py: Implement raising a big-int to a negative power.

Before this patch raising a big-int to a negative power would just return
0.  Now it returns a floating-point number with the correct value.

py/mpz.c

@@ -946,10 +946,6 @@ bool mpz_is_pos(const mpz_t *z) {
     return z->len > 0 && z->neg == 0;
 }
 
-bool mpz_is_neg(const mpz_t *z) {
-    return z->len > 0 && z->neg != 0;
-}
-
 bool mpz_is_odd(const mpz_t *z) {
     return z->len > 0 && (z->dig[0] & 1) != 0;
 }

py/mpz.h

@@ -112,6 +112,7 @@ size_t mpz_set_from_str(mpz_t *z, const char *str, size_t len, bool neg, unsigne
 void mpz_set_from_bytes(mpz_t *z, bool big_endian, size_t len, const byte *buf);
 
 static inline bool mpz_is_zero(const mpz_t *z) { return z->len == 0; }
+static inline bool mpz_is_neg(const mpz_t *z) { return z->len != 0 && z->neg != 0; }
 int mpz_cmp(const mpz_t *lhs, const mpz_t *rhs);
 
 void mpz_abs_inpl(mpz_t *dest, const mpz_t *z);

py/objint_longlong.c

@@ -191,6 +191,13 @@ mp_obj_t mp_obj_int_binary_op(mp_uint_t op, mp_obj_t lhs_in, mp_obj_t rhs_in) {
 
         case MP_BINARY_OP_POWER:
         case MP_BINARY_OP_INPLACE_POWER: {
+            if (rhs_val < 0) {
+                #if MICROPY_PY_BUILTINS_FLOAT
+                return mp_obj_float_binary_op(op, lhs_val, rhs_in);
+                #else
+                mp_raise_ValueError("negative power with no float support");
+                #endif
+            }
             long long ans = 1;
             while (rhs_val > 0) {
                 if (rhs_val & 1) {

py/objint_mpz.c

@@ -290,6 +290,13 @@ mp_obj_t mp_obj_int_binary_op(mp_uint_t op, mp_obj_t lhs_in, mp_obj_t rhs_in) {
 
             case MP_BINARY_OP_POWER:
             case MP_BINARY_OP_INPLACE_POWER:
+                if (mpz_is_neg(zrhs)) {
+                    #if MICROPY_PY_BUILTINS_FLOAT
+                    return mp_obj_float_binary_op(op, mpz_as_float(zlhs), rhs_in);
+                    #else
+                    mp_raise_ValueError("negative power with no float support");
+                    #endif
+                }
                 mpz_pow_inpl(&res->mpz, zlhs, zrhs);
                 break;
 

tests/float/int_big_float.py

@@ -18,6 +18,10 @@ print("%.5g" % (i / 1.2))
 # this should delegate to complex
 print("%.5g" % (i * 1.2j).imag)
 
+# negative power should produce float
+print("%.5g" % (i ** -1))
+print("%.5g" % ((2 + i - i) ** -3))
+
 try:
     i / 0
 except ZeroDivisionError: