add overflow/underflow failures to **

src/core/Exception.pm

@@ -2018,6 +2018,14 @@ my class X::Numeric::DivideByZero is Exception {
     }
 }
 
+my class X::Numeric::Overflow is Exception {
+    method message() { "Numeric overflow" }
+}
+
+my class X::Numeric::Underflow is Exception {
+    method message() { "Numeric underflow" }
+}
+
 my class X::Numeric::Confused is Exception {
     has $.num;
     has $.base;

src/core/Int.pm

@@ -258,10 +258,12 @@ multi sub infix:<%>(int $a, int $b) returns int {
 multi sub infix:<**>(Int:D \a, Int:D \b) {
     b >= 0 ?? nqp::pow_I(nqp::decont(a), nqp::decont(b), Num, Int)
            !! 1 / nqp::pow_I(nqp::decont(a), nqp::decont(-b), Num, Int)
+    or a == 0 ?? 0 !! fail X::Numeric::Overflow.new;
 }
 
 multi sub infix:<**>(int $a, int $b) returns int {
     nqp::pow_i($a, $b)
+        or $a == 0 ?? 0 !! fail X::Numeric::Overflow.new;
 }
 
 multi sub infix:<lcm>(Int:D \a, Int:D \b) returns Int {

src/core/Num.pm

@@ -350,11 +350,14 @@ multi sub infix:<%>(num $a, num $b) {
     nqp::mod_n($a, $b)
 }
 
+# (If we get 0 here, must be underflow, since floating overflow provides Inf.)
 multi sub infix:<**>(Num:D \a, Num:D \b) {
     nqp::p6box_n(nqp::pow_n(nqp::unbox_n(a), nqp::unbox_n(b)))
+        or a == 0e0 || b.abs == Inf ?? 0e0 !! fail X::Numeric::Underflow.new;
 }
 multi sub infix:<**>(num $a, num $b) {
     nqp::pow_n($a, $b)
+        or $a == 0e0 || $b.abs == Inf ?? 0e0 !! fail X::Numeric::Underflow.new;
 }
 
 # Here we sort NaN in with string "NaN"

src/core/Rat.pm

@@ -213,6 +213,7 @@ multi sub infix:<**>(Rational \a, Int \b) {
             a.numerator ** -b,
             a,
             b
+    or a == 0.0 ?? 0.0 !! fail X::Numeric::Overflow.new;  # can this happen?
 }
 
 multi sub infix:<==>(Rational:D \a, Rational:D \b) {

src/core/core_prologue.pm

@@ -6,6 +6,8 @@ my class WhateverCode { ... }
 my class Cursor { ... }
 my class Failure { ... }
 my class Rakudo::Internals { ... }
+my class X::Numeric::Overflow { ... }
+my class X::Numeric::Underflow { ... }
 
 # Stub these or we can't use any sigil other than $.
 my role Positional { ... }