Deduce facet values for arguments to generic fns receiving a facet type #4865
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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.
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.
The fuzzer tests are too good, and see this immediately. I will have to either do more here or add some context.TODOs, but then I also hit an eval phase check, so this isn't ready yet I guess. |
danakj
commented
Feb 3, 2025
| class Ginger {} | ||
| impl Ginger as Goat {} | ||
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| fn WalkGoat[T:! Goat](g: T) {} |
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Note to self: add a test with a generic interface, and the generic argument comes in a later parameter:
fn CallGenericMethod[T:! type](U:! Generic(T), t: T) {}
fn G(t: GenericParam) { CallGenericMethod(GenericParam, t); }
From @zygoloid here: https://github.com/carbon-language/carbon-lang/pull/4882/files#r1939902344
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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:
Here T is a generic parameter that is a facet value, but the
valparameter's type is the facet value converted from a FacetType to a TypeType, withas type. The result of that conversion is a FacetAccessType. So the deduction code sees a FacetAccessType for the type ofval.We make deduction undo the
as typeconversion to move back to theTparameter 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 #4863.Tests with
impl forallcan 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 #4863.