Fix bug in and rename throw_nil, warn_nil.

In warn_nil, the return value was undefined on exception -- I think.

While informative, the names throw_nil and warn_nil don't work very
well in conditionals:

  if (warn_nil<f>(...)) g();

sounds like g should be called if f returns nil and a warning is
issued. But it is actually the other way around; g is called if f is
successful.

  if (prefer<f>(...)) g();

sounds less like that.

1 files changed, 4 insertions, 5 deletions

diff --git a/c/common.h b/c/common.h
index a4f73e5..6b29ed5 100644
--- a/c/common.h
+++ b/c/common.h
@@ -55,7 +55,7 @@ std::optional<T> maybe_make(U... xs)
 
 /* Call Windows API function, throwing error on NULL. */
 template <auto F, typename... T>
-inline auto throw_nil(T... xs)
+inline auto require(T... xs)
 {
 	auto r = F(xs...);
 	if (!r) throw Win32Error(GetLastError());
@@ -64,15 +64,14 @@ inline auto throw_nil(T... xs)
 
 /* Call Windows API function, showing a warning on NULL. */
 template <auto F, typename... T>
-inline auto warn_nil(T... xs)
+inline auto prefer(T... xs)
 {
-	decltype(F(std::declval<T>()...)) r;
 	try {
-		r = throw_nil<F>(xs...);
+		return require<F>(xs...);
 	} catch (Win32Error& e) {
 		EBMessageBox(e.twhat(), TEXT("System Error"), MB_ICONWARNING);
+		return (decltype(F(std::declval<T>()...)))NULL;
 	}
-	return r;
 }
 
 /* Return integer scaled for current DPI. */