Most declaration forms (data types, values, type families, etc) support forward declaration in hs-boot/hsig files. However, pattern synonyms do not. This seems like a major omission!

Some problems to solve:

• The obvious syntax pattern Foo :: pat_ty is insufficient to specify whether or not a pattern is bidirectional or unidirectional. How should this be represented syntactically?
• What is the interaction with bundling? Should it be possible to export a bundle of abstract pattern synonyms, with the intent that this means an implementation must also have bundled them together
• See also #12717; abstract pattern synonym should be implementable with a plain old constructor