Fix displaying some UnionAll types

Author: JamesWrigley

docs/src/_changelog.md

@@ -9,6 +9,9 @@ Changelog](https://keepachangelog.com).
 
 ## Unreleased
 
+### Fixed
+- Fixed the display of `UnionAll` types such as `Pair.body` ([#1203]).
+
 ### Changed
 - Replaced JSON.jl with a vendored copy of
   [JSONX](https://github.com/JuliaIO/JSON.jl/tree/master/vendor) ([#1200]). This

src/display.jl

@@ -140,7 +140,10 @@ display_dict(x) = _display_dict(x)
 
 # remove x from the display queue
 function undisplay(x, kernel)
-    i = findfirst(isequal(x), kernel.displayqueue)
+    # Note that we intentionally create an anonymous function as a comparator
+    # instead of using the Base.Fix version of `isequal(x)`. This avoids an
+    # edge-case with Type's: https://github.com/JuliaLang/IJulia.jl/issues/1098
+    i = findfirst(y -> isequal(x, y), kernel.displayqueue)
     i !== nothing && splice!(kernel.displayqueue, i)
     return x
 end

test/kernel.jl

@@ -332,6 +332,10 @@ end
                 @test msg_ok(execute(client, "ans == 42"))
                 @test kernel.ans
 
+                # Test an edge-case with displaying Type's:
+                # https://github.com/JuliaLang/IJulia.jl/issues/1098
+                @test msg_ok(execute(client, "Pair.body"))
+
                 # Test shutdown_request
                 @test msg_ok(shutdown(client))
                 @test timedwait(() -> shutdown_called, 10) == :ok