Pair API: allow any type of Pair

Do not restrict the first argument to be a `DataType`/`UnionAll`, as
`make` is about to start accepting non-type objects as first argument.

Author: rfourquet

src/sampling.jl

@@ -574,12 +574,10 @@ pair_to_make((a, b)::Pair) =
 
 pair_to_make(x) = x
 
-@inline Sampler(RNG::Type{<:AbstractRNG}, p::Pair{<:Union{DataType,UnionAll}},
-                r::Repetition) =
-                    Sampler(RNG, pair_to_make(p), r)
+@inline Sampler(RNG::Type{<:AbstractRNG}, p::Pair, r::Repetition) =
+    Sampler(RNG, pair_to_make(p), r)
 
 # nothing can be inferred when only the pair type is available
-@inline gentype(::Type{<:Pair{<:Union{DataType,UnionAll}}}) = Any
+@inline gentype(::Type{<:Pair}) = Any
 
-@inline gentype(p::Pair{<:Union{DataType,UnionAll}}) =
-    gentype(pair_to_make(p))
+@inline gentype(p::Pair) = gentype(pair_to_make(p))

test/runtests.jl

@@ -570,7 +570,7 @@ end
 @testset "rand(T => x) & rand(T => (x, y, ...))" begin
     @test rand(Complex => Int) isa Complex{Int}
     @test rand(Pair => (String, Int8)) isa Pair{String,Int8}
-    @test_throws ArgumentError rand(1=>2) # should not call rand(make(1, 2))
+    @test_throws MethodError rand(1=>2) # calls rand(make(1, 2))
 
     @test rand(Complex => Int, 3) isa Vector{Complex{Int}}
     @test rand(Pair => (String, Int8), Set, 3) isa Set{Pair{String,Int8}}