>But the before-test is strictly negative

I actually did this the other day on a piece of code. I was feeling a bit lazy. I didn't write the test and I figured that making the type checker catch it was enough. I still didn't write a test after either though.

Anecdotally I've always found that tests which cover real life bugs are in the class of test with the highest chance of catching future regressions. So even if it does exist, I'm still mildly skeptical of the idea that tests that catch bugs that compilers have been provoked into also catching are strictly negative.

>And yet I see TDD practitioners as the primary source of such tests

I find the precise opposite to be true. TDD are more likely to tie requirements to tests because they write them directly after getting requirements. Test-after practitioners more likely tie implementation to the test.

It's always possible to write a shit implementation-tied test with TDD, but the person who writes a shit test with TDD will write a shit implementation tied test after too. What did TDD have to do with that? Nothing.

>if you are dogmatically writing a test for every intermediate change, you will end up with lots of extra tests that assert things in order to satisfy the TDD dogma rather than the specific needs of the problem.

I find that this only really happens when you practice TDD with very loose typing. If you practice strict typing, the tests will invariably be narrowed down to ones which address the specific needs of the problem.

Again - without TDD and writing the test after, loose typing is still a shit show. So, I see this as another issue which is about something separate to TDD.

>Obviously this can be avoided with judgement - but if you have sound independent judgement you don't need to adhere to specific philosophies

I think this is conflating "TDD is a panacea" with "if it is valuable to write a test, it's always better to write it before". I've never thought the former, but the examples you've listed here look to me only like examples of where TDD didn't save somebody from making a mistake that was about a separate issue (types, poor quality test). None of them are examples of "actually writing the test after would have been better".

>When implementing to a spec you are absolutely right, but a very small amount of software is completely or even mostly specified in advance.

Why on earth would you do that? If I write even a single line of production code I have specified what that line of code is going to do. I have watched juniors flail around and do this when getting vague specs but seniors generally try to nail down a user story tight with a combination of code investigation, spiking and dialog with stakeholders before writing code that they would otherwise have to toss in the trash can if it wasn't fit for purpose.

To me this isn't related to TDD either. Whether or not I practice TDD, I don't fuck around writing or changing production code if I don't know precisely what result it is I want to achieve. Ever.

Future requirements will probably remain vague but never the ones I'm implementing right now.

>I agree this can lead to brittle tests and lack of spec adherence, but if you are iterating on intermediate state and writing tests as you go, the structure of the code you wrote 30 seconds ago is very much influencing the test you're writing now.

Only if the spec is changing too. This sometimes happens if I discover some issue by looking at the code, but in general my test remains relatively static while the code underneath it iterates.

This obviously wouldn't happen if you wrote implementation-tied tests rather than specification-tied tests but... maybe just don't do that?

>Another issue is that fault injection tests basically require coupling to the implementation

All tests requiring coupling to implementation in some way. The goal is to loosely couple as possible while maximizing speed, ease of use, etc. I'm not really sure why fault injection should be treated as special. If you need to refactor the test harness to allow it, that's probably a really good idea.

>The way I prefer to write these is to write the implementation first, then write the fuzz test - add a few bugs in the implementation, and fix/enhance the fuzz test until it catches them. Fuzz testing is one of the best bang-for-buck testing

Fuzz testing is great and having preferences is fine, but once again fuzz testing says little about the efficacy of TDD (fuzz tests can be written both before and after) and preference for test-after I find tends to mean little more than "I like my old habits".

>Fuzz testing is one of the best bang-for-buck testing methodologies there is, and in my experience it's very hard to write a really good fuzz test unless you already have most of your implementation

In my experience you can (I've done TDD with property tests and, well, I see fuzz testing as simply a subset of that). I also don't see any particular reason why you can't.

These methodologies I find do provide bang for the buck if you're writing a very specific kind of code. I will know if I'm writing that type of code in advance.

>This change alters a lock-free data structure to add a monotonicity invariant,

If I'm reading it correctly, this looks like a class of bugs we discussed that is fixed by tightening up the typing. In which case, no test is strictly necessary, although I'd argue that it probably would not hurt either.

>This change moves a memory layout - again, I don't know how I would have written a test for this, besides something wild like querying smaps (not portable) to see if the final page of the arena allocation had faulted in.

I can't tell if this is refactoring or you're fixing a bug. Is there a scenario which would reproduce a bug? If so, quite possibly a test would help. I've rarely been terribly sympathetic to the view that "writing a test to replicate this bug is too hard" is a good reason for not doing it. Programming is hard. I find that A) bugs often tend to cluster in scenarios that the testing infrastructure is ill equipped to reproduce the scenario B) once you upgrade the testing infrastructure to handle those scenario types those bugs often poof...stop recurring.

>This change was written more in the way you recommend - but the whole change is basically a test. I debugged this by reading the code and thinking about it, then wrote up a pretty complicated fuzz test to help find any future races. I'm guessing that you would not consider adding debug asserts to be a violation of "write the test first"

I would file that under "tightening up typing" again and also file it under "the decision to write a test at all is distinct from the decision to write a test first".

>I don't think TDD would have led to better results in these cases

Again, I see no examples where writing a test after would have been better. There are just a few where you could argue that not writing a test at all is the correct course of action.