diff --git a/src/bin/pgbench/exprparse.y b/src/bin/pgbench/exprparse.y
index 5d0d97272f..4948ff3b81 100644
--- a/src/bin/pgbench/exprparse.y
+++ b/src/bin/pgbench/exprparse.y
@@ -28,6 +28,7 @@ static PgBenchExpr *make_func(yyscan_t yyscanner, int fnumber, PgBenchExprList *
 
 %}
 
+%pure-parser
 %expect 0
 %name-prefix="expr_yy"
 
@@ -263,5 +264,7 @@ make_func(yyscan_t yyscanner, int fnumber, PgBenchExprList *args)
 
 /* First, get rid of "#define yyscan_t" from pgbench.h */
 #undef yyscan_t
+/* ... and the yylval macro, which flex will have its own definition for */
+#undef yylval
 
 #include "exprscan.c"
diff --git a/src/bin/pgbench/exprscan.l b/src/bin/pgbench/exprscan.l
index d069c5b05b..825dacc0af 100644
--- a/src/bin/pgbench/exprscan.l
+++ b/src/bin/pgbench/exprscan.l
@@ -47,6 +47,7 @@ extern void expr_yyset_column(int column_no, yyscan_t yyscanner);
 
 /* Except for the prefix, these options should match psqlscan.l */
 %option reentrant
+%option bison-bridge
 %option 8bit
 %option never-interactive
 %option nodefault
@@ -117,15 +118,15 @@ newline			[\n]
 ","				{ return ','; }
 
 :{alnum}+		{
-					yylval.str = pg_strdup(yytext + 1);
+					yylval->str = pg_strdup(yytext + 1);
 					return VARIABLE;
 				}
 {digit}+		{
-					yylval.ival = strtoint64(yytext);
+					yylval->ival = strtoint64(yytext);
 					return INTEGER;
 				}
 {alpha}{alnum}*	{
-					yylval.str = pg_strdup(yytext);
+					yylval->str = pg_strdup(yytext);
 					return FUNCTION;
 				}
 
@@ -169,6 +170,7 @@ expr_yyerror_more(yyscan_t yyscanner, const char *message, const char *more)
 {
 	PsqlScanState state = yyget_extra(yyscanner);
 	int			error_detection_offset = expr_scanner_offset(state) - 1;
+	YYSTYPE		lval;
 	char	   *full_line;
 	size_t		l;
 
@@ -179,7 +181,7 @@ expr_yyerror_more(yyscan_t yyscanner, const char *message, const char *more)
 	 */
 	if (!last_was_newline)
 	{
-		while (yylex(yyscanner))
+		while (yylex(&lval, yyscanner))
 			 /* skip */ ;
 	}
 
@@ -210,6 +212,7 @@ bool
 expr_lex_one_word(PsqlScanState state, PQExpBuffer word_buf, int *offset)
 {
 	int			lexresult;
+	YYSTYPE		lval;
 
 	/* Must be scanning already */
 	Assert(state->scanbufhandle != NULL);
@@ -228,7 +231,7 @@ expr_lex_one_word(PsqlScanState state, PQExpBuffer word_buf, int *offset)
 	state->start_state = INITIAL;
 
 	/* And lex. */
-	lexresult = yylex(state->scanner);
+	lexresult = yylex(&lval, state->scanner);
 
 	/*
 	 * Save start offset of word, if any.  We could do this more efficiently,
diff --git a/src/bin/pgbench/pgbench.h b/src/bin/pgbench/pgbench.h
index a9db9c5b83..101b0bdbf3 100644
--- a/src/bin/pgbench/pgbench.h
+++ b/src/bin/pgbench/pgbench.h
@@ -17,11 +17,17 @@
  * This file is included outside exprscan.l, in places where we can't see
  * flex's definition of typedef yyscan_t.  Fortunately, it's documented as
  * being "void *", so we can use a macro to keep the function declarations
- * here looking like the definitions in exprscan.l.  exprparse.y also
- * uses this to be able to declare things as "yyscan_t".
+ * here looking like the definitions in exprscan.l.  exprparse.y and
+ * pgbench.c also use this to be able to declare things as "yyscan_t".
  */
 #define yyscan_t  void *
 
+/*
+ * Likewise, we can't see exprparse.y's definition of union YYSTYPE here,
+ * but for now there's no need to know what the union contents are.
+ */
+union YYSTYPE;
+
 /* Types of expression nodes */
 typedef enum PgBenchExprType
 {
@@ -85,7 +91,7 @@ struct PgBenchExprList
 extern PgBenchExpr *expr_parse_result;
 
 extern int	expr_yyparse(yyscan_t yyscanner);
-extern int	expr_yylex(yyscan_t yyscanner);
+extern int	expr_yylex(union YYSTYPE *lvalp, yyscan_t yyscanner);
 extern void expr_yyerror(yyscan_t yyscanner, const char *str) pg_attribute_noreturn();
 extern void expr_yyerror_more(yyscan_t yyscanner, const char *str,
 				  const char *more) pg_attribute_noreturn();
diff --git a/src/bin/psql/psqlscan.l b/src/bin/psql/psqlscan.l
index 26ad45c7f9..93c7355960 100644
--- a/src/bin/psql/psqlscan.l
+++ b/src/bin/psql/psqlscan.l
@@ -35,6 +35,12 @@
 %{
 #include "psqlscan_int.h"
 
+/*
+ * We must have a typedef YYSTYPE for yylex's first argument, but this lexer
+ * doesn't presently make use of that argument, so just declare it as int.
+ */
+typedef int YYSTYPE;
+
 /*
  * Set the type of yyextra; we use it as a pointer back to the containing
  * PsqlScanState.
@@ -64,6 +70,7 @@ extern void psql_yyset_column(int column_no, yyscan_t yyscanner);
 %}
 
 %option reentrant
+%option bison-bridge
 %option 8bit
 %option never-interactive
 %option nodefault
@@ -1002,7 +1009,7 @@ psql_scan(PsqlScanState state,
 		yy_switch_to_buffer(state->scanbufhandle, state->scanner);
 
 	/* And lex. */
-	lexresult = yylex(state->scanner);
+	lexresult = yylex(NULL, state->scanner);
 
 	/*
 	 * Check termination state and return appropriate result info.
diff --git a/src/bin/psql/psqlscanslash.l b/src/bin/psql/psqlscanslash.l
index e3cef7c5c5..a89ce15ad4 100644
--- a/src/bin/psql/psqlscanslash.l
+++ b/src/bin/psql/psqlscanslash.l
@@ -26,6 +26,12 @@
 %{
 #include "psqlscan_int.h"
 
+/*
+ * We must have a typedef YYSTYPE for yylex's first argument, but this lexer
+ * doesn't presently make use of that argument, so just declare it as int.
+ */
+typedef int YYSTYPE;
+
 /*
  * Set the type of yyextra; we use it as a pointer back to the containing
  * PsqlScanState.
@@ -62,7 +68,9 @@ extern void slash_yyset_column(int column_no, yyscan_t yyscanner);
 
 %}
 
+/* Except for the prefix, these options should match psqlscan.l */
 %option reentrant
+%option bison-bridge
 %option 8bit
 %option never-interactive
 %option nodefault
@@ -447,7 +455,7 @@ psql_scan_slash_command(PsqlScanState state)
 	state->start_state = xslashcmd;
 
 	/* And lex. */
-	yylex(state->scanner);
+	yylex(NULL, state->scanner);
 
 	/* There are no possible errors in this lex state... */
 
@@ -521,7 +529,7 @@ psql_scan_slash_option(PsqlScanState state,
 		state->start_state = xslashargstart;
 
 	/* And lex. */
-	lexresult = yylex(state->scanner);
+	lexresult = yylex(NULL, state->scanner);
 
 	/* Save final state for a moment... */
 	final_state = state->start_state;
@@ -648,7 +656,7 @@ psql_scan_slash_command_end(PsqlScanState state)
 	state->start_state = xslashend;
 
 	/* And lex. */
-	yylex(state->scanner);
+	yylex(NULL, state->scanner);
 
 	/* There are no possible errors in this lex state... */