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432ee89dda
In open discussion[1] we decided that "identical" rewrites would mean
that for a given LHS value, all RHS have the same value (after
evaluation), rather than requiring the RHS to all have the same
syntactic value. This means the following is valid, since the value of
`.Y` is known to be `()` while resolving the rewrite constraints of `T`.
So both rewrites of `.X` are resolved to `.X = ()`:
```
fn Identical(T:! I where .X = () and .X = .Y and .Y = ()) {}
```
The implementation of this clarification, along with test cases encoding
it, is done in https://github.com/carbon-language/carbon-lang/pull/5686.
Clarify this in the language design documents, and improve some other
clarity while we're there:
- The prose talks about a facet `T`, but the examples were using `A` as
its name. Change the facet to be `T`. This means changing the `.T`
associated constant (and `.U` and `.V`) to be `.X` (and `.Y` and `.Z`).
While doing this, use `I` for the interface name instead of `C`, which
we use more commonly for a class type name.
- Correct the comments in the cycle example that claim we find `.Y then
.Y* then .Y**`. In this example `.Y = .Z* and .Z = .Y*` which adds _two_
levels of pointers when evaluating `.Y`: `.Y => .Z* => (.Y*)* => .Y**`
[1]
https://docs.google.com/document/d/1Yt-i5AmF76LSvD4TrWRIAE_92kii6j5yFiW-S7ahzlg/edit?tab=t.0#heading=h.qti4vn50zwy
---------
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Co-authored-by: Carbon Infra Bot <carbon-external-infra@google.com>
Generics
This directory contains the collection of documents describing the generics feature of Carbon:
- Overview - A high-level description of the generics design, with pointers to other design documents that dive deeper into individual topics
- Goals - The motivation and principles guiding the design direction
- Terminology - A glossary establishing common terminology for describing the design
- Detailed design - In-depth description
- Appendix: Coherence - Describes the rationale for Carbon's choice to have coherent generics, and the alternatives.
- Appendix: Rewrite constraints - Describes the detailed rules governing rewrite constraints, and why resolving them terminates.
- Appendix: Witness tables - Describes an implementation strategy for checked generics, and Carbon's rationale for only using it for dynamic dispatch.