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From what I've observed semantic merge issues always get picked up at compile time - hence only the compile phase need pass to allow a queued merge through, but

From what I've observed semantic merge issues always get picked up at compile time - hence only the compile phase need pass to allow a queued merge through, but I guess if you're working a lot with weakly-typed or interpreted languages it'd be different. It's hard not to think there should be some way of automatically flagging likely issues where, say, commits in one PR refer to particular identifiers that were modified in parallel PRs, but it definitely goes well beyond what diff-merge tools typically do today.

skissane 3y ago
> From what I've observed semantic merge issues always get picked up at compile time - hence only the compile phase need pass to allow a queued merge through, but I guess if you're working a lot with weakly-typed or interpreted languages it'd be different. I agree that 90% of the time, with strongly typed languages, a compile is enough. But even in those languages, you can still get semantic merge issues which aren’t compilation-related. Suppose your app has a config file. The first PR renames one of the configuration keys in the config file, the second PR adds some new code which reads that configuration key. The combination compiles fine, but the second PR fails in integration tests, because the config key it was expecting wasn’t there any more. Of course this is a sign of a design flaw - the name of the config key should occur only once in the program (say as a constant), so both PRs would use the constant, and the second PR would get the change in constant value from the first, or else the first renames the constant so the second doesn’t compile. Or even a higher-level API which deals with the semantics of what the config represents instead of just key-value strings. But a lot of large legacy code bases contain design flaws like this, and fixing them can be a lot of work, so they don’t get fixed, at least not any time soon.
- wizofaus 3y ago
  Sure, there are always going to be occasions where a combination of two bits of work done separately cause unexpected behaviour changes - but generally if there are integration/automated E2E tests that are capable of picking that sort of thing up I'd expect they typically get triggered after the mainline merge anyway. OTOH, I can understand at least wanting to ensure that it's not possible for two consecutive merges to the mainline branch to cause it stop building.

skissane 3y ago

> From what I've observed semantic merge issues always get picked up at compile time - hence only the compile phase need pass to allow a queued merge through, but I guess if you're working a lot with weakly-typed or interpreted languages it'd be different. I agree that 90% of the time, with strongly typed languages, a compile is enough. But even in those languages, you can still get semantic merge issues which aren’t compilation-related. Suppose your app has a config file. The first PR renames one of the configuration keys in the config file, the second PR adds some new code which reads that configuration key. The combination compiles fine, but the second PR fails in integration tests, because the config key it was expecting wasn’t there any more. Of course this is a sign of a design flaw - the name of the config key should occur only once in the program (say as a constant), so both PRs would use the constant, and the second PR would get the change in constant value from the first, or else the first renames the constant so the second doesn’t compile. Or even a higher-level API which deals with the semantics of what the config represents instead of just key-value strings. But a lot of large legacy code bases contain design flaws like this, and fixing them can be a lot of work, so they don’t get fixed, at least not any time soon.

wizofaus 3y ago
Sure, there are always going to be occasions where a combination of two bits of work done separately cause unexpected behaviour changes - but generally if there are integration/automated E2E tests that are capable of picking that sort of thing up I'd expect they typically get triggered after the mainline merge anyway. OTOH, I can understand at least wanting to ensure that it's not possible for two consecutive merges to the mainline branch to cause it stop building.