Crash in parser involving an assign call inside multi-assign

[oprypin]

Code to cause a compiler crash:

a, a.<=

Index out of bounds (IndexError)
  from src/array.cr:1253:11 in 'pop'
  from src/compiler/crystal/syntax/parser.cr:214:19 in 'parse_multi_assign'
  from src/compiler/crystal/syntax/parser.cr:112:13 in 'parse_expressions_internal'
  from src/compiler/crystal/syntax/parser.cr:104:29 in 'parse_expressions'
  from src/compiler/crystal/syntax/parser.cr:83:21 in 'parse'
  from src/compiler/crystal/compiler.cr:245:7 in 'parse'
  from src/compiler/crystal/compiler.cr:228:11 in 'parse'
  from src/compiler/crystal/compiler.cr:167:14 in 'compile'
  from src/compiler/crystal/command/eval.cr:29:14 in 'eval'
  from src/compiler/crystal/command.cr:97:7 in 'run'
  from src/compiler/crystal/command.cr:49:5 in 'run'
  from src/compiler/crystal/command.cr:48:3 in 'run'
  from src/compiler/crystal.cr:11:1 in '__crystal_main'
  from src/crystal/main.cr:110:5 in 'main_user_code'
  from src/crystal/main.cr:96:7 in 'main'
  from src/crystal/main.cr:119:3 in 'main'
  from /usr/lib/libc.so.6 in '__libc_start_main'
  from /home/blaxpirit/repos/crystal/.build/crystal in '_start'
  from ???
Error: you've found a bug in the Crystal compiler. Please open an issue, including source code that will allow us to reproduce the bug: https://github.com/crystal-lang/crystal/issues

This error is not new.

And if you try to add more to this weird source snippet, it instead shows some unhelpful error messages; seems to be in a really weird state.

I couldn't find the cause.

Found with fuzzing :) - one of the few remaining issues in that area