also make non-default constructible colors unignorable

Author: Tsche

include/chroma

@@ -5,6 +5,7 @@
 #include <meta>
 #include <ranges>
 #include <string_view>
+#include <type_traits>
 #include <vector>
 
 namespace chroma {
@@ -79,7 +80,9 @@ template <typename T>
 struct callable_from;
 
 template <typename Color>
-concept ignorable = not requires { Color::ignorable; } || Color::ignorable;
+concept ignorable =
+    (not requires { Color::ignorable; } and std::is_default_constructible_v<Color>) or
+    Color::ignorable;
 
 enum struct viability { accepted, rejected, ignored };
 

test/unignorable.pass.cpp

@@ -14,4 +14,21 @@ void fnc1(callable_from<A> = {}) {}
 void test() {
   auto a = A();
   fnc1(a.make_context());
+}
+
+struct B {
+  static constexpr auto error_message = "not in B";
+
+  B() = delete;
+  explicit B(int) {}
+  operator callable_from<B>() const {
+    return chroma::color<B>();
+  }
+};
+
+void fnc2(callable_from<B> = {}) {}
+
+void test2() {
+  auto b = B(123);
+  fnc2(b);
 }