Suppress variable-set-but-not-used warnings from clang 15.

clang 15+ will issue a set-but-not-used warning when the only
use of a variable is in autoincrements (e.g., "foo++;").
That's perfectly sensible, but it detects a few more cases that
we'd not noticed before.  Silence the warnings with our usual
methods, such as PG_USED_FOR_ASSERTS_ONLY, or in one case by
actually removing a useless variable.

One thing that we can't nicely get rid of is that with %pure-parser,
Bison emits "yynerrs" as a local variable that falls foul of this
warning.  To silence those, I inserted "(void) yynerrs;" in the
top-level productions of affected grammars.

Per recently-established project policy, this is a candidate
for back-patching into out-of-support branches: it suppresses
annoying compiler warnings but changes no behavior.  Hence,
back-patch to 9.5, which is as far as these patches go without
issues.  (A preliminary check shows that the prior branches
need some other set-but-not-used cleanups too, so I'll leave
them for another day.)

Discussion: https://postgr.es/m/514615.1663615243@sss.pgh.pa.us

5 files changed, 8 insertions, 6 deletions

diff --git a/src/backend/access/gist/gistxlog.c b/src/backend/access/gist/gistxlog.c
index f4dfaaa1b8..144e7ab8a0 100644
--- a/src/backend/access/gist/gistxlog.c
+++ b/src/backend/access/gist/gistxlog.c
@@ -257,7 +257,7 @@ gistRedoPageUpdateRecord(XLogReaderState *record)
 		char	   *begin;
 		char	   *data;
 		Size		datalen;
-		int			ninserted = 0;
+		int			ninserted PG_USED_FOR_ASSERTS_ONLY = 0;
 
 		data = begin = XLogRecGetBlockData(record, 0, &datalen);
 
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 8522765489..23c7a64844 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2073,7 +2073,7 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, bool opportunistic)
 	XLogRecPtr	NewPageEndPtr = InvalidXLogRecPtr;
 	XLogRecPtr	NewPageBeginPtr;
 	XLogPageHeader NewPage;
-	int			npages = 0;
+	int			npages pg_attribute_unused() = 0;
 
 	LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);
 
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index d36b82f4df..9478602be8 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -775,6 +775,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 stmtblock:	stmtmulti
 			{
 				pg_yyget_extra(yyscanner)->parsetree = $1;
+				(void) yynerrs;		/* suppress compiler warning */
 			}
 		;
 
diff --git a/src/backend/utils/adt/array_typanalyze.c b/src/backend/utils/adt/array_typanalyze.c
index 78153d232f..2ed823d86f 100644
--- a/src/backend/utils/adt/array_typanalyze.c
+++ b/src/backend/utils/adt/array_typanalyze.c
@@ -217,7 +217,6 @@ compute_array_stats(VacAttrStats *stats, AnalyzeAttrFetchFunc fetchfunc,
 {
 	ArrayAnalyzeExtraData *extra_data;
 	int			num_mcelem;
-	int			null_cnt = 0;
 	int			null_elem_cnt = 0;
 	int			analyzed_rows = 0;
 
@@ -321,8 +320,7 @@ compute_array_stats(VacAttrStats *stats, AnalyzeAttrFetchFunc fetchfunc,
 		value = fetchfunc(stats, array_no, &isnull);
 		if (isnull)
 		{
-			/* array is null, just count that */
-			null_cnt++;
+			/* ignore arrays that are null overall */
 			continue;
 		}
 
diff --git a/src/bin/pgbench/exprparse.y b/src/bin/pgbench/exprparse.y
index b3a2d9bfd3..4315990d0d 100644
--- a/src/bin/pgbench/exprparse.y
+++ b/src/bin/pgbench/exprparse.y
@@ -60,7 +60,10 @@ static PgBenchExpr *make_func(yyscan_t yyscanner, int fnumber, PgBenchExprList *
 
 %%
 
-result: expr				{ expr_parse_result = $1; }
+result: expr				{
+								expr_parse_result = $1;
+								(void) yynerrs; /* suppress compiler warning */
+							}
 
 elist:                  	{ $$ = NULL; }
 	| expr 					{ $$ = make_elist($1, NULL); }