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/convert.cpp

@@ -36,6 +36,7 @@
 #include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/inst.h"
 #include "toolchain/sem_ir/inst_kind.h"
+#include "toolchain/sem_ir/pattern.h"
 #include "toolchain/sem_ir/type.h"
 #include "toolchain/sem_ir/type_info.h"
 #include "toolchain/sem_ir/typed_insts.h"
@@ -2221,32 +2222,18 @@ auto ConvertForExplicitAs(Context& context, Parse::NodeId as_node,
 }
 
 // TODO: Consider moving this to pattern_match.h.
-auto ConvertCallArgs(Context& context, SemIR::LocId call_loc_id,
-                     SemIR::InstId self_id,
+auto ConvertCallArgs(Context& context, SemIR::InstId self_id,
                      llvm::ArrayRef<SemIR::InstId> arg_refs,
                      SemIR::InstId return_arg_id, const SemIR::Function& callee,
                      SemIR::SpecificId callee_specific_id, bool is_desugared)
     -> SemIR::InstBlockId {
-  auto param_patterns =
-      context.inst_blocks().GetOrEmpty(callee.param_patterns_id);
-  auto return_pattern_id = callee.return_pattern_id;
-
   // The caller should have ensured this callee has the right arity.
-  CARBON_CHECK(arg_refs.size() == param_patterns.size());
-
-  if (callee.self_param_id.has_value() && !self_id.has_value()) {
-    CARBON_DIAGNOSTIC(MissingObjectInMethodCall, Error,
-                      "missing object argument in method call");
-    CARBON_DIAGNOSTIC(InCallToFunction, Note, "calling function declared here");
-    context.emitter()
-        .Build(call_loc_id, MissingObjectInMethodCall)
-        .Note(callee.latest_decl_id(), InCallToFunction)
-        .Emit();
-    self_id = SemIR::ErrorInst::InstId;
-  }
+  CARBON_CHECK(
+      (self_id.has_value() ? 1 : 0) + arg_refs.size() ==
+      context.inst_blocks().GetOrEmpty(callee.param_patterns_id).size());
 
   return CallerPatternMatch(context, callee_specific_id, callee.self_param_id,
-                            callee.param_patterns_id, return_pattern_id,
+                            callee.param_patterns_id, callee.return_pattern_id,
                             self_id, arg_refs, return_arg_id, is_desugared);
 }