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Properly diagnose ambiguous .Self in T impls X where... (#7132)

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A `where` expression nested inside a `T impls X` constraint makes
`.Self` ambiguous on the right-hand side of the `where` if `T` is
anything other than `.Self`. After the `where`, the value of a `.Self`
could be `T` or could be the value of `.Self` before the `impls`
constraint: the so-called top-level value of `.Self`.

Implicit use of `.Self` in designators is always allowed, and they are
bound (and replaced by a reference) to the inner-most possible value of
`.Self`. On the right-hand side of the nested `where` above, they have
the value `T as X`.

`.Self impls ...` is also always allowed, since it acts more as a
keyword here, and it always refers to the inner-most possible value of
`.Self`.

Any other explicit use of `.Self` is diagnosed when ambiguous, in any
kind of constraint. This is done in the handling of `WhereExpr` since it
has enough context to allow `.Self impls` (which is an explicit use)
while disallowing other explicit uses. And because it has non-canonical
instructions to work with, so it is able to diagnose errors with precise
locations.

Since `.Self` is no longer going to be marked with depth modifiers, the
eval of `WhereExpr` does not need an input facet value instruction
representing `.Self` to compare with, as they are now going to all be
equivalent. So revert it back to just looking for the `PeriodSelf` name
id, through a shared helper being introduced as `IsPeriodSelf`. And drop
the period self InstId from the `WhereExpr` instruction. This causes
most of the formatted SemIR changes.

Move helpers for working with and replacing `.Self` to their own file,
out of the `facet_type.h` header/cpp files. These are working with
`.Self` facet values more than facet types, though `.Self` is a name
that only exists inside the scope of a facet type.
This commit is contained in:
Dana Jansens
2026-05-01 21:22:32 +00:00
committed by GitHub
parent 0fcbe7c6b5
commit 46bb0fecd4
46 changed files with 1008 additions and 678 deletions
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toolchain/check/eval.cpp
+11 -56
View File
@@ -22,6 +22,7 @@
#include "toolchain/check/import_ref.h"
#include "toolchain/check/inst.h"
#include "toolchain/check/name_lookup.h"
#include "toolchain/check/period_self.h"
#include "toolchain/check/type.h"
#include "toolchain/check/type_completion.h"
#include "toolchain/diagnostics/diagnostic.h"
@@ -2487,17 +2488,6 @@ auto TryEvalTypedInst<SemIR::Temporary>(EvalContext& eval_context,
return MakeConstantResult(eval_context.context(), temporary, phase);
}
// Returns whether `const_id` is the same constant facet value as
// `facet_value_inst_id`.
//
// Compares with the canonical facet value of `const_id`, dropping any `as type`
// conversions.
static auto IsSameFacetValue(Context& context, SemIR::ConstantId const_id,
SemIR::InstId facet_value_inst_id) -> bool {
auto canon_const_id = GetCanonicalFacetOrTypeValue(context, const_id);
return canon_const_id == context.constant_values().Get(facet_value_inst_id);
}
static auto AddRequirementBase(Context& context,
SemIR::RequirementBaseFacetType base,
SemIR::FacetTypeInfo* info, Phase* phase)
@@ -2569,7 +2559,6 @@ static auto AddRequirementRewrite(Context& context,
static auto AddRequirementImpls(Context& context, SemIR::LocId loc_id,
SemIR::RequirementImpls impls,
SemIR::InstId period_self_id,
SemIR::FacetTypeInfo* info, Phase* phase)
-> void {
auto lhs_id = context.constant_values().GetConstantInstId(impls.lhs_id);
@@ -2588,8 +2577,7 @@ static auto AddRequirementImpls(Context& context, SemIR::LocId loc_id,
auto facet_type = context.insts().GetAs<SemIR::FacetType>(rhs_id);
const auto& rhs = context.facet_types().Get(facet_type.facet_type_id);
if (IsSameFacetValue(context, context.constant_values().Get(lhs_id),
period_self_id)) {
if (IsPeriodSelf(context, lhs_id)) {
// A facet type with `.Self impls <RHS facet type>`. Whatever the RHS facet
// type constrains for `.Self` gets forwarded to the output facet type to
// also constrain `.Self`. Nothing on the RHS of `impls` can extend the
@@ -2646,35 +2634,6 @@ static auto AddRequirementImpls(Context& context, SemIR::LocId loc_id,
// facet that it could possibly refer to, the `T as X` from the `impls`
// constraint, which eliminates any ambiguity in the resulting facet type.
class SubstPeriodSelfDiagnoseExplicitCallbacks
: public SubstPeriodSelfCallbacks {
public:
explicit SubstPeriodSelfDiagnoseExplicitCallbacks(
Context* context, SemIR::LocId loc_id,
SemIR::ConstantId period_self_replacement_id, Phase* phase)
: SubstPeriodSelfCallbacks(context, loc_id,
period_self_replacement_id),
phase_(phase) {}
auto ShouldReplace(bool implicit) -> bool override {
if (!implicit && *phase_ != Phase::UnknownDueToError) {
CARBON_DIAGNOSTIC(
AmbiguousPeriodSelf, Error,
"`.Self` is ambiguous after nested `where` in `<type> "
"impls ...` clause.");
context().emitter().Emit(loc_id(), AmbiguousPeriodSelf);
*phase_ = Phase::UnknownDueToError;
}
return implicit;
}
Phase* phase_;
};
SubstPeriodSelfDiagnoseExplicitCallbacks
callbacks_should_replace_explicit_is_error(
&context, loc_id, context.constant_values().Get(lhs_facet_or_type),
phase);
class SubstPeriodSelfImplicitOnlyCallbacks
: public SubstPeriodSelfCallbacks {
public:
@@ -2692,21 +2651,21 @@ static auto AddRequirementImpls(Context& context, SemIR::LocId loc_id,
&context, loc_id, context.constant_values().Get(lhs_facet_or_type));
auto self_impls_interface = [&](SemIR::SpecificInterface si) {
return SubstPeriodSelf(context,
callbacks_should_replace_explicit_is_error, si);
return SubstPeriodSelf(context, callbacks_should_replace_implicit_only,
si);
};
auto self_impls_constraint = [&](SemIR::SpecificNamedConstraint sc) {
return SubstPeriodSelf(context,
callbacks_should_replace_explicit_is_error, sc);
return SubstPeriodSelf(context, callbacks_should_replace_implicit_only,
sc);
};
auto type_impls_interface =
[&](SemIR::FacetTypeInfo::TypeImplsInterface impls)
-> SemIR::FacetTypeInfo::TypeImplsInterface {
auto self =
SubstPeriodSelf(context, callbacks_should_replace_explicit_is_error,
SubstPeriodSelf(context, callbacks_should_replace_implicit_only,
context.constant_values().Get(impls.self_type));
auto interface =
SubstPeriodSelf(context, callbacks_should_replace_explicit_is_error,
SubstPeriodSelf(context, callbacks_should_replace_implicit_only,
impls.specific_interface);
return {context.constant_values().GetInstId(self), interface};
};
@@ -2714,10 +2673,10 @@ static auto AddRequirementImpls(Context& context, SemIR::LocId loc_id,
[&](SemIR::FacetTypeInfo::TypeImplsNamedConstraint impls)
-> SemIR::FacetTypeInfo::TypeImplsNamedConstraint {
auto self =
SubstPeriodSelf(context, callbacks_should_replace_explicit_is_error,
SubstPeriodSelf(context, callbacks_should_replace_implicit_only,
context.constant_values().Get(impls.self_type));
auto constraint =
SubstPeriodSelf(context, callbacks_should_replace_explicit_is_error,
SubstPeriodSelf(context, callbacks_should_replace_implicit_only,
impls.specific_named_constraint);
return {context.constant_values().GetInstId(self), constraint};
};
@@ -2777,10 +2736,6 @@ auto TryEvalTypedInst<SemIR::WhereExpr>(EvalContext& eval_context,
Phase phase = Phase::Concrete;
SemIR::FacetTypeInfo info;
if (typed_inst.period_self_id == SemIR::ErrorInst::InstId) {
return SemIR::ErrorInst::ConstantId;
}
// Note that these requirement instructions don't have a constant value. That
// means we have to look for errors inside them, we can't just look to see if
// their constant value is an error.
@@ -2803,7 +2758,7 @@ auto TryEvalTypedInst<SemIR::WhereExpr>(EvalContext& eval_context,
}
case CARBON_KIND(SemIR::RequirementImpls impls): {
AddRequirementImpls(eval_context.context(), SemIR::LocId(inst_id),
impls, typed_inst.period_self_id, &info, &phase);
impls, &info, &phase);
break;
}
case CARBON_KIND(SemIR::RequirementEquivalent _): {