From c9d8d219d6fa064b7b056ac3e3244bdb2c50be6a Mon Sep 17 00:00:00 2001
From: JamesWrigley <james@puiterwijk.org>
Date: Sun, 15 Mar 2026 12:33:10 +0100
Subject: [PATCH] Support different subtypes of AbstractDict in
 merge()/mergewith()

The merge functions internally use the _typeddict() helper function, which
returns a new Dict with the correct promoted key/value types. But the fact that
it always returns a Dict means that the default implementations of
merge()/mergewith() cannot be used for new AbstractDict subtypes that wish
those functions to return their own custom type.
---
 base/abstractdict.jl | 4 ++--
 test/dict.jl         | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/base/abstractdict.jl b/base/abstractdict.jl
index cdea331d49c51..d36c2dbf83eec 100644
--- a/base/abstractdict.jl
+++ b/base/abstractdict.jl
@@ -406,10 +406,10 @@ promoteK(K) = K
 promoteV(V) = V
 promoteK(K, d, ds...) = promoteK(promote_type(K, keytype(d)), ds...)
 promoteV(V, d, ds...) = promoteV(promote_type(V, valtype(d)), ds...)
-function _typeddict(d::AbstractDict, others::AbstractDict...)
+function _typeddict(d::T, others::AbstractDict...) where {T <: AbstractDict}
     K = promoteK(keytype(d), others...)
     V = promoteV(valtype(d), others...)
-    Dict{K,V}(d)
+    T.name.wrapper{K, V}(d)
 end
 
 """
diff --git a/test/dict.jl b/test/dict.jl
index a3f3ed0de5305..03e403f27a8c0 100644
--- a/test/dict.jl
+++ b/test/dict.jl
@@ -1349,8 +1349,8 @@ end
     @test d == IdDict(1 => 6, 2 => 5, 3 => 10)
     @inferred mergewith(+, d1, d2, d3)
     d = mergewith(+, d1, d2, d3)
-    @test d isa Dict{Int, Float64}
-    @test d == Dict(1 => 6, 2 => 5, 3 => 10)
+    @test d isa IdDict{Int, Float64}
+    @test d == IdDict(1 => 6, 2 => 5, 3 => 10)
 end
 
 @testset "misc error/io" begin