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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/lower/testdata/interop/cpp/class/export/method.carbon
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+4 -4
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@@ -17,7 +17,7 @@ library "[[@TEST_NAME]]";
import Cpp;
class A {
fn F[self: Self]() { self; }
fn F(self) { self; }
}
inline Cpp '''
@@ -32,7 +32,7 @@ library "[[@TEST_NAME]]";
import Cpp;
class A {
fn F[self: Self](a: i32) -> i32 {
fn F(self, a: i32) -> i32 {
self;
return a;
}
@@ -251,8 +251,8 @@ fn CallCallF() { Cpp.CallF(); }
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define void @_CF__carbon_thunk.A.Main(ptr %self, ptr %_, ptr %_1) #4 !dbg !33 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: %.loc6_35.2 = load i32, ptr %_, align 4, !dbg !40
// CHECK:STDOUT: %A.F.call = call i32 @_CF.A.Main(ptr %self, i32 %.loc6_35.2), !dbg !40
// CHECK:STDOUT: %.loc6_29.2 = load i32, ptr %_, align 4, !dbg !40
// CHECK:STDOUT: %A.F.call = call i32 @_CF.A.Main(ptr %self, i32 %.loc6_29.2), !dbg !40
// CHECK:STDOUT: store i32 %A.F.call, ptr %_1, align 4, !dbg !40
// CHECK:STDOUT: ret void, !dbg !40
// CHECK:STDOUT: }