> Did anyone like my previous idea of having a way of telling the compiler that we want template instantiation to fail?

At the risk of stating the obvious, the difficulty there is that such an idea is a sledgehammer which could be a major change to template programming; and such an idea needs serious, strong motivation which goes far beyond being a workaround to make a different proposed feature's edge cases work. Particularly as at least a few of the potential use-cases for it can already be achieved in the language as it is now.