allow fieldcount(T) == 0

Author: vchuravy

src/array.jl

@@ -45,8 +45,9 @@ end
 #    these are stored with a selector at the end (handled by Julia).
 # 3. bitstype unions (`Union{Int, Float32}`, etc)
 #    these are stored contiguously and require a selector array (handled by us)
+# As well as "mutable singleton" types like `Symbol` that use pointer-identity
 @inline function check_eltype(name, T)
-  if !Base.allocatedinline(T)
+  if !(Base.allocatedinline(T) || Base.datatype_fieldcount(T) == 0)
     explanation = explain_eltype(T)
     error("""
       $name only supports element types that are allocated inline.