doFoldKnownClass folds class_exists() whenever the name is a literal the
symbol table knows. That table also holds traits, so a trait name folded
to true while PHP answers false:
trait Helper {}
class_exists('Helper'); // folded to true
$name = 'Helper';
class_exists($name); // reaches php::fn::class_exists, answers false
The same program therefore gives two different answers for the same
trait, decided only by whether the argument is a literal.
The runtime side is already right, and deliberately so: traits are
compile-time AST templates in TypePHP, which is why
tests/compiler/stdlib/class_exists.phpt expects trait_exists() to be
false. Only the constant fold disagreed - with PHP and with the
compiler's own runtime.
A trait name now folds to false. Classes and enums keep folding to true,
which matches PHP: an enum is a class, a trait is not.
class_exists.phpt gains the literal and non-literal trait cases, and
ClassExistsTraitFoldTest pins the fold decision in the generated C++.
Giandonn