Possible bug in CASE evaluation

I want to draw attention to this thread on -general:
CAMQ5dGq4SuJPbhT2-9XLAPAsvKNUL2-bb0cPyci2Fp+pfSfc3g(at)mail(dot)gmail(dot)com

Would you concur that this is a bug?

The fine manual says about CASE:

If the condition's result is true, the value of the CASE expression
is the result that follows the condition, and the remainder of the
CASE expression is not processed.

But:

test=> CREATE FUNCTION zero() RETURNS integer IMMUTABLE LANGUAGE SQL AS 'SELECT 0';
CREATE FUNCTION
test=> SELECT CASE WHEN (SELECT zero()) = 0 THEN -1 ELSE 1/zero() END;
ERROR: division by zero

Yours,
Laurenz Albe