Unfortunately, even though the return type assertion syntax is accepted, it is broken:
*julia> **compute_val(1)*
*ERROR: ArgumentError: typeassert: too few arguments (expected 2)*
* in compute_val(::Int64) at ./REPL[2]:1*
* in eval(::Module, ::Any) at ./boot.jl:225*
* in macro expansion at ./REPL.jl:92 [inlined]*
* in (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at ./event.jl:46*
