Base: typeassert displaysize(::IOContext) return value

Improve abstract type inference. This is OK because `displaysize` is
documented to always return a `Tuple{Int, Int}` value.

Also add tests.

Author: nsajko

base/show.jl

@@ -442,7 +442,7 @@ 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)
+displaysize(io::IOContext) = haskey(io, :displaysize) ? io[:displaysize]::Tuple{Int,Int} : displaysize(io.io)::Tuple{Int,Int}
 
 show_circular(io::IO, @nospecialize(x)) = false
 function show_circular(io::IOContext, @nospecialize(x))

test/show.jl

@@ -1708,6 +1708,13 @@ end
         "[3.141592653589793 3.141592653589793; 3.141592653589793 3.141592653589793]"
 end
 
+@testset "`displaysize` return type inference" begin
+    @test Tuple{Int, Int} === Base.infer_return_type(displaysize, Tuple{})
+    @test Tuple{Int, Int} === Base.infer_return_type(displaysize, Tuple{IO})
+    @test Tuple{Int, Int} === Base.infer_return_type(displaysize, Tuple{IOContext})
+    @test Tuple{Int, Int} === Base.infer_return_type(displaysize, Tuple{Base.TTY})
+end
+
 @testset "Array printing with limited rows" begin
     arrstr = let buf = IOBuffer()
         function (A, rows)