Does the following fallacy have a name?
If A were not true then it would be impossible to determine whether B is true or not, therefore A is true.
It sounds like a pretty silly argument when written explicitly, but I've come across it in science from time to time. It sometimes happens that if one assumes a hypothesis (A) to be true, then one can make progress on some important unanswered question (B). This sometimes leads the hypothesis to be treated as a principle (i.e. a postulate that must be accepted without explanation). It can then become difficult to question A, because this also means questioning all the progress that has been made towards B under the assumption that it's true.
I note that this form also comes up quite often as a (perfectly valid!) step in solving logic puzzles. Its validity in this case comes from knowing the puzzle has a unique solution, so one can correctly conclude that it won't be left unknown. Please note though that this point about logic puzzles is just an observation - my question is about the situation in the paragraphs above, which are about the cases where we do not know the problem has a solution, so we cannot correctly make this inference.
I guess it's a form of appeal to consequences, but I'm wondering if it has a more specific name.
It also seems related to the streetlight effect, though it's perhaps even stronger.