Saxon

The optimizer trace shows

OPT : At line 22 of file:/Users/mike/Desktop/temp/test.xsl
OPT : Extracted global variable vv:gg1636506029 := data((((current()) treat as node())/child::element(Q{http://www.w3.org/1999/XSL/Transform}field))/attribute::attribute(Q{}name))

It seems to have failed to recognize that an expression calling current() can't be promoted to global.

The error message arises when current() is called as a normal function - it should be rewritten as a variable reference. It's misleading because this isn't a dynamic function call, just a static function call happening at runtime.

The problem is that XSLIf doesn't call typeCheck() on the then and else subexpressions (new 4.0 feature), and it's during the typeCheck call that calls on current() are resolved.