I found a bug that only appears based on file name. If the filename starts with u-z or U-Z, no bug.
➜ y ponyc
Building builtin -> /usr/local/lib/pony/0.2.1-748-gf504469/packages/builtin
Building . -> /Users/sean/Private/Code/pony/y
Building ponytest -> /usr/local/lib/pony/0.2.1-748-gf504469/packages/ponytest
Building time -> /usr/local/lib/pony/0.2.1-748-gf504469/packages/time
Building collections -> /usr/local/lib/pony/0.2.1-748-gf504469/packages/collections
Building promises -> /usr/local/lib/pony/0.2.1-748-gf504469/packages/promises
Generating
Reachability
Selector painting
Data prototypes
Data types
Function prototypes
Descriptors
Functions
Optimising
Writing ./y.o
Linking ./y
➜ y ./y
1 test started, 0 complete: logger/error started
2 tests started, 0 complete: logger/info started
3 tests started, 0 complete: logger/fine started
4 tests started, 0 complete: logger/warn started
5 tests started, 0 complete: logger/object started
5 tests started, 1 complete: logger/error complete
5 tests started, 2 complete: logger/warn complete
5 tests started, 3 complete: logger/fine complete
5 tests started, 4 complete: logger/object complete
5 tests started, 5 complete: logger/info complete
---- Passed: logger/error
---- Passed: logger/warn
---- Passed: logger/info
---- Passed: logger/fine
---- Passed: logger/object
----
---- 5 tests ran.
---- Passed: 5
➜ y mv z.pony a.pony
➜ y ponyc
Building builtin -> /usr/local/lib/pony/0.2.1-748-gf504469/packages/builtin
Building . -> /Users/sean/Private/Code/pony/y
Building ponytest -> /usr/local/lib/pony/0.2.1-748-gf504469/packages/ponytest
Building time -> /usr/local/lib/pony/0.2.1-748-gf504469/packages/time
Building collections -> /usr/local/lib/pony/0.2.1-748-gf504469/packages/collections
Building promises -> /usr/local/lib/pony/0.2.1-748-gf504469/packages/promises
Assertion failed: (t->frame->package != NULL), function package_hygienic_id, file src/libponyc/pkg/package.c, line 950.
[1] 55533 illegal hardware instruction ponyc