Reporting back: The problem was a bug in the bytecode compiler, and
commit 88d8ba00e0 is the repair.

The debugger works by modifying a program so that local variables are
mutable. Meanwhile, Philip's program uses the contracts in a way that
expands to `call-with-immediate-continuation-mark`. The compiler
mishandled `call-with-immediate-continuation-mark` on a `lambda` whose
argument variable is mutable.

At Sat, 21 Apr 2018 07:38:32 -0600, Matthew Flatt wrote:
> Thanks for the crash report! It doesn't immediately point to the
> problem, but it's good starting point. I'll ask you for more debugging
> help off-list, and we can report back here afterwards.
> At Sat, 21 Apr 2018 05:11:10 -0500, Philip McGrath wrote:
> > In a particular module, I can consistently trigger a segfault by opening
> > DrRacket's graphical debugger, setting a breakpoint, and evaluating an
> > expression at the REPL. I am using Racket 6.12 on Mac OS 10.13.2: I have
> > not (yet) tried to reproduce this on other platforms.
> > 
> > Unfortunately I don't have anything approaching a minimal example: the file
> > that's triggering the issue is part of a large library.
> > 
> > I've attached the crash report in case that is illuminating. I'm happy to
> > help try to find the problem to the extent I can.
> > 
> > -Philip

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