Submitted by: @pavel-zotov

Following script:

select 1 as static_sql_result from rdb$database where true union all
select 2 as static_sql_result from rdb$database where true = true union all
select 3 as static_sql_result from rdb$database where true is true union all
select 4 as static_sql_result from rdb$database where true is distinct from false
;

set heading on;
set list on;
set echo on;
set term ^;
execute block returns( dynamic_sql_result int) as
begin
for execute statement ( 'select 1 from rdb$database where :param_b' ) ( param_b := true ) into dynamic_sql_result
do
suspend;
end
^

execute block returns( dynamic_sql_result int) as
begin
for execute statement ( 'select 2 from rdb$database where true = :param_b' ) ( param_b := true ) into dynamic_sql_result
do
suspend;
end
^

execute block returns( dynamic_sql_result int) as
begin
for execute statement ( 'select 3 from rdb$database where true IS :param_b' ) ( param_b := true ) into dynamic_sql_result -- <<<<<<< { NB }
do
suspend;

end
^

execute block returns( dynamic_sql_result int) as
begin
for execute statement ( 'select 4 from rdb$database where true IS distinct from :param_b' ) ( param_b := false ) into dynamic_sql_result
do
suspend;

end
^
set term ;^
commit;

-- will issue:

execute block returns( dynamic_sql_result int) as
begin
for execute statement ( 'select 3 from rdb$database where true IS :param_b' ) ( param_b := true ) into dynamic_sql_result
do
suspend;

end
^

Statement failed, SQLSTATE = 42000
Dynamic SQL Error
-SQL error code = -104
-Token unknown - line 1, column 42
-?
-At block line: 3, col: 5

Checked on: WI-V3.0.5.33115, WI-T4.0.0.1479