Bug 15782 – [Reg 2.071-devel] Alias no longer strips qualifiers from user defined types

Since a week or so vibe.d's test suite is no longer compilable w/ the dmd-master. A meta-programming test fails with static assert ("(A, const(B), const(A))" == "(A, B)") indicating that some compiler semantics changed. https://github.com/rejectedsoftware/vibe.d/blob/0b4f51ef7a7bc880a7ce37356fcf0c0a5f4b19b5/source/vibe/internal/meta/codegen.d#L95

Introduced in: https://github.com/D-Programming-Language/phobos/pull/3985 Now the Alias template properly holds the aliased type qualifier. It would affect to staticIndexOf, Erase, EraseAll, NoDuplicates, Replace, ReplaceAll, and DerivedToFront behaviors at least.

If that test had used builtin types it would never have passed, it was a bug in Alias when dealing with qualified user-defined types. I understand that it's frustrating to have old code break, but having templates like Alias & NoDuplicates strip qualifiers for user-defined types but not builtins is definitely not an option. So we have 2 choices: it always strips or it never strips. The second seems vastly preferable to me.

I think it would be OK to make the change if it were only for Alias (which was package protected and undocumented anyhow), but I don't like that those breakages just happen, and it's definitely not OK to change all the other functions in std.meta w/o a proper deprecation. Also see issue 11480

Apologies for not noticing the changes, I didn't notice the bug in the old implementation. What is the way forward here? The old behaviour was unexpected (why remove qualifiers?), inconsistent (didn't happen for builtins) and undocumented. I would argue that it's a straightforward bug and we should just say it's now fixed, perhaps adding a warning and a special changelog entry.

(In reply to John Colvin from comment #5) > Apologies for not noticing the changes, I didn't notice the bug in the old > implementation. No problem, it took me 20 minutes to find the change (b/c it's more or less a compiler bug, see issue 11480). > What is the way forward here? The old behaviour was unexpected (why remove > qualifiers?), inconsistent (didn't happen for builtins) and undocumented. I > would argue that it's a straightforward bug and we should just say it's now > fixed, perhaps adding a warning and a special changelog entry. But it's been like that since ages and might now subtly break metagen code (e.g. serialization frameworks). The amount of time necessary to debug that some binary protocol no longer works b/c the semantics of Replace were fixed justifies that we put some more effort into this. I'd try sth. along this line, though it might raise too many errors. ---- deprecated("message about Alias") template Alias(T) if (isAffected!T && !is(T == Unqual!T)) { alias Alias = Unqual!T; } template Alias(T) if (!isAffected!T || is(T == Unqual!T)) { alias Alias = T; } ----

Commits pushed to stable at https://github.com/D-Programming-Language/phobos https://github.com/D-Programming-Language/phobos/commit/5fb53481fccbe800f083aeaa867d0dd541efbe67 fix Issue 15782 - Alias no longer strips qualifiers from user defined types - introduce OldAlias for internal usage - deprecate unqualifying of UDTs - unfortunately deprecation message isn't shown in many use-cases https://github.com/D-Programming-Language/phobos/commit/fed12920a6bfc6cbb37da8e7568cf555005632f1 Merge pull request #4094 from MartinNowak/fix15782 fix Issue 15782 - Alias no longer strips qualifiers from user defined types

Commits pushed to master at https://github.com/D-Programming-Language/phobos https://github.com/D-Programming-Language/phobos/commit/5fb53481fccbe800f083aeaa867d0dd541efbe67 fix Issue 15782 - Alias no longer strips qualifiers from user defined types https://github.com/D-Programming-Language/phobos/commit/fed12920a6bfc6cbb37da8e7568cf555005632f1 Merge pull request #4094 from MartinNowak/fix15782