Fix type inference of eltype(vi, spl) (Turing#2151)

src/abstract_varinfo.jl

@@ -363,7 +363,14 @@ Determine the default `eltype` of the values returned by `vi[spl]`.
     This method is considered legacy, and is likely to be deprecated in the future.
 """
 function Base.eltype(vi::AbstractVarInfo, spl::Union{AbstractSampler,SampleFromPrior})
-    return eltype(Core.Compiler.return_type(getindex, Tuple{typeof(vi),typeof(spl)}))
+    T = Base.promote_op(getindex, typeof(vi), typeof(spl))
+    if T === Union{}
+        # In this case `getindex(vi, spl)` errors
+        # Let us throw a more descriptive error message
+        # Ref https://github.com/TuringLang/Turing.jl/issues/2151
+        return eltype(vi[spl])
+    end
+    return eltype(T)
 end
 
 # TODO: Should relax constraints on `vns` to be `AbstractVector{<:Any}` and just try to convert

src/varinfo.jl

@@ -1444,7 +1444,7 @@ function getindex(vi::TypedVarInfo, spl::Sampler)
     # Gets the ranges as a NamedTuple
     ranges = _getranges(vi, spl)
     # Calling getfield(ranges, f) gives all the indices in `vals` of the `vn`s with symbol `f` sampled by `spl` in `vi`
-    return vcat(_getindex(vi.metadata, ranges)...)
+    return reduce(vcat, _getindex(vi.metadata, ranges))
 end
 # Recursively builds a tuple of the `vals` of all the symbols
 @generated function _getindex(metadata, ranges::NamedTuple{names}) where {names}