Difference between CAST(v AS t) and v::t

Hi all,

while writing test cases for my SQL pretty printer tool[1], I found what seems
a discrepancy in the "Type Casts" documentation[2]: it states that the two
syntaxes are equivalent, but while

  CREATE TABLE contracts (
    ...
    company_id uuid NOT NULL,
    validity daterange NOT NULL,
    
    EXCLUDE USING gist (CAST(company_id AS text) WITH =, validity WITH &&)
  )

works, the following

  CREATE TABLE contracts (
    ...
    company_id uuid NOT NULL,
    validity daterange NOT NULL,
    
    EXCLUDE USING gist (company_id::text WITH =, validity WITH &&)
  )

is rejected with the message "syntax error at or near "::"".

Am I misreading the documentation, or is it missing some special case?

Thanks in advance for any explanation,
ciao, lele.

[1] https://github.com/lelit/pg_query
[2] https://www.postgresql.org/docs/10/static/sql-expressions.html#SQL-SYNTAX-TYPE-CASTS
-- 
nickname: Lele Gaifax | Quando vivrò di quello che ho pensato ieri
real: Emanuele Gaifas | comincerò ad aver paura di chi mi copia.
lele@metapensiero.it  |                 -- Fortunato Depero, 1929.