I wrote the post Write Less with More – Part 8 – PL/SQL in the WITH Clause in November 2015, when the latest released Oracle version was 12.1.
In that post I explained about PL/SQL in the WITH Clause – a new 12.1 feature – and demonstrated it using the following example:

todo8

Since then Oracle 12.2 was released, and introduced a new feature that enables solving this task in a simpler way – the VALIDATE_CONVERSION function. This function gets an expression and a data type, and returns 1 if the expression can be converted to the data type and 0 if not.
Using the same setup from the original post, the requested query becomes as simple as:

> select *
 from people
 where general_info is not null
 and validate_conversion(general_info as date, 'dd/mm/yyyy') = 1;
PERSON_ID FIRST_NAME LAST_NAME GENERAL_INFO
---------- ---------- --------------- --------------------
 102 Paul McCartney 18/6/1942
 202 Ella Fitzgerald 15/6/1996
 203 Etta James 20/1/2012

In addition to introducing the new VALIDATE_CONVERSION function, the older CAST and some of the TO_* conversion functions have been enhanced in Oracle 12.2 and include a DEFAULT ON CONVERSION ERROR clause, so when data type conversion fails we can get some default value instead of an error.

> select p.person_id,
 p.first_name,
 p.last_name,
 to_date(p.general_info default null on conversion error, 'dd/mm/yyyy') my_date
 from people p;
 PERSON_ID FIRST_NAME LAST_NAME MY_DATE
---------- ---------- --------------- ----------
 101 John Lennon
 102 Paul McCartney 18/06/1942
 103 Ringo Starr
 104 George Harisson
 201 Louis Armstrong
 202 Ella Fitzgerald 15/06/1996
 203 Etta James 20/01/2012
 317 Julie Andrews
8 rows selected.