Make Broadcast.result_style work on styles with fields. (#50938)

Fixes #50937.

---------

Co-authored-by: Jameson Nash <vtjnash@gmail.com>

Author: tpapp

base/broadcast.jl

@@ -441,7 +441,9 @@ Base.Broadcast.DefaultArrayStyle{1}()
 function result_style end
 
 result_style(s::BroadcastStyle) = s
-result_style(s1::S, s2::S) where S<:BroadcastStyle = S()
+function result_style(s1::S, s2::S) where S<:BroadcastStyle
+    s1 ≡ s2 ? s1 : error("inconsistent broadcast styles, custom rule needed")
+end
 # Test both orders so users typically only have to declare one order
 result_style(s1, s2) = result_join(s1, s2, BroadcastStyle(s1, s2), BroadcastStyle(s2, s1))
 
@@ -457,7 +459,8 @@ result_join(::Any, ::Any, s::BroadcastStyle, ::Unknown) = s
 result_join(::AbstractArrayStyle, ::AbstractArrayStyle, ::Unknown, ::Unknown) =
     ArrayConflict()
 # Fallbacks in case users define `rule` for both argument-orders (not recommended)
-result_join(::Any, ::Any, ::S, ::S) where S<:BroadcastStyle = S()
+result_join(::Any, ::Any, s1::S, s2::S) where S<:BroadcastStyle = result_style(s1, s2)
+
 @noinline function result_join(::S, ::T, ::U, ::V) where {S,T,U,V}
     error("""
 conflicting broadcast rules defined

test/broadcast.jl

@@ -1142,6 +1142,19 @@ end
     @test CartesianIndex(1,2) .+ [CartesianIndex(3,4), CartesianIndex(5,6)] == [CartesianIndex(4, 6), CartesianIndex(6, 8)]
 end
 
+struct MyBroadcastStyleWithField <: Broadcast.BroadcastStyle
+    i::Int
+end
+# asymmetry intended
+Base.BroadcastStyle(a::MyBroadcastStyleWithField, b::MyBroadcastStyleWithField) = a
+
+@testset "issue #50937: styles that have fields" begin
+    @test Broadcast.result_style(MyBroadcastStyleWithField(1), MyBroadcastStyleWithField(1)) ==
+        MyBroadcastStyleWithField(1)
+    @test_throws ErrorException Broadcast.result_style(MyBroadcastStyleWithField(1),
+                                                       MyBroadcastStyleWithField(2))
+end
+
 # test that `Broadcast` definition is defined as total and eligible for concrete evaluation
 import Base.Broadcast: BroadcastStyle, DefaultArrayStyle
 @test Base.infer_effects(BroadcastStyle, (DefaultArrayStyle{1},DefaultArrayStyle{2},)) |>