fix tuple_type_head and tuple_type_tail on UnionAll

Ref #22911
(cherry picked from commit 763d008)

Author: JeffBezanson

base/essentials.jl

@@ -59,13 +59,13 @@ end
 argtail(x, rest...) = rest
 tail(x::Tuple) = argtail(x...)
 
-tuple_type_head(T::UnionAll) = tuple_type_head(T.body)
+tuple_type_head(T::UnionAll) = (@_pure_meta; UnionAll(T.var, tuple_type_head(T.body)))
 function tuple_type_head(T::DataType)
     @_pure_meta
     T.name === Tuple.name || throw(MethodError(tuple_type_head, (T,)))
     return unwrapva(T.parameters[1])
 end
-tuple_type_tail(T::UnionAll) = tuple_type_tail(T.body)
+tuple_type_tail(T::UnionAll) = (@_pure_meta; UnionAll(T.var, tuple_type_tail(T.body)))
 function tuple_type_tail(T::DataType)
     @_pure_meta
     T.name === Tuple.name || throw(MethodError(tuple_type_tail, (T,)))

test/tuple.jl

@@ -258,3 +258,7 @@ end
 for n = 0:15
     @test ntuple(identity, Val{n}) == ntuple(identity, n)
 end
+
+# https://github.com/JuliaLang/julia/issues/21026#issuecomment-317113307
+const VecTuple21026{T} = Tuple{VecElement{T}}
+@test convert(VecTuple21026, (1,)) === (VecElement(1),)