Add backslash method specialized for SparseMatrixCSCs and a test ch…

…ecking that that specialized method gets called, a simple if inelegant fix for #16548.

base/sparse/linalg.jl

@@ -856,6 +856,32 @@ end
 scale!(A::SparseMatrixCSC, b::Number) = (scale!(A.nzval, b); A)
 scale!(b::Number, A::SparseMatrixCSC) = (scale!(b, A.nzval); A)
 
+function (\)(A::SparseMatrixCSC, B::AbstractVecOrMat)
+    m, n = size(A)
+    if m == n
+        if istril(A)
+            if istriu(A)
+                return Diagonal(A) \ B
+            else
+                return LowerTriangular(A) \ B
+            end
+        elseif istriu(A)
+            return UpperTriangular(A) \ B
+        end
+        if ishermitian(A)
+            try
+                return cholfact(Hermitian(A)) \ B
+            catch e
+                isa(e, PosDefException) || rethrow(e)
+                return ldltfact(Hermitian(A)) \ B
+            end
+        end
+        return lufact(A) \ B
+    else
+        return qrfact(A) \ B
+    end
+end
+
 function factorize(A::SparseMatrixCSC)
     m, n = size(A)
     if m == n

test/sparsedir/sparse.jl

@@ -1385,3 +1385,6 @@ end
 @test !issparse([rand(10,10)  rand(10,10)])
 @test !issparse([rand(10,10); rand(10,10)])
 @test !issparse([rand(10,10)  rand(10,10); rand(10,10) rand(10,10)])
+
+# Test temporary fix for issue #16548 in PR #16979. Brittle. Expect to remove with `\` revisions.
+@test which(\, (SparseMatrixCSC, AbstractVecOrMat)).module == Base.SparseArrays