fix false alarm when constant variable was initialized by default value

expected/plpgsql_check_active-15.out

@@ -588,3 +588,16 @@ select * from plpgsql_check_function('f1');
 
 drop function f1();
 drop procedure p1(int, int);
+create or replace function f1()
+returns int as $$
+declare c constant int default 100;
+begin
+  return c;
+end;
+$$ language plpgsql;
+-- should be ok
+select * from plpgsql_check_function('f1');
+ plpgsql_check_function 
+------------------------
+(0 rows)
+

sql/plpgsql_check_active-15.sql

@@ -361,3 +361,14 @@ select * from plpgsql_check_function('f1');
 
 drop function f1();
 drop procedure p1(int, int);
+
+create or replace function f1()
+returns int as $$
+declare c constant int default 100;
+begin
+  return c;
+end;
+$$ language plpgsql;
+
+-- should be ok
+select * from plpgsql_check_function('f1');

src/assign.c

@@ -162,7 +162,13 @@ plpgsql_check_target(PLpgSQL_checkstate *cstate, int varno, Oid *expected_typoid
 {
 	PLpgSQL_datum *target = cstate->estate->datums[varno];
 
-	plpgsql_check_is_assignable(cstate->estate, varno);
+	/*
+	 * The target should be not constant, but we can allow assignment to
+	 * constant variable at block statement - it is using default value.
+	 */
+	if (cstate->estate->err_stmt->cmd_type != PLPGSQL_STMT_BLOCK)
+		plpgsql_check_is_assignable(cstate->estate, varno);
+
 	plpgsql_check_record_variable_usage(cstate, varno, true);
 
 	switch (target->dtype)