fix JuliaLang#37701: support keys(::IOContext) (JuliaLang#37753)

NEWS.md

@@ -114,6 +114,8 @@ New library features
 * New `append!(vector, collections...)` and `prepend!(vector, collections...)` methods accept multiple
  collections to be appended or prepended ([#36227]).
 * The postfix operator `'ᵀ` can now be used as an alias for `transpose` ([#38043]).
+* `keys(io::IO)` has been added, which returns all keys of `io` if `io` is an `IOContext` and an empty
+ `Base.KeySet` otherwise ([#37753]).
 
 Standard library changes
 ------------------------

base/show.jl

@@ -341,6 +341,8 @@ getindex(io::IOContext, key) = getindex(io.dict, key)
 getindex(io::IO, key) = throw(KeyError(key))
 get(io::IOContext, key, default) = get(io.dict, key, default)
 get(io::IO, key, default) = default
+keys(io::IOContext) = keys(io.dict)
+keys(io::IO) = keys(ImmutableDict{Symbol,Any}())
 
 displaysize(io::IOContext) = haskey(io, :displaysize) ? io[:displaysize]::Tuple{Int,Int} : displaysize(io.io)
 

test/show.jl

@@ -20,6 +20,9 @@ showstr(x, kv::Pair...) = sprint((io,x) -> show(IOContext(io, :limit => true, :d
  @test ioc.io == io
  @test ioc.dict == Base.ImmutableDict(Base.ImmutableDict{Symbol, Any}(:x, 1),
  :y => 2)
+ @test Base.ImmutableDict((key => ioc[key] for key in keys(ioc))...) == ioc.dict
+ @test keys(IOBuffer()) isa Base.KeySet
+ @test length(keys(IOBuffer())) == 0
 end
 
 @test replstr(Array{Any}(undef, 2)) == "2-element Vector{Any}:\n #undef\n #undef"