Crystal: `a = yield || return nil` -> can't execute ...

[chaniks]

Mac OS X 10.11.6 / Crystal 0.19.2 and also master (0.19.2+48)
(NOT reproducible on playground by the way.)

Code:

def foo(&block)
  a = yield || return nil
  # .. some code ..
end

foo { nil } # error

## -> also
def bar
  a = nil || return "?"
end

bar() # error

Produces:

can't execute `a = if __temp_77 = yield
  __temp_77
else
  return nil
end` at ~/git/crystal/bugs/src/nil_or_return.cr:2:3 (Exception)
[4367046482] *CallStack::unwind:Array(Pointer(Void)) +82
[4367046385] *CallStack#initialize:Array(Pointer(Void)) +17
[4367046344] *CallStack::new:CallStack +40
[4367026505] *raise<Exception>:NoReturn +25
[4367026465] *raise<String>:NoReturn +17
[4367025105] __crystal_main +945
[4367038280] main +40

Work Around:

def foo(&block)
  (a = yield) || return nil
  # .. some code ..
end

or just use if/unless

[david50407]

def foo(&block)
  a = yield || return nil
  # .. some code ..
end

is extracted as:

def foo(&block)
  a = if __tmp = yield
    __tmp
  else
    return nil
  end
  # .. some code ..
end

This breaks the flow, it returned from foo method but not assignment value to a.

And,

def foo(&block)
  (a = yield) || return nil
  # .. some code ..
end

extract as:

def foo(&block)
  if a = yield
    a
  else
    return nil
  end
  # .. some code ..
end

is fine, it doesn't break the code

[chaniks]

Hmm.. Now I see it more clearly.
Thank you for the clarification. I'm very new to this language.

Additionally, it seems the compiler breaks only when the right-hand value of if-clause is nil.
(Same thing happens when I put a variable instead of yield, or nil value itself directly.)

def foo(&block)
  a = if __tmp = yield
    __tmp
  else
    return nil 
  end 
  # .. some code ..
end

foo { "bar" }  # ok
foo {  nil  }  # error

[asterite]

@chaniks Thank you for reporting this!

It's still a compiler bug and should be fixed. There's a similar issue, #821, which in the comments have more or less this code, but now I know a way to fix this, I think.

[chaniks]

@asterite Oh my God.. This fast? I couldn't even finish my reply.

The issue explained me everything. Now it's _crystal clear_. 👍