fix bug when first field contains "=>", and add tests (tkelman)

base/show.jl

@@ -273,11 +273,12 @@ print(io::IO, n::Unsigned) = print(io, dec(n))
 
 show(io::IO, p::Ptr) = print(io, typeof(p), " @0x$(hex(UInt(p), Sys.WORD_SIZE>>2))")
 
+show_pair_with_arrow(p::Pair) =
+    typeof(p.first)  == typeof(p).parameters[1] &&
+    typeof(p.second) == typeof(p).parameters[2]
+
 function show(io::IO, p::Pair)
-    if typeof(p.first) != typeof(p).parameters[1] ||
-       typeof(p.second) != typeof(p).parameters[2]
-        return show_default(io, p)
-    end
+    show_pair_with_arrow(p) || return show_default(io, p)
 
     isa(p.first,Pair) && print(io, "(")
     show(io, p.first)
@@ -1373,9 +1374,13 @@ function alignment(io::IO, x::Rational)
 end
 
 function alignment(io::IO, x::Pair)
-    m = match(r"^(.*?=)(>.*)$", sprint(0, show, x, env=io))
-    m === nothing ? (length(sprint(0, show, x, env=io)), 0) :
-                    (length(m.captures[1]), length(m.captures[2]))
+    s = sprint(0, show, x, env=io)
+    if show_pair_with_arrow(x)
+        left = length(sprint(0, show, x.first, env=io)) + 2isa(x.first, Pair) # 2 for parens
+        (left+1, length(s)-left-1) # +1 for the "=" part of "=>"
+    else
+        (0, length(s)) # as for x::Any
+    end
 end
 
 const undef_ref_str = "#undef"

test/show.jl

@@ -818,3 +818,15 @@ end
     @test sprint(show, Val(:Float64)) == "Val{:Float64}()" # Val of a symbol
     @test sprint(show, Val(true))     == "Val{true}()"     # Val of a value
 end
+
+@testset "alignment for pairs" begin  # (#22899)
+    @test replstr([1=>22,33=>4]) == "2-element Array{Pair{Int64,Int64},1}:\n  1=>22\n 33=>4 "
+    # first field may have "=>" in its representation
+    @test replstr(Pair[(1=>2)=>3, 4=>5]) ==
+        "2-element Array{Pair,1}:\n (1=>2)=>3\n      4=>5"
+    @test replstr(Any[Dict(1=>2)=> (3=>4), 1=>2]) ==
+        "2-element Array{Any,1}:\n Dict(1=>2)=>(3=>4)\n          1=>2     "
+    # left-alignment when not using the "=>" symbol
+    @test replstr(Pair{Integer,Int}[1=>2, 33=>4]) ==
+        "2-element Array{Pair{Integer,Int64},1}:\n Pair{Integer,Int64}(1, 2) \n Pair{Integer,Int64}(33, 4)"
+end