julia> # removing the `S` here fixes the segfault
       function g(x::T) where {T,S}
           cf = @cfunction identity Ref{T} (Ref{T},)
           GC.@preserve cf begin
               fptr = Base.unsafe_convert(Ptr{Cvoid}, cf)
               ccall(fptr, Ref{T}, (Ref{T},), x)
           end
       end
g (generic function with 1 method)

julia> g(1)
[1]    35506 segmentation fault  julia

julia> versioninfo()
Julia Version 0.7.0-beta.9
Commit d7bf89fc1c (2018-06-25 13:53 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin17.5.0)
  CPU: Intel(R) Core(TM) i7-7920HQ CPU @ 3.10GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-6.0.0 (ORCJIT, skylake)