From @masak

<masak> m​: sub foo { 2 }; say 1 ?? foo !! 3
<camelia> rakudo-moar 315ec6​: OUTPUT«===SORRY!===␤Found ?? but no !! [...]
<masak> that's an LTA error message (even knowing what's going on there)
<masak> std​: sub foo { 2 }; say 1 ?? foo !! 3
<camelia> std 14ad63b​: OUTPUT«===SORRY!===␤Found ?? but no !! [...]
<masak> and STD.pm6 is no better. hm.
<masak> I ran into it in code like this​:
<masak> m​: module M { enum A is export <B C> }; import M; say 1 ?? B !! C
* masak submits ?? !! LTA rakudobug

Ideally, I would like to see an error message that acknowledges that a
'!!' is actually there, but that it was gobbled by the listop -- much
like this error message warns about a listop gobbling a block​:

<masak> m​: for lines { .say }
<camelia> rakudo-moar 315ec6​: OUTPUT«===SORRY!===␤Function lines needs
parens to avoid gobbling block [...]

From @usev6

This fails now with a typed exception and a fitting error message​:

$ perl6 -e 'sub foo { 2 }; say 1 ?? foo !! 3'
===SORRY!=== Error while compiling -e
Your !! was gobbled by the expression in the middle; please parenthesize
at -e​:1
------> sub foo { 2 }; say 1 ?? foo !!⏏ 3
  expecting any of​:
  infix
  infix stopper
  postfix

I added a test for the typed exception to S03-operators/ternary.t. I'm closing this ticket as 'resolved'.

@usev6 - Status changed from 'new' to 'resolved'