Expected syntax error

[notoria]

?- X = a(!;!).
   error(syntax_error(incomplete_reduction),read_term/3:1).
?- X = +(!;!).
   error(syntax_error(incomplete_reduction),read_term/3:1).
?- op(200, fy, a).
   true.
?- X = a(!;!).
   X = a (!;!). % unexpected.
?- X = \(!;!).
   X = \ (!;!). % unexpected.

Expected syntax error.

[UWN]

What is even more surprising is the following:

?- current_op(Pri,Fix,+).
   Pri = 500, Fix = yfx
;  Pri = 200, Fix = fy.
?- X = +(!;!).
error(syntax_error(incomplete_reduction),read_term/3:1).
?- current_op(Pri,Fix,\).
   Pri = 200, Fix = fy.
?- X = \(!;!). 
   X = \ (!;!), unexpected. % as you have pointed out
   syntax_error(...).

And by adding an infix, we get:

?- op(500,yfx,\).
   true.
?- X = \(!;!).
error(syntax_error(incomplete_reduction),read_term/3:1).

[UWN]

And, for completeness:

?- op(0,yfx,+).
   true.
?- X = +(!;!).
   X = + (!;!), unexpected.

[UWN]

Seems to be again an LR-artefact.

s#362