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Avoid symbolic witnesses for .Self in an impl decl (#7564)

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Point symbolic witnesses into `.Self` written inside an impl decl at the
impl that is being declared. This is tricky because the impl does not
yet exist. So we use a new instruction `ImplSelfWitness` which _will_ be
replaced by the `ImplWitness` once it becomes available. The
`ImplSelfWitness` acts like a symbolic witness, except it does not
perform lookup, since we know which impl we will get a witness from.

This prevents us from finding other impls when performing lookups into
`.Self` in an impl decl, which produces incorrect/incoherent results.
This commit is contained in:
Dana Jansens
2026-07-29 16:25:23 +00:00
committed by GitHub
parent 2ac425e591
commit f51c075f8b
94 changed files with 3221 additions and 2797 deletions
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toolchain/check/handle_where.cpp
+25
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@@ -12,10 +12,12 @@
#include "toolchain/check/period_self.h"
#include "toolchain/check/subst.h"
#include "toolchain/check/type.h"
#include "toolchain/check/type_completion.h"
#include "toolchain/check/unused.h"
#include "toolchain/sem_ir/declared_facet_type.h"
#include "toolchain/sem_ir/ids.h"
#include "toolchain/sem_ir/inst.h"
#include "toolchain/sem_ir/specific_interface.h"
#include "toolchain/sem_ir/typed_insts.h"
namespace Carbon::Check {
@@ -90,6 +92,29 @@ auto HandleParseNode(Context& context, Parse::WhereOperandId node_id) -> bool {
auto period_self =
MakePeriodSelfFacetValue(context, node_id, period_self_type_id);
if (context.node_stack().PeekIs(Parse::NodeKind::ImplTypeAs) ||
context.node_stack().PeekIs(Parse::NodeKind::ImplDefaultSelfAs)) {
// We are in an impl declaration. We want to find the interface being
// impl'd, which will have to be on the LHS of the `where`. We want to catch
// lookups into that interface with `.Self` so we identify it with `.Self`.
auto identified_id = TryToIdentifyFacetType(
context, node_id, context.constant_values().Get(period_self),
context.types().GetTypeInstId(period_self_type_id),
/*allow_partially_identified=*/false);
if (identified_id.has_value()) {
const auto& identified =
context.identified_facet_types().Get(identified_id);
if (identified.is_valid_impl_as_target()) {
auto& decl_impl_interface = context.declaring_impl_decls().back();
if (decl_impl_interface != SemIR::SpecificInterface::None) {
CARBON_CHECK(decl_impl_interface ==
identified.impl_as_target_interface());
}
decl_impl_interface = identified.impl_as_target_interface();
}
}
}
// Going to put each requirement on `args_type_info_stack`, so we can have an
// inst block with the varying number of requirements but keeping other
// instructions on the current inst block from the `inst_block_stack()`.