From a2e601db066a486140a00a4a732ea050bde2d185 Mon Sep 17 00:00:00 2001
From: Max Horn <max@quendi.de>
Date: Wed, 15 May 2024 22:36:36 +0200
Subject: [PATCH] Add base_ring_type for MatrixElem; fix it for MatSpace
 (#1705)

This is a useful "inverse" for `dense_matrix_type`.
---
 src/Matrix.jl               | 4 +++-
 test/generic/Matrix-test.jl | 6 ++++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/Matrix.jl b/src/Matrix.jl
index 960c2ad6e5..9f4761e7fd 100644
--- a/src/Matrix.jl
+++ b/src/Matrix.jl
@@ -10,10 +10,12 @@
 #
 ###############################################################################
 
-base_ring_type(::Type{MatSpace{T}}) where T <: NCRingElement = parent_type(T)
+base_ring_type(::Type{<:MatSpace{T}}) where T <: NCRingElement = parent_type(T)
 
 base_ring(a::MatSpace{T}) where {T <: NCRingElement} = a.base_ring::parent_type(T)
 
+base_ring_type(::Type{<:MatrixElem{T}}) where T <: NCRingElement = parent_type(T)
+
 base_ring(a::MatrixElem{T}) where {T <: NCRingElement} = a.base_ring::parent_type(T)
 
 function check_parent(a::MatElem, b::MatElem, throw::Bool = true)
diff --git a/test/generic/Matrix-test.jl b/test/generic/Matrix-test.jl
index 567a063c62..08a1c6dba7 100644
--- a/test/generic/Matrix-test.jl
+++ b/test/generic/Matrix-test.jl
@@ -122,6 +122,9 @@ end
    @test elem_type(S) == Generic.MatSpaceElem{elem_type(R)}
    @test elem_type(Generic.MatSpace{elem_type(R)}) == Generic.MatSpaceElem{elem_type(R)}
    @test parent_type(Generic.MatSpaceElem{elem_type(R)}) == Generic.MatSpace{elem_type(R)}
+   @test base_ring_type(Generic.MatSpace{elem_type(R)}) == typeof(R)
+   @test base_ring_type(Generic.MatSpaceElem{elem_type(R)}) == typeof(R)
+   @test base_ring_type(S) == typeof(R)
    @test base_ring(S) == R
    @test nrows(S) == 3
    @test ncols(S) == 3
@@ -132,6 +135,7 @@ end
 
    @test isa(f, MatElem)
    @test parent_type(f) == typeof(S)
+   @test base_ring_type(f) == typeof(R)
 
    g = S(2)
 
@@ -348,6 +352,8 @@ end
    @test dense_matrix_type(typeof(R)) == elem_type(S)
    @test dense_matrix_type(typeof(R(1))) == elem_type(S)
 
+   @test base_ring_type(dense_matrix_type(R)) == typeof(R)
+
    @test isa(S(), MatElem)
    @test isa(S(ZZ(1)), MatElem)
    @test isa(S(one(R)), MatElem)