Precompilation crash with promotion

[rdeits]

This was lovingly distilled from the nightly failures of GeometryTypes.jl. Create the following module inside .julia/v0.7/Foo/src/Foo.jl:

__precompile__()

module Foo

immutable OffsetInteger{O, T <: Integer} <: Integer
    i::T
    function (::Type{OffsetInteger{O, T}}){O, T}(x::T)
        new{O, T}(x - O)
    end
end

Base.promote_type{T <: Int, OI <: OffsetInteger}(::Type{T}, ::Type{OI}) = T
Base.promote_type{T <: Int, OI <: OffsetInteger}(::Type{OI}, ::Type{T}) = T

end

and then run using Foo in Julia master to see:

julia> using Foo

WARNING: deprecated syntax "immutable" at /Users/rdeits/.julia/v0.7/Foo/src/Foo.jl:5.
Use "struct" instead.
INFO: Precompiling module Foo.

WARNING: deprecated syntax "immutable" at /Users/rdeits/.julia/v0.7/Foo/src/Foo.jl:5.
Use "struct" instead.

ERROR: fatal: error thrown and no exception handler available.
Base.MethodError(f=typeof(Base.promote_type)(), args=(Core.TypeofBottom(), Int64), world=0x00000000000055f1)
rec_backtrace at /Users/rdeits/Projects/julia/src/stackwalk.c:84
record_backtrace at /Users/rdeits/Projects/julia/src/task.c:245 [inlined]
jl_throw at /Users/rdeits/Projects/julia/src/task.c:564
jl_method_error_bare at /Users/rdeits/Projects/julia/src/gf.c:1469
jl_method_error at /Users/rdeits/Projects/julia/src/gf.c:1487
jl_lookup_generic_ at /Users/rdeits/Projects/julia/src/gf.c:1900
jl_apply_generic at /Users/rdeits/Projects/julia/src/gf.c:1920
uv_write at ./stream.jl:803
unsafe_write at ./stream.jl:832
print at ./strings/io.jl:122 [inlined]
#with_output_color#525 at ./util.jl:404
jl_apply at /Users/rdeits/Projects/julia/src/./julia.h:1424 [inlined]
jl_f__apply at /Users/rdeits/Projects/julia/src/builtins.c:426
#with_output_color at ./<missing>:0
jl_apply at /Users/rdeits/Projects/julia/src/./julia.h:1424 [inlined]
jl_f__apply at /Users/rdeits/Projects/julia/src/builtins.c:426
#print_with_color#526 at ./util.jl:417
jl_apply at /Users/rdeits/Projects/julia/src/./julia.h:1424 [inlined]
jl_f__apply at /Users/rdeits/Projects/julia/src/builtins.c:426
#print_with_color at ./<missing>:0
display_error at ./client.jl:133
unknown function (ip: 0x11c94de9d)
display_error at ./client.jl:142
unknown function (ip: 0x11c94d8c6)
do_call at /Users/rdeits/Projects/julia/src/interpreter.c:75
eval at /Users/rdeits/Projects/julia/src/interpreter.c:242
eval_body at /Users/rdeits/Projects/julia/src/interpreter.c:539
jl_toplevel_eval_body at /Users/rdeits/Projects/julia/src/interpreter.c:511
jl_toplevel_eval_flex at /Users/rdeits/Projects/julia/src/toplevel.c:569
jl_toplevel_eval_in at /Users/rdeits/Projects/julia/src/builtins.c:496
eval at ./boot.jl:236
jlcall_eval_18290 at /Users/rdeits/Projects/julia/usr/lib/julia/sys.dylib (unknown line)
_start at ./client.jl:419
jlcall__start_19148 at /Users/rdeits/Projects/julia/usr/lib/julia/sys.dylib (unknown line)
true_main at /Users/rdeits/Projects/julia/./julia (unknown line)
main at /Users/rdeits/Projects/julia/./julia (unknown line)

(this also crashes Julia itself).

I've seen this on OSX, Linux, and Windows. v0.6-rc2 seems to be fine.

julia> versioninfo()
Julia Version 0.7.0-DEV.439
Commit e36e412 (2017-06-01 18:48 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin15.6.0)
  CPU: Intel(R) Core(TM) i7-2860QM CPU @ 2.50GHz
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Sandybridge)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.9.1 (ORCJIT, sandybridge)

[rdeits]

Even simpler case:

__precompile__()

module Foo

struct OffsetInteger
end

Base.promote_type(::Type{T}, ::Type{OI}) where {T <: Int, OI <: OffsetInteger} = T
Base.promote_type(::Type{OI}, ::Type{T}) where {T <: Int, OI <: OffsetInteger} = T

end

[JeffBezanson]

I'm not sure if this is the only source of the problem, but you should define promote_rule instead of promote_type. That also has the feature that you only need to define it in one argument order instead of both.

[rdeits]

Thanks, @JeffBezanson.

Using promote_rule does fix the problem here, but I assume that using promote_type probably shouldn't cause Julia to barf like this?

[martinholters]

Has nothing to do with precompilation (needs a keytroke at the third prompt to trigger the error):

julia> struct OffsetInteger end

julia> Base.promote_type(::Type{T}, ::Type{OI}) where {T <: Int, OI <: OffsetInteger} = T

julia> ERROR (in the keymap): MethodError: promote_type(::Type{Int64}, ::Type{Union{}}) is ambiguous. Candidates:
  promote_type(::Type{T}, ::Type{OI}) where {T<:Int64, OI<:OffsetInteger} in Main at REPL[2]:1
  promote_type(::Type{T}, ::Type{Union{}}) where T in Base at promotion.jl:131
Possible fix, define
  promote_type(::Type{T<:Int64}, ::Type{Union{}})
Stacktrace:
 [1] uv_error at ./libuv.jl:68 [inlined]
 [2] displaysize(::Base.TTY) at ./stream.jl:377
 [3] width(::Base.Terminals.TTYTerminal) at ./Terminals.jl:74
 [4] edit_insert(::Base.LineEdit.PromptState, ::String) at ./LineEdit.jl:461
 [5] (::Base.LineEdit.##101#131)(::Base.LineEdit.MIState, ::Base.REPL.LineEditREPL, ::String) at ./LineEdit.jl:1397
 [6] (::Base.LineEdit.##13#14{Base.LineEdit.##101#131,String})(::Base.LineEdit.MIState, ::Base.REPL.LineEditREPL) at ./LineEdit.jl:740
 [7] prompt!(::Base.Terminals.TTYTerminal, ::Base.LineEdit.ModalInterface, ::Base.LineEdit.MIState) at ./LineEdit.jl:1618
 [8] run_interface(::Base.Terminals.TTYTerminal, ::Base.LineEdit.ModalInterface) at ./LineEdit.jl:1578
 [9] run_frontend(::Base.REPL.LineEditREPL, ::Base.REPL.REPLBackendRef) at ./REPL.jl:943
 [10] run_repl(::Base.REPL.LineEditREPL, ::Base.##523#524) at ./REPL.jl:180
 [11] _start() at ./client.jl:415
julia> 
ERROR: MethodError: promote_type(::Type{Int64}, ::Type{Union{}}) is ambiguous. Candidates:
  
Stacktrace:fatal: error thrown and no exception handler available.
Base.MethodError(f=typeof(Base.promote_type)(), args=(Int64, Core.TypeofBottom()), world=0x00000000000055ec)
[...]

It's a bit unfortunate that the error message in the OP didn't make it obvious that an ambiguity is the problem, but otherwise I don't think there is much we can do to prevent messing up promote_type from breaking Julia, ref #22132.