Support isassigned(::RefValue)

Author: omus

src/Compat.jl

@@ -1423,6 +1423,11 @@ if VERSION < v"0.6.0-dev.2283"
     end
 end
 
+# https://github.com/JuliaLang/julia/pull/18082
+if VERSION < v"0.6.0-dev.2347"
+    Base.isassigned(x::Base.RefValue) = isdefined(x, :x)
+end
+
 include("to-be-deprecated.jl")
 
 end # module Compat

test/runtests.jl

@@ -1768,4 +1768,8 @@ end
 @test Compat.readline(IOBuffer("x\n"), chomp=true) == "x"
 @test Compat.readline(IOBuffer("x\n"), chomp=false) == "x\n"
 
+# PR 18082
+@test !isassigned(Ref{String}())
+@test isassigned(Ref{String}("Test"))
+
 include("to-be-deprecated.jl")