Use simple assert for Nout check

This is an internal sanity check that shouldn't need to throw an error
back to the user

src/typeutils/recursive_maps.jl

@@ -206,7 +206,7 @@ const YS{Nout,T} = Union{Val{Nout},NTuple{Nout,T}}
 function recursive_map(
     f::F, ys::YS{Nout,T}, xs::NTuple{Nin,T}, config::InactiveConfig=InactiveConfig()
 ) where {F,Nout,Nin,T}
-    check_nout(ys)
+    @assert (Nout == 1) || (Nout == 2)
     newys = if isinactivetype(T, config)
         recursive_map_inactive(nothing, ys, xs, config)
     elseif isvectortype(T) || isbitstype(T)
@@ -226,7 +226,7 @@ function recursive_map(
     config::InactiveConfig=InactiveConfig(),
 ) where {F,Nout,Nin,T}
     # determine whether to continue recursion, copy/share, or retrieve from cache
-    check_nout(ys)
+    @assert (Nout == 1) || (Nout == 2)
     newys = if isinactivetype(T, config)
         recursive_map_inactive(seen, ys, xs, config)
     elseif isbitstype(T)  # no object identity to to track in this branch
@@ -581,19 +581,13 @@ end
     else   # may not show in coverage but is covered via accumulate_into! TODO: ensure coverage via VectorSpace once implemented
         cache = seen[x1]::NTuple{(Nout + Nin - 1),T}
         cachedtail = cache[(Nout+1):end]
-        check_identical(cachedtail, xtail)  # check compatible layout
+        check_identical(cachedtail, xtail)  # check compatible structure
         cache[1:Nout]
     end
     return newys::NTuple{Nout,T}
 end
 
 ## argument validation
-@inline function check_nout(::YS{Nout}) where {Nout}
-    if Nout > 2
-        throw_nout()
-    end
-end
-
 Base.@propagate_inbounds function check_initialized(x, i, initialized=true)
     if isinitialized(x, i) != initialized
         throw_initialized()  # TODO: hit this when VectorSpace implemented
@@ -644,7 +638,7 @@ end
 end
 
 @noinline function throw_identical()
-    msg = "recursive_map(!) called on objects whose layout don't match"
+    msg = "recursive_map(!) called on objects whose structure don't match"
     throw(ArgumentError(msg))
 end