Support implicit conversion from a type value to a facet value #4863
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If a type satisfies the requirements of a FacetType, then that type
as a value can be converted to a FacetValue, which binds the type value
to the FacetType.
For instance, if the class A implements an interface B, then
fn F(b:! B) {}
can be called with the type `A`
F(A);
This does not handle yet receiving non-type values matching a FacetType,
as that requires deducing the required FacetValue for the caller's
argument. Follow-up work will do this step.
danakj
added a commit
to danakj/carbon-lang
that referenced
this pull request
Jan 29, 2025
The fn receiving a facet type needs to deduce a type from a FacetAccessType, which is the SemIR type representing the parameter type that is a generic parameter. For example: ``` fn F[T: Interface](val: T); ``` Here T is a generic parameter that is a facet value, but the `val` parameter's type is the facet value converted from a FacetType to a TypeType, with `as type`. The result of that conversion is a FacetAccessType. So the deduction code sees a FacetAccessType for the type of `val`. We make deduction undo the `as type` conversion to move back to the `T` parameter declaration, which has type FacetType in order to deduce the required facet value (which is itself a type constrained by the FacetType). And when we have a facet value (of type FacetType) to be deduced, we will also convert the argument if it is of an appropriate type to a FacetValue that matches the FacetType using the changes from PR carbon-language#4863. Tests with `impl forall` can cause impl deduction to recurse forever and crash, so those tests are omitted in this PR and they will come in follow-up work that address the infinite recursion. This PR is written on top of carbon-language#4863.
chandlerc
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If a type satisfies the requirements of a FacetType, then that type as a value can be converted to a FacetValue, which binds the type value to the FacetType.
For instance, if the class A implements an interface B, then
can be called with the type
AThis does not handle yet receiving non-type values matching a FacetType, as that requires deducing the required FacetValue for the caller's argument. Follow-up work will do this step.