Julia: Segfault with `ccall` and a type parameter

[fcard]

Well this happened:

julia> try_substrtod(::Type{T}, s) where T = 
           ccall(:jl_try_substrtod, Nullable{T},
                 (Ptr{UInt8},Csize_t,Csize_t), s, 0, sizeof(s))
try_substrtod (generic function with 1 method)

julia> try_substrtod(s) =
           ccall(:jl_try_substrtod, Nullable{Float64},
                 (Ptr{UInt8},Csize_t,Csize_t), s, 0, sizeof(s))
try_substrtod (generic function with 2 methods)

julia> try_substrtod("10.0")
Nullable{Float64}(10.0)

julia> try_substrtod(Float64, "10.0")

signal (11): Segmentation fault
while loading no file, in expression starting on line 0
julia_struct_has_layout at /home/fabio/Source/Git/Languages/julia/src/cgutils.cpp:442 [inlined]
julia_struct_to_llvm at /home/fabio/Source/Git/Languages/julia/src/cgutils.cpp:470
verify_ccall_sig at /home/fabio/Source/Git/Languages/julia/src/ccall.cpp:1415 [inlined]
emit_ccall at /home/fabio/Source/Git/Languages/julia/src/ccall.cpp:1536
emit_expr at /home/fabio/Source/Git/Languages/julia/src/codegen.cpp:4041
emit_function at /home/fabio/Source/Git/Languages/julia/src/codegen.cpp:5963
jl_compile_linfo at /home/fabio/Source/Git/Languages/julia/src/codegen.cpp:1201
jl_compile_for_dispatch at /home/fabio/Source/Git/Languages/julia/src/gf.c:1627
jl_compile_method_internal at /home/fabio/Source/Git/Languages/julia/src/julia_internal.h:294 [inlined]
jl_call_method_internal at /home/fabio/Source/Git/Languages/julia/src/julia_internal.h:341 [inlined]
jl_apply_generic at /home/fabio/Source/Git/Languages/julia/src/gf.c:2225
do_call at /home/fabio/Source/Git/Languages/julia/src/interpreter.c:75
eval at /home/fabio/Source/Git/Languages/julia/src/interpreter.c:242
jl_interpret_toplevel_expr at /home/fabio/Source/Git/Languages/julia/src/interpreter.c:34
jl_toplevel_eval_flex at /home/fabio/Source/Git/Languages/julia/src/toplevel.c:577
jl_toplevel_eval_in at /home/fabio/Source/Git/Languages/julia/src/builtins.c:484
eval at ./boot.jl:235
unknown function (ip: 0x7fe252cf9d8f)
jl_call_fptr_internal at /home/fabio/Source/Git/Languages/julia/src/julia_internal.h:326 [inlined]
jl_call_method_internal at /home/fabio/Source/Git/Languages/julia/src/julia_internal.h:345 [inlined]
jl_apply_generic at /home/fabio/Source/Git/Languages/julia/src/gf.c:2225
eval_user_input at ./REPL.jl:66
unknown function (ip: 0x7fe0410e1576)
jl_call_fptr_internal at /home/fabio/Source/Git/Languages/julia/src/julia_internal.h:326 [inlined]
jl_call_method_internal at /home/fabio/Source/Git/Languages/julia/src/julia_internal.h:345 [inlined]
jl_apply_generic at /home/fabio/Source/Git/Languages/julia/src/gf.c:2225
macro expansion at ./REPL.jl:97 [inlined]
#1 at ./event.jl:73
unknown function (ip: 0x7fe0410d7d2f)
jl_call_fptr_internal at /home/fabio/Source/Git/Languages/julia/src/julia_internal.h:326 [inlined]
jl_call_method_internal at /home/fabio/Source/Git/Languages/julia/src/julia_internal.h:345 [inlined]
jl_apply_generic at /home/fabio/Source/Git/Languages/julia/src/gf.c:2225
jl_apply at /home/fabio/Source/Git/Languages/julia/src/julia.h:1410 [inlined]
start_task at /home/fabio/Source/Git/Languages/julia/src/task.c:261
unknown function (ip: 0xffffffffffffffff)
Allocations: 2263084 (Pool: 2261779; Big: 1305); GC: 1
[1]    9293 segmentation fault  julia

I've posted this on discourse already but since @yuyichao confirmed that there is a bug here somewhere I decided to make it a proper issue.

julia> versioninfo()
Julia Version 0.6.0-pre.alpha.182
Commit 696e1d9fc4* (2017-03-19 17:11 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i7-4510U CPU @ 2.00GHz
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Haswell)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.9.1 (ORCJIT, haswell)

[vtjnash]

It's supposed to throw a Julia error, since that's not a valid function signature in C, but it looks like the detection code may be dereferencing the pointers in the wrong order.

[StefanKarpinski]

When this is fixed, it will definitely be good to add this as a regression test.