implement fieldnames reflection for Tuples (#16540)

fix #14858

Author: vtjnash

base/reflection.jl

@@ -57,6 +57,7 @@ end
 Get the name of field `i` of a `DataType`.
 """
 fieldname(t::DataType, i::Integer) = t.name.names[i]::Symbol
+fieldname{T<:Tuple}(t::Type{T}, i::Integer) = i < 1 || i > nfields(t) ? throw(BoundsError(t, i)) : Int(i)
 
 """
     fieldnames(x::DataType)
@@ -71,6 +72,7 @@ function fieldnames(v)
     return fieldnames(t)
 end
 fieldnames(t::DataType) = Symbol[fieldname(t, n) for n in 1:nfields(t)]
+fieldnames{T<:Tuple}(t::Type{T}) = Int[n for n in 1:nfields(t)]
 
 isconst(s::Symbol) = ccall(:jl_is_const, Int32, (Ptr{Void}, Any), C_NULL, s) != 0
 

test/reflection.jl

@@ -251,6 +251,10 @@ tlayout = TLayout(5,7,11)
 @test_throws BoundsError fieldname(TLayout, 4)
 @test_throws BoundsError fieldoffset(TLayout, 4)
 
+@test fieldnames((1,2,3)) == fieldnames(NTuple{3, Int}) == [fieldname(NTuple{3, Int}, i) for i = 1:3] == [1, 2, 3]
+@test_throws BoundsError fieldname(NTuple{3, Int}, 0)
+@test_throws BoundsError fieldname(NTuple{3, Int}, 4)
+
 import Base: isstructtype, type_alignment, return_types
 @test !isstructtype(Union{})
 @test !isstructtype(Union{Int,Float64})