show: consolidate wheres with {} in printing

Always a bit more compact in this form, and somewhat easier to implement
too (thus keeping this consistent with the corrected typealias printing).

(cherry picked from commit ee816ef)

Author: vtjnash

base/show.jl

@@ -628,7 +628,7 @@ end
 
 function make_wheres(io::IO, env::SimpleVector, @nospecialize(x::Type))
     seen = IdSet()
-    wheres = []
+    wheres = TypeVar[]
     # record things printed by the context
     if io isa IOContext
         for (key, val) in io.dict
@@ -827,22 +827,30 @@ function show(io::IO, @nospecialize(x::Type))
     end
 
     x = x::UnionAll
-    if x.var.name === :_ || io_has_tvar_name(io, x.var.name, x)
-        counter = 1
-        while true
-            newname = Symbol(x.var.name, counter)
-            if !io_has_tvar_name(io, newname, x)
-                newtv = TypeVar(newname, x.var.lb, x.var.ub)
-                x = UnionAll(newtv, x{newtv})
-                break
+    wheres = TypeVar[]
+    let io = IOContext(io)
+        while x isa UnionAll
+            var = x.var
+            if var.name === :_ || io_has_tvar_name(io, var.name, x)
+                counter = 1
+                while true
+                    newname = Symbol(var.name, counter)
+                    if !io_has_tvar_name(io, newname, x)
+                        var = TypeVar(newname, var.lb, var.ub)
+                        x = x{var}
+                        break
+                    end
+                    counter += 1
+                end
+            else
+                x = x.body
             end
-            counter += 1
+            push!(wheres, var)
+            io = IOContext(io, :unionall_env => var)
         end
+        show(io, x)
     end
-
-    show(IOContext(io, :unionall_env => x.var), x.body)
-    print(io, " where ")
-    show(io, x.var)
+    show_wheres(io, wheres)
 end
 
 # Check whether 'sym' (defined in module 'parent') is visible from module 'from'

test/show.jl

@@ -634,7 +634,7 @@ end
 # `where` syntax
 @test_repr "A where T<:B"
 @test_repr "A where T<:(Array{T} where T<:Real)"
-@test_repr "Array{T} where T<:Array{S} where S<:Real"
+@test_repr "Array{T} where {S<:Real, T<:Array{S}}"
 @test_repr "x::Array{T} where T"
 @test_repr "(a::b) where T"
 @test_repr "a::b where T"
@@ -1568,12 +1568,12 @@ end
 end
 
 let x = TypeVar(:_), y = TypeVar(:_)
-    @test repr(UnionAll(x, UnionAll(y, Pair{x,y}))) == "Pair{_1, _2} where _2 where _1"
+    @test repr(UnionAll(x, UnionAll(y, Pair{x,y}))) == "Pair{_1, _2} where {_1, _2}"
     @test repr(UnionAll(x, UnionAll(y, Pair{UnionAll(x,Ref{x}),y}))) == "Pair{Ref{_1} where _1, _1} where _1"
     x = TypeVar(:a)
     y = TypeVar(:a)
     z = TypeVar(:a)
-    @test repr(UnionAll(z, UnionAll(x, UnionAll(y, Tuple{x,y,z})))) == "Tuple{a1, a2, a} where a2 where a1 where a"
+    @test repr(UnionAll(z, UnionAll(x, UnionAll(y, Tuple{x,y,z})))) == "Tuple{a1, a2, a} where {a, a1, a2}"
 end
 
 @testset "showarg" begin