When unqualified lookup finds a name from an enclosing generic, create a
`specific_constant` if it's a constant, or a `specific_inst` otherwise,
to refer to that name from the inner generic. This allows consumers of
generic instructions to always use a specific of the current scope's
generic when accessing the constant value or type of a dependent
instruction.
Fixes crashes in dominance and in `GetExprCategory` caused by using
specifics for the wrong generic.
Assisted-by: Gemini via Antigravity
When identifying a facet type, defer evaluation of LookupImplWitnesses
by redirecting them to evaluate into IdentifiedWitness. Then the caller
of identify will need to substitute the IdentifiedWitnesses, either with
witnesses of their own, or with LookupImplWitness to cause a lookup
operation.
This allows:
- An `impl as` on a named constraint can subst the witnesses in
designators from the named constraint with a witness to the impl itself.
This prevents them from looking for and finding some other overlapping
impl. (This PR)
- Impl lookup to collect rewrite/equality requirements from all named
constraints and then use the impls it has already found to evaluate any
designators in those requirements instead of performing impl lookup
again (creating a cycle). (Done in
https://github.com/carbon-language/carbon-lang/pull/7944)
- Similarly, an `impl as` can collect impls and equality requirements
from all named constraints and verify them correctly by replacing
witnesses that should refer to the impl with the correct witness instead
of performing a lookup and finding an overlapping impl. (Done in
https://github.com/carbon-language/carbon-lang/pull/7937)
- ImplWitnessAccess to be able to find rewrites from a named constraint,
not just from the initial DeclaredFacetType. (TBD)
To do this we need to introduce the concept of "identified" specifics.
In order to identify a facet type, we must construct a specific for each
require decl in a named constraint. These specifics will have
IdentifiedWitnesses inside them, but their args would otherwise match a
specific for the require decl constructed in another context. To avoid
polluting the canonical specific namespace, we introduce a second
namespace of specifics that are "identified" since they are created by
identifying.
As we have introduced a new global state that modifies eval behaviour,
we must disable that state when resolving a specific (as per
https://github.com/carbon-language/carbon-lang/pull/7923). But inside an
"identified" specific, we preserve the behaviour so that the witnesses
modified by identify (through SubstPeriodSelf or through evaluating a
require specific) have their evaluation deferred.
Support for `fn` expressions with the same feature set as `fn`
declarations. In particular, there is no support for captures,
positional parameters, `=>` bodies, or `auto` return types.
Note that this results in two parse nodes for each `=>` token. That's a
bug, but it matches the behavior of function declarations, and will be
fixed in a follow-up change.
Assisted-by: Gemini via Antigravity
Richard Smith
Dana Jansens
Geoff Romer