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

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>

toolchain/check/deduce.cpp

@@ -14,6 +14,7 @@
 #include "toolchain/diagnostics/diagnostic.h"
 #include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/impl.h"
+#include "toolchain/sem_ir/pattern.h"
 #include "toolchain/sem_ir/type.h"
 #include "toolchain/sem_ir/typed_insts.h"
 
@@ -180,6 +181,12 @@ class DeductionContext {
     worklist_.AddAll(param, arg, want_value);
   }
 
+  template <typename ElementId>
+  auto AddAll(llvm::ArrayRef<ElementId> params, llvm::ArrayRef<ElementId> args,
+              bool want_value) -> void {
+    worklist_.AddAll(params, args, want_value);
+  }
+
   template <typename ParamT, typename ArgT>
   auto AddAll(ParamT param, ArgT arg) -> void {
     worklist_.AddAll(param, arg);
@@ -581,17 +588,24 @@ auto DeduceGenericCallArguments(
     Context& context, SemIR::LocId loc_id, SemIR::GenericId generic_id,
     SemIR::SpecificId enclosing_specific_id,
     [[maybe_unused]] SemIR::InstBlockId implicit_param_patterns_id,
-    SemIR::InstBlockId param_patterns_id,
-    [[maybe_unused]] SemIR::InstId self_id,
+    SemIR::InstBlockId param_patterns_id, SemIR::InstId self_id,
     llvm::ArrayRef<SemIR::InstId> arg_ids) -> SemIR::SpecificId {
   DeductionContext deduction(&context, loc_id, generic_id,
                              enclosing_specific_id,
                              /*diagnose=*/true);
 
-  // Prepare to perform deduction of the explicit parameters against their
-  // arguments.
-  // TODO: Also perform deduction for type of self.
-  deduction.AddAll(param_patterns_id, arg_ids, /*want_value=*/false);
+  // Prepare to perform deduction of the parameters against the explicit
+  // arguments. When `self` is provided as the method-call receiver it is
+  // prepended to the explicit argument list to match the leading `self`
+  // parameter pattern.
+  llvm::ArrayRef<SemIR::InstId> self_refs = {};
+  if (self_id.has_value()) {
+    self_refs = self_id;
+  }
+  deduction.AddAll(context.inst_blocks().GetOrEmpty(param_patterns_id),
+                   llvm::ArrayRef<SemIR::InstId>(llvm::to_vector<8>(
+                       llvm::concat<const SemIR::InstId>(self_refs, arg_ids))),
+                   /*want_value=*/false);
 
   if (!deduction.Deduce() || !deduction.CheckDeductionIsComplete()) {
     return SemIR::SpecificId::None;