avoid kind type dispatch bugs in tuple_type_head and tuple_type_tail (J…

…uliaLang#27776)

base/essentials.jl

@@ -141,31 +141,21 @@ end
 argtail(x, rest...) = rest
 tail(x::Tuple) = argtail(x...)
 
-# TODO: a better / more infer-able definition would pehaps be
-# tuple_type_head(T::Type) = fieldtype(T::Type{<:Tuple}, 1)
-tuple_type_head(T::UnionAll) = (@_pure_meta; UnionAll(T.var, tuple_type_head(T.body)))
-function tuple_type_head(T::Union)
- @_pure_meta
- return Union{tuple_type_head(T.a), tuple_type_head(T.b)}
-end
-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_head(T::Type) = (@_pure_meta; fieldtype(T::Type{<:Tuple}, 1))
 
-tuple_type_tail(T::UnionAll) = (@_pure_meta; UnionAll(T.var, tuple_type_tail(T.body)))
-function tuple_type_tail(T::Union)
+function tuple_type_tail(T::Type)
  @_pure_meta
- return Union{tuple_type_tail(T.a), tuple_type_tail(T.b)}
-end
-function tuple_type_tail(T::DataType)
- @_pure_meta
- T.name === Tuple.name || throw(MethodError(tuple_type_tail, (T,)))
- if isvatuple(T) && length(T.parameters) == 1
- return T
+ if isa(T, UnionAll)
+ return UnionAll(T.var, tuple_type_tail(T.body))
+ elseif isa(T, Union)
+ return Union{tuple_type_tail(T.a), tuple_type_tail(T.b)}
+ else
+ T.name === Tuple.name || throw(MethodError(tuple_type_tail, (T,)))
+ if isvatuple(T) && length(T.parameters) == 1
+ return T
+ end
+ return Tuple{argtail(T.parameters...)...}
  end
- return Tuple{argtail(T.parameters...)...}
 end
 
 tuple_type_cons(::Type, ::Type{Union{}}) = Union{}