resolver;
+		ConflictTypeResolver *conftyperesolver = NULL;

nit - let's call names with '_name' suffix. s/resolver/resolver_name/
nit - let's consistently put a period on end of every comment.   

~~~

UpdateSubConflictResolvers:

N8.
+void
+UpdateSubConflictResolvers(List *conflict_resolvers, Oid subid)
+{
+	Datum		values[Natts_pg_subscription_conflict];
+	bool		nulls[Natts_pg_subscription_conflict];
+	bool		replaces[Natts_pg_subscription_conflict];
+	HeapTuple	oldtup;
+	HeapTuple	newtup = NULL;
+	Relation	pg_subscription_conflict;
+	char	   *cur_conflict_res;
+	Datum		datum;

nit - Many of those variables can be declared at a lower scope. See the nitpick attachment.
nit - Put periods on all the comments in this func.

~~~

ResetSubConflictResolver:

N9.
+void
+ResetSubConflictResolver(Oid subid, char *conflict_type)
+{

nit - change the parameter /conflict_type/conflict_type_name/
nit - add periods a end of all comments
nit - cleanup some var names

~~~

N10.
+/*
+ * Set Conflict Resolvers on the subscription
+ */
+void
+SetSubConflictResolvers(Oid subId, List *conflict_resolvers)

nit - add period to function comment
nit - add period to the body comments too
nit - put some variables at a lower scope

~~~

N11.
+/*
+ * Remove the subscription conflict resolvers for the subscription id
+ */
+void
+RemoveSubConflictResolvers(Oid subid)

nit - add period to function comment.
nit - add period to other code comments.

======
src/bin/pg_dump/pg_dump.c 

N12.
+static bool is_default_resolver(const char *confType, const char *confRes);
+static void destroyConcflictResolverList(SimplePtrList *list);

nit - improve the param names. /confType/conf_type_name/ and /confRes/conf_resolver_name/

~~~

getSubscriptions:

N13.
nit - add periods to comments.

~

N14.
+	PQExpBuffer confQuery;
 	PGresult   *res;
+	PGresult   *confRes;

nit - let's rename these to "conf_query" amd "conf_res" (to match the existing vars)

~

N15.
-				ntups;
+				j,

nit - This 'j' can be a for-loop variable.

~
      
N16.
+		appendPQExpBuffer(confQuery,
+						  "SELECT conftype, confres FROM pg_catalog.pg_subscription_conflict "
+						  "WHERE confsubid = %u order by conftype;", subinfo[i].dobj.catId.oid);
+		confRes = ExecuteSqlQuery(fout, confQuery->data, PGRES_TUPLES_OK);

"order by" should be uppercase.

~~~

N17.
+static bool
+is_default_resolver(const char *confType, const char *confRes)

nit - call this param '_name'
nit - this function is easier to read if you just expand out each conflict type. (see nits attachment for what I mean)

~~~

N18:
+static void
+destroyConcflictResolverList(SimplePtrList *conflictlist)

nit - typo in this function name "Concflict"

======
src/bin/pg_dump/pg_dump.h  

N19.
+
+typedef struct ConflictTypeResolver
+{
+	const char *conflict_type;
+	const char *resolver;
+} ConflictTypeResolver;
+

nit - remove excess spaces
nit - change the field name to be 'conflict_type_name' and 'conflict_resolver_name' 
                  
======
src/bin/psql/describe.c  

describeSubscriptions:

N20.
+
+		/* Add conflict resolvers information from pg_subscription_conflict */
+		if (pset.sversion >= 180000)

nit - add period to the comments.                     

======
src/include/nodes/parsenodes.h    

N21.
+	List	   *resolvers;		/* List of conflict resolvers */
 } CreateSubscriptionStmt;

nit - this comment should be same as the other one in this file, and give a proper description what these list element really are.
                             
======
src/include/replication/conflict.h            

N22.
+extern ConflictType ValidateConflictType(const char *conflict_type);
+extern ConflictType ValidateConflictTypeAndResolver(const char *conflict_type,
+													const char *conflict_resolver);
+extern List *GetDefaultConflictResolvers(void);
+extern void ResetSubConflictResolver(Oid subid, char *conflict_type);

nit - Those function parameters that are names should be called XXX_name. e.g.
/conflict_type/conflict_type_name/
/conflict_resolver/conflict_resolver_name/
  
======
src/test/regress/sql/subscription.sql         

N23.
+-- creating subscription with no explicit conflict resolvers should
+-- configure default conflict resolvers
+CREATE SUBSCRIPTION regress_testsub CONNECTION 'dbname=regress_doesnotexist' PUBLICATION testpub WITH (connect = false);

nit - add  word "ok" in the comment.

~

N24.
+-- ok - valid conflict types and resolvers
+ALTER SUBSCRIPTION regress_testsub CONFLICT RESOLVER (insert_exists = 'apply_remote', update_missing = 'skip', delete_origin_differs = 'keep_local' );

nit - add "alter with" in the comment

~

N25.
+-- ok - valid conflict types and resolvers
+ALTER SUBSCRIPTION regress_testsub CONFLICT RESOLVER (update_exists = 'keep_local', delete_missing = 'error', update_origin_differs = 'error');

nit - add "alter with" in the comment

Also, maybe you should say why there are multiple tests doing the same thing (this and the previous one)

~

N26.
+-- fail - reset with an invalid conflit type
+ALTER SUBSCRIPTION regress_testsub RESET CONFLICT RESOLVER for 'foo';

nit - comment "reset for..."
nit - typo /conflit/conflict/
nit - use uppercase FOR

~

N27.
+-- ok - valid conflict type
+ALTER SUBSCRIPTION regress_testsub RESET CONFLICT RESOLVER for 'insert_exists';

nit - comment "reset for..."
nit - use uppercase FOR

======

FYI - Most of those nit review comments above have already been changed in the accompanying nitpicks attachment.

======
Kind Regards,
Peter Smith.
Fujitsu Australia.