Inverse of a sparse/diagonal singular matrix should be a sparse/diagonal matrix of Infs. * liboctave/array/CSparse.cc(inverse): if the matrix is null, return an error. If it is singular, return a sparse matrix of Infs. * liboctave/array/dSparse.cc(inverse): if the matrix is null, return an error. If it is singular, return a sparse matrix of Infs. * liboctave/array/CDiagMatrix.cc(inverse): if the matrix is null, return an error. If it is singular, return a diagonal matrix of Infs. * liboctave/array/dDiagMatrix.cc(inverse): if the matrix is null, return an error. If it is singular, return a diagonal matrix of Infs. * libinterp/corefcn/inv.cc: set rcond to zero for singular diagonal matrices. Add tests for the above cases. * test/diag-perm.tst: modify a test for the inversion of a singular diagonal matrix.

--- a/test/diag-perm.tst	Sat May 18 13:18:00 2019 +0200
+++ b/test/diag-perm.tst	Tue May 14 17:00:52 2019 +0200
@@ -265,11 +265,12 @@
 %! assert (full (D - A), D - full (A));
 
 ## inverse preserves diagonal structure even for singular matrices (bug #46103)
+## but set all the diagonal elements to Inf (bug #56232)
 %!test
 %! x = diag (1:3);
 %! assert (inv (x), diag ([1 1/2 1/3]));
-%! x = diag (0:2);
-%! assert (inv (x), diag ([Inf 1 1/2]));
+%!warning <matrix singular> A = inv (diag (0:2));
+%! assert (A, diag ([Inf Inf Inf]));
 
 ## assignment to diagonal elements preserves diagonal structure (bug #36932)
 %!test