constructing a tuple from an iterator may fail when it's not supposed to

[nsajko]

Constructing tuples is buggy because of how it depends on Base.tuple_type_tail:

julia> const A = Tuple{Val{T},T,T} where {T}
Tuple{Val{T}, T, T} where T

julia> const t = (Val(Any), 2, 2.0)
(Val{Any}(), 2, 2.0)

julia> const v = [Val(Any), 2, 2.0]
3-element Vector{Any}:
  Val{Any}()
 2
 2.0

julia> A(v)
ERROR: TypeError: in typeassert, expected Tuple{T, T} where T, got a value of type Tuple{Int64, Float64}
Stacktrace:
 [1] _totuple
   @ ./tuple.jl:474 [inlined]
 [2] _totuple
   @ ./tuple.jl:473 [inlined]
 [3] (Tuple{Val{T}, T, T} where T)(itr::Vector{Any})
   @ Base ./tuple.jl:455
 [4] top-level scope
   @ REPL[4]:1

julia> t isa A
true

julia> versioninfo()
Julia Version 1.11.0-DEV.1451
Commit f117a500ca9 (2024-02-01 15:38 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 8 × AMD Ryzen 3 5300U with Radeon Graphics
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver2)
Threads: 1 default, 0 interactive, 1 GC (on 8 virtual cores)

xref: #27736 (comment)

Working on a PR that should fix this by constructing tuples without relying on tuple_type_tail.

[vtjnash]

I have slowly tried to remove a lot of these calls in Base for that reason (in favor of fieldtypes, though fieldtypes can still suffer the same bug), so more work in that direction is probably good. We do have a predicate now (Base.widen_diagonal) to correct these types however as well in construction.