From 70b043a329f13738e55907d33c6eaa386e13a51c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Beno=C3=AEt=20Legat?= <benoit.legat@gmail.com>
Date: Wed, 14 Feb 2024 21:51:05 +0100
Subject: [PATCH] Fix promotion of outer product (#263)

* Fix promotion of outer product

* Update matmul.jl

---------

Co-authored-by: Oscar Dowson <odow@users.noreply.github.com>
---
 src/implementations/LinearAlgebra.jl |  4 ++--
 test/matmul.jl                       | 19 +++++++++++++++++++
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/src/implementations/LinearAlgebra.jl b/src/implementations/LinearAlgebra.jl
index d577e6c..00ff2b6 100644
--- a/src/implementations/LinearAlgebra.jl
+++ b/src/implementations/LinearAlgebra.jl
@@ -213,14 +213,14 @@ function promote_array_mul(
     ::Type{<:AbstractVector{S}},
     ::Type{<:LinearAlgebra.Adjoint{T,<:AbstractVector{T}}},
 ) where {S,T}
-    return Matrix{promote_sum_mul(S, T)}
+    return Matrix{promote_operation(*, S, T)}
 end
 
 function promote_array_mul(
     ::Type{<:AbstractVector{S}},
     ::Type{<:LinearAlgebra.Transpose{T,<:AbstractVector{T}}},
 ) where {S,T}
-    return Matrix{promote_sum_mul(S, T)}
+    return Matrix{promote_operation(*, S, T)}
 end
 
 function promote_array_mul(
diff --git a/test/matmul.jl b/test/matmul.jl
index 7392066..339b87e 100644
--- a/test/matmul.jl
+++ b/test/matmul.jl
@@ -403,3 +403,22 @@ end
     y = MA.@rewrite sum(A[i, :] * LinearAlgebra.transpose(x[i, :]) for i in 1:2)
     @test y == BigInt[10 14; 14 20]
 end
+
+struct Monomial end
+LinearAlgebra.transpose(m::Monomial) = m
+LinearAlgebra.adjoint(m::Monomial) = m
+MA.promote_operation(::typeof(*), ::Type{Monomial}, ::Type{Monomial}) = Monomial
+Base.:*(m::Monomial, ::Monomial) = m
+
+# `Monomial` does not implement `+`, we should check that it does not prevent
+# to do outer products of vectors
+@testset "Vector*Transpose{Vector}_issue_256 with Monomial" begin
+    m = Monomial()
+    a = [m, m]
+    for f in [LinearAlgebra.transpose, LinearAlgebra.adjoint]
+        b = f(a)
+        T = MA.promote_operation(*, typeof(a), typeof(b))
+        @test T == typeof(a * b)
+        @test T == typeof(MA.operate(*, a, b))
+    end
+end