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Compiler hang on this complicated example #24016
Description
TypeScript Version: 2.9.0-dev.20180510
NodeJS Version: 9.11.1
OS Version: Mac OS X 10.13.4
Search Terms: hangs
Code
It's a pretty involved and complicated code. Put it up as a gist for better readability.
https://gist.github.com/shivawu/22053a9292ac694bfc87c97ef3c652bf
Note the last line, if it's commented it out, it compiles just fine. That particular line hangs the compiler.
Expected behavior:
It should type check just fine.
Actual behavior:
The compiler hangs. Taking 100% CPU, memory keeps increasing non-stop.
Playground Link: I would skip that since the code would hang the browser tab :)
Activity
ahejlsberg commentedon Oct 10, 2018
With the compiler currently in
masterwe don't hang on the original example (probably because of #26558), but it still takes 7.5s to type check and allocates over 700,000 types.The core issue here is the pattern of using indexed access types to write self-referencing conditional types. It is a Turing Complete device (#14833) and along with that comes the halting problem. It is effectively impossible for us to know if evaluation of such types will ever terminate. For this reason I strongly discourage use of the pattern.