Reject zero substitutions in Laurent denominators

M2/Macaulay2/e/ringmap.cpp

@@ -5,6 +5,7 @@
 #include "matrix-con.hpp"
 #include "polyring.hpp"
 #include "relem.hpp"
+#include "exceptions.hpp"
 
 #include <iostream>
 RingMap::RingMap(const Matrix *m) : R(m->get_ring())
@@ -147,7 +148,10 @@ ring_elem RingMap::eval_term(const Ring *sourceK,  // source coeff ring
     {
       int v = first_var + i.var();
       if (v >= nvars || _elem[v].is_zero)
-        return R->from_long(0);  // The result is zero.
+        {
+          if (i.exponent() < 0) throw exc::division_by_zero_error();
+          return R->from_long(0);  // The result is zero.
+        }
     }
 
   // If K is a coeff ring of R, AND map is an identity on K,

M2/Macaulay2/tests/normal/subst.m2

@@ -4,6 +4,13 @@ I = matrix{{x^2,y^2}}
 P=matrix({{2,3,4}})
 substitute(f,P)
 substitute(I,P)
+
+S = QQ[t,Inverses=>true,MonomialOrder=>Lex]
+g = matrix{{t^-1,1/2}}
+assert(try (sub(g,t=>0); false) else true)
+assert(sub(matrix{{t,1/2}}, t=>0) == matrix{{0_QQ,1/2}})
+assert(sub(t^-1, t=>2) == 1/2)
+
 end
 -- Local Variables:
 -- compile-command: "make -C $M2BUILDDIR/Macaulay2/packages/Macaulay2Doc/test subst.out"