Assert type for displaysize

Author: timholy

base/show.jl

@@ -340,7 +340,7 @@ getindex(io::IO, key) = throw(KeyError(key))
 get(io::IOContext, key, default) = get(io.dict, key, default)
 get(io::IO, key, default) = default
 
-displaysize(io::IOContext) = haskey(io, :displaysize) ? io[:displaysize] : displaysize(io.io)
+displaysize(io::IOContext) = haskey(io, :displaysize) ? io[:displaysize]::Tuple{Int,Int} : displaysize(io.io)
 
 show_circular(io::IO, @nospecialize(x)) = false
 function show_circular(io::IOContext, @nospecialize(x))