Things that make developing Psalm complicated

This is a somewhat informal list that might aid others.

Statement analysis

Type inference
what effect do different PHP elements (function calls, if/for/foreach statements etc.) have on the types of things
Especially loops
loops are hard to reason about - break and continue are a pain
Also dealing with literal strings/ints/floats
Code liveness detection
what effect do different PHP elements have on whether code is in scope, whether code is redundant
Logical assertions
what effect do different PHP elements have on user-asserted logic in if conditionals, ternarys etc.
Generics & Templated code
Figuring out how templated code should work (@template tags), how much it should work like it does in other languages (Hack, TypeScript etc.)

Supporting formal PHPDoc annotations
Supporting informal PHPDoc annotations e.g. ArrayIterator|string[] to denote an ArrayIterator over strings
non-Composer projects e.g. WordPress

Parser-based reflection
requires scanning everything necessary for analysis
Forking processes (non-windows)
mostly handled by code borrowed from Phan, but can introduce subtle issues, also requires to think about how to make work happen in processes
Caching things
see below

Invalidating analysis results
requires tracking what methods/properties are used in what other files, and invalidating those results when linked methods change
Partial parsing
Reparsing bits of files that have changed, which is hard

Making Psalm fast
see above
Handling temporary file changes
Dealing with malformed PHP code
When people write code, it's not always pretty as they write it. A language server needs to deal with that bad code somehow

Adding/replacing code
Figuring out what changed, making edits that could have been made by a human
Minimal diffs
hard to change more than you need