From eb959193a50e5c28faff5cd1cd55b4cc86ca203b Mon Sep 17 00:00:00 2001
From: Tim Holy <tim.holy@gmail.com>
Date: Sat, 13 Aug 2016 06:19:17 -0500
Subject: [PATCH] Provide a more useful error message for calling map on a Dict
 with a non-Pair return type

---
 base/dict.jl | 3 +++
 test/dict.jl | 5 +++++
 2 files changed, 8 insertions(+)

diff --git a/base/dict.jl b/base/dict.jl
index d66b93377c7f8..69c41ece8d8cc 100644
--- a/base/dict.jl
+++ b/base/dict.jl
@@ -848,3 +848,6 @@ function similar(t::ImmutableDict)
     end
     return t
 end
+
+_similar_for{P<:Pair}(c::Dict, ::Type{P}, itr, isz) = similar(c, P)
+_similar_for(c::Associative, T, itr, isz) = throw(ArgumentError("for Associatives, similar requires an element type of Pair;\n  if calling map, consider a comprehension instead"))
diff --git a/test/dict.jl b/test/dict.jl
index 45dcbbe998c8d..0af6b4d3807d7 100644
--- a/test/dict.jl
+++ b/test/dict.jl
@@ -388,6 +388,11 @@ d = Dict('a'=>1, 'b'=>1, 'c'=> 3)
 @test [d[k] for k in keys(d)] == [d[k] for k in eachindex(d)] ==
       [v for (k, v) in d] == [d[x[1]] for (i, x) in enumerate(d)]
 
+# generators, similar
+d = Dict(:a=>"a")
+@test @inferred(map(identity, d)) == d
+@test @inferred(map(p->p.first=>p.second[1], d)) == Dict(:a=>'a')
+@test_throws ArgumentError map(p->p.second, d)
 
 # Issue 12451
 @test_throws ArgumentError Dict(0)