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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/testdata/impl/fail_impl_bad_interface.carbon
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@@ -163,8 +163,8 @@ impl {.a: bool} as type where .Self impls I {}
// CHECK:STDOUT: %.loc8_36: type = type_literal type [concrete = type]
// CHECK:STDOUT: %.Self.as_type: type = facet_access_type %.Self.ref [symbolic_self = constants.%.Self.as_type]
// CHECK:STDOUT: %.loc8_24: type = converted %.Self.ref, %.Self.as_type [symbolic_self = constants.%.Self.as_type]
// CHECK:STDOUT: %.loc8_18: type = where_expr %.Self [concrete = constants.%type] {
// CHECK:STDOUT: requirement_base_facet_type type
// CHECK:STDOUT: %.loc8_18: type = where_expr [concrete = constants.%type] {
// CHECK:STDOUT: requirement_base_facet_type %.loc8_13
// CHECK:STDOUT: requirement_impls %.loc8_24, %.loc8_36
// CHECK:STDOUT: }
// CHECK:STDOUT: }
@@ -214,8 +214,8 @@ impl {.a: bool} as type where .Self impls I {}
// CHECK:STDOUT: %I.ref: type = name_ref I, file.%I.decl [concrete = constants.%I.type]
// CHECK:STDOUT: %.Self.as_type: type = facet_access_type %.Self.ref [symbolic_self = constants.%.Self.as_type]
// CHECK:STDOUT: %.loc10_31: type = converted %.Self.ref, %.Self.as_type [symbolic_self = constants.%.Self.as_type]
// CHECK:STDOUT: %.loc10_25: type = where_expr %.Self [concrete = constants.%type_where] {
// CHECK:STDOUT: requirement_base_facet_type type
// CHECK:STDOUT: %.loc10_25: type = where_expr [concrete = constants.%type_where] {
// CHECK:STDOUT: requirement_base_facet_type %.loc10_20
// CHECK:STDOUT: requirement_impls %.loc10_31, %I.ref
// CHECK:STDOUT: }
// CHECK:STDOUT: }