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Move self to the explicit () parameter list (proposal #7016) (#7272)

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Implements proposal #7016: `self` moves from the deduced implicit list
(`fn F[self: Self]()`) to the front of the explicit list. Its type may
be written explicitly (`fn F(self: Self)`) or omitted, in which case it
defaults to `Self` (`fn F(self)`, `fn F(ref self)`); `self` in the
implicit list is rejected.

Throughout checking, `self` is modeled as the first explicit parameter.
Because a method is just a function whose first parameter is `self`, it
can also be called as an ordinary function with the receiver passed
explicitly (`Type.M(obj, ...)`), not only as `obj.M(...)`. A new
`SemIR::CallArgParamPatterns` helper chooses the parameters matched
against the explicit arguments, excluding a leading `self` only when it
is supplied as a method-call receiver; arity checking, conversion, and
generic deduction use it. The resulting SemIR and lowering are
unchanged: `self` is still `call_param0`, and witnesses, thunks, and
vtables are unaffected.

An omitted `self` type is parsed as a `SelfBindingPattern` node with no
type expression; checking synthesizes the `Self` type so it behaves
exactly like `self: Self`. However, the exact spelling used must match
between a forward declaration and a definition, following #3763's rules
around declaration matching.

Generated functions, thunks, and C++ interop import/export build `self`
as the first explicit parameter, and the `self`-type override (e.g.
Derived->Base for a virtual override) applies to the explicit `self`.
Placement is validated by new diagnostics: `SelfInImplicitParamList`,
`SelfNotFirstParam`, and `SelfOutsideParamList`. The benchmark source
generator and the documentation adopt the `(self)` shorthand; the
prelude, the examples, and the test data are migrated in the following
commits.

Assisted-by: Claude Code with Claude Opus 4.7

---------

Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>
This commit is contained in:
Chandler Carruth
2026-06-10 15:30:43 +00:00
committed by GitHub
co-authored by josh11b
parent fb05da761f
commit 7871237c15
296 changed files with 6101 additions and 4899 deletions
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toolchain/parse/testdata/generics/impl/basic.carbon
Vendored
+7 -9
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@@ -9,7 +9,7 @@
// TIP: bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/generics/impl/basic.carbon
impl i32 as Interface {
fn Add[self: Self](b: Self) -> Self;
fn Add(self, b: Self) -> Self;
}
// CHECK:STDOUT: - filename: basic.carbon
@@ -22,19 +22,17 @@ impl i32 as Interface {
// CHECK:STDOUT: {kind: 'ImplDefinitionStart', text: '{', subtree_size: 5},
// CHECK:STDOUT: {kind: 'FunctionIntroducer', text: 'fn'},
// CHECK:STDOUT: {kind: 'IdentifierNameMaybeBeforeSignature', text: 'Add'},
// CHECK:STDOUT: {kind: 'ImplicitParamListStart', text: '['},
// CHECK:STDOUT: {kind: 'SelfValueName', text: 'self'},
// CHECK:STDOUT: {kind: 'SelfTypeNameExpr', text: 'Self'},
// CHECK:STDOUT: {kind: 'LetBindingPattern', text: ':', subtree_size: 3},
// CHECK:STDOUT: {kind: 'ImplicitParamList', text: ']', subtree_size: 5},
// CHECK:STDOUT: {kind: 'ExplicitParamListStart', text: '('},
// CHECK:STDOUT: {kind: 'SelfValueName', text: 'self'},
// CHECK:STDOUT: {kind: 'SelfBindingPattern', text: 'self', subtree_size: 2},
// CHECK:STDOUT: {kind: 'PatternListComma', text: ','},
// CHECK:STDOUT: {kind: 'IdentifierNameNotBeforeSignature', text: 'b'},
// CHECK:STDOUT: {kind: 'SelfTypeNameExpr', text: 'Self'},
// CHECK:STDOUT: {kind: 'LetBindingPattern', text: ':', subtree_size: 3},
// CHECK:STDOUT: {kind: 'ExplicitParamList', text: ')', subtree_size: 5},
// CHECK:STDOUT: {kind: 'ExplicitParamList', text: ')', subtree_size: 8},
// CHECK:STDOUT: {kind: 'SelfTypeNameExpr', text: 'Self'},
// CHECK:STDOUT: {kind: 'ReturnType', text: '->', subtree_size: 2},
// CHECK:STDOUT: {kind: 'FunctionDecl', text: ';', subtree_size: 15},
// CHECK:STDOUT: {kind: 'ImplDefinition', text: '}', subtree_size: 21},
// CHECK:STDOUT: {kind: 'FunctionDecl', text: ';', subtree_size: 13},
// CHECK:STDOUT: {kind: 'ImplDefinition', text: '}', subtree_size: 19},
// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
// CHECK:STDOUT: ]