From a887fabefc9d5c01374d16e38001b9595da2f361 Mon Sep 17 00:00:00 2001
From: Jeff Bezanson <jeff.bezanson@gmail.com>
Date: Thu, 10 May 2018 17:04:17 -0400
Subject: [PATCH] don't allow promoting `Some{T}` and `Some{S}`

add promotion rule for `Union{Nothing, Missing, T}`

fixes the bug part of #26927
---
 base/missing.jl | 2 ++
 base/some.jl    | 2 +-
 test/some.jl    | 8 +++++++-
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/base/missing.jl b/base/missing.jl
index 6fb7556aa9af49..dd6ec8d8f0a7a2 100644
--- a/base/missing.jl
+++ b/base/missing.jl
@@ -35,6 +35,8 @@ end
 promote_rule(::Type{Union{Nothing, Missing}}, ::Type{Any}) = Any
 promote_rule(::Type{Union{Nothing, Missing}}, ::Type{T}) where {T} =
     Union{Nothing, Missing, T}
+promote_rule(::Type{Union{Nothing, Missing, S}}, ::Type{T}) where {T,S} =
+    Union{Nothing, Missing, promote_type(T, S)}
 
 convert(::Type{Union{T, Missing}}, x) where {T} = convert(T, x)
 # To fix ambiguities
diff --git a/base/some.jl b/base/some.jl
index f75a493d91703d..851ee2a53f2544 100644
--- a/base/some.jl
+++ b/base/some.jl
@@ -12,7 +12,7 @@ struct Some{T}
     value::T
 end
 
-promote_rule(::Type{Some{S}}, ::Type{Some{T}}) where {S,T} = Some{promote_type(S, T)}
+promote_rule(::Type{Some{T}}, ::Type{Some{S}}) where {T, S<:T} = Some{T}
 promote_rule(::Type{Some{T}}, ::Type{Nothing}) where {T} = Union{Some{T}, Nothing}
 
 convert(::Type{Some{T}}, x::Some) where {T} = Some{T}(convert(T, x.value))
diff --git a/test/some.jl b/test/some.jl
index a4d37d8df03892..4f9f407a1728b5 100644
--- a/test/some.jl
+++ b/test/some.jl
@@ -2,7 +2,9 @@
 
 ## promote()
 
-@test promote_type(Some{Int}, Some{Float64}) === Some{Float64}
+@test promote_type(Some{Int}, Some{Float64}) == Some
+@test promote_type(Some{Int}, Some{Real}) == Some{Real}
+@test promote_type(Some{Int}, Nothing) == Union{Some{Int},Nothing}
 
 ## convert()
 
@@ -75,6 +77,10 @@ for v in (nothing, missing)
     @test coalesce(v, nothing) === nothing
     @test coalesce(v, missing, v) === v
     @test coalesce(v, nothing, v) === v
+
+    # issue #26927
+    @test all(coalesce.([missing, nothing, Some(nothing), Some(missing)], "replacement") .===
+              [ "replacement", "replacement", nothing, missing ])
 end
 
 # notnothing()