implement convert from NamedTuple to Tuple (#49879)

See the discussion from Discourse:


https://discourse.julialang.org/t/construct-tuple-containing-namedtuple-from-tuple

Author: baumgold

base/namedtuple.jl

@@ -182,6 +182,7 @@ nextind(@nospecialize(t::NamedTuple), i::Integer) = Int(i)+1
 
 convert(::Type{NT}, nt::NT) where {names, NT<:NamedTuple{names}} = nt
 convert(::Type{NT}, nt::NT) where {names, T<:Tuple, NT<:NamedTuple{names,T}} = nt
+convert(::Type{NT}, t::Tuple) where {NT<:NamedTuple} = NT(t)
 
 function convert(::Type{NamedTuple{names,T}}, nt::NamedTuple{names}) where {names,T<:Tuple}
     NamedTuple{names,T}(T(nt))::NamedTuple{names,T}

test/namedtuple.jl

@@ -94,6 +94,9 @@ end
 
     conv_res = @test_throws MethodError convert(NamedTuple{(:a,),Tuple{I}} where I<:AbstractString, (;a=1))
     @test conv_res.value.f === convert && conv_res.value.args === (AbstractString, 1)
+
+    conv6 = convert(NamedTuple{(:a,),Tuple{NamedTuple{(:b,), Tuple{Int}}}}, ((1,),))
+    @test conv6 === (a = (b = 1,),)
 end
 
 @test NamedTuple{(:a,:c)}((b=1,z=2,c=3,aa=4,a=5)) === (a=5, c=3)