Fix another obscure pattern-synonym crash

Authored by simonpj on Jan 5 2018, 3:12 AM.

Description

Fix another obscure pattern-synonym crash

This one, discovered by Iceland Jack (Trac Trac #14507), shows
that a pattern-bound coercion can show up in the argument
type(s) of the matcher of a pattern synonym.

The error message isn't great, but at least we now rightly
reject the program.