From 709f421e8a39c690434db8a44f10e1b4a55f5bf1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Wed, 9 Oct 2024 23:31:09 +0200
Subject: [PATCH] Some MatrixRing fixes (#1852)

* Error on MatrixRing of size -1
* Fix characteristic of `matrix_ring(R, 0)`
---
 src/MatRing.jl               | 1 +
 src/generic/MatRing.jl       | 1 +
 test/generic/MatRing-test.jl | 4 ++++
 3 files changed, 6 insertions(+)

diff --git a/src/MatRing.jl b/src/MatRing.jl
index a0a1fab9b6..f778d255f7 100644
--- a/src/MatRing.jl
+++ b/src/MatRing.jl
@@ -66,6 +66,7 @@ end
 
 
 function characteristic(a::MatRing)
+   iszero(a.n) && return 1
    return characteristic(base_ring(a))
 end
 
diff --git a/src/generic/MatRing.jl b/src/generic/MatRing.jl
index 23c18760ee..4273b04a71 100644
--- a/src/generic/MatRing.jl
+++ b/src/generic/MatRing.jl
@@ -211,6 +211,7 @@ end
 function matrix_ring(R::AbstractAlgebra.NCRing, n::Int; cached::Bool = true)
    # TODO: the 'cached' argument is ignored and mainly here for backwards compatibility
    # (and perhaps future compatibility, in case we need it again)
+   @req n >= 0 "n must be a non-negative integer"
    T = elem_type(R)
    return MatRing{T}(R, n)
 end
diff --git a/test/generic/MatRing-test.jl b/test/generic/MatRing-test.jl
index cc726de152..e8bbe18c6f 100644
--- a/test/generic/MatRing-test.jl
+++ b/test/generic/MatRing-test.jl
@@ -1,5 +1,7 @@
 @testset "Generic.MatAlg.constructors" begin
    R, t = polynomial_ring(QQ, "t")
+   @test_throws ArgumentError matrix_ring(R, -1)
+
    S = matrix_ring(R, 3)
 
    @test S === matrix_ring(R, 3)
@@ -177,6 +179,8 @@ end
    @test is_zero_column(M, 1)
 
    @test degree(M) == 2
+
+   @test characteristic(matrix_ring(R, 0)) == 1
 end
 
 @testset "Generic.MatAlg.size/axes" begin