can't infer type in some constructors

[kleinschmidt]

julia> @code_warntype ConjugatePriors.NormalInverseGamma(1., 1., 1., 1.)
Variables:
  #self# <optimized out>
  mu::Float64
  v0::Float64
  sh::Float64
  r::Float64
  T::Any

Body:
  begin 
      T::Any = (Base.promote_type)(Float64, (Base.promote_type)(Float64, (Base.promote_type)(Float64, Float64)::Any)::Any)::Any # line 20:
      return ((Core.apply_type)(ConjugatePriors.NormalInverseGamma, T::Any)::Type{ConjugatePriors.NormalInverseGamma{_}} where _)((T::Any)(mu::Float64)::Any, (T::Any)(v0::Float64)::Any, (T::Any)(sh::Float64)::Any, (T::Any)(r::Float64)::Any)::ConjugatePriors.NormalInverseGamma{_} where _
  end::ConjugatePriors.NormalInverseGamma{_} where _

I think this comes from having constructors like

ConjugatePriors.jl/src/normalinversegamma.jl

Lines 18 to 21 in 36fe174

	function NormalInverseGamma(mu::Real, v0::Real, sh::Real, r::Real)
	T = promote_type(typeof(mu), typeof(v0), typeof(sh), typeof(r))
	return NormalInverseGamma{T}(T(mu),T(v0),T(sh),T(r))
	end

where the type is computed at runtime (with promote_type), but I could be wrong about that.

When all the arguments are of the same type, adding a method like NormalInverseGamma(::T, ::T, ::T, ::T) where {T<:Real}) gets rid of the type instability. Another possible fix would be to use the inner constructor with the explicit type parameter whenever possible (e.g., in posterior_canon, which is where I was bitten by this).