Re: [C++ PATCH] Detect UB in shifts in constexpr functions

[Jason Merrill]

Please give diagnostics explaining what's wrong with the shift rather 
than the generic "is not a constant expression".

+      tree t = build_int_cst (unsigned_type_node, uprec - 1);
+      t = fold_build2 (MINUS_EXPR, unsigned_type_node, t, rhs);
+      tree ulhs = fold_convert (unsigned_type_for (lhstype), lhs);
+      t = fold_build2 (RSHIFT_EXPR, TREE_TYPE (ulhs), ulhs, t);
+      if (tree_int_cst_lt (integer_one_node, t))

This could also use a comment explaining the logic.

Jason