Try to work around constexpr limits

src/lib/math/pcurves/pcurves_impl/pcurves_impl.h

@@ -530,6 +530,13 @@ class AffineCurvePoint {
       FieldElement m_y;
 };
 
+template <typename FE>
+const FE& inv_2() {
+   // TODO could be a constexpr, but some compilers get cranky
+   static const FE INV_2 = FE::from_word(2).invert();
+   return INV_2;
+}
+
 template <typename FieldElement, typename Params>
 class ProjectiveCurvePoint {
    public:
@@ -538,8 +545,6 @@ class ProjectiveCurvePoint {
       // recreate it here from the words.
       static constexpr FieldElement A = FieldElement::from_words(Params::AW);
 
-      static constexpr FieldElement INV_2 = FieldElement::from_word(2).invert();
-
       static constexpr bool A_is_zero = A.is_zero().as_bool();
       static constexpr bool A_is_minus_3 = (A == FieldElement::constant(-3)).as_bool();
 
@@ -733,7 +738,7 @@ class ProjectiveCurvePoint {
                   w *= ny4;
                }
             }
-            ny *= Self::INV_2;
+            ny *= inv_2<FieldElement>();
             return Self(nx, ny, nz);
          } else {
             Self pt = (*this);