fix(cpp): of two overloads alike but for a const, like protobuf's DynamicCastMessage(const MessageLite*) and DynamicCastMessage(MessageLite*), a call reaches the one C++ picks

[colbymchenry]

Summary

Follow-up from #2616, filed as a task chip there.

The bug. Overload fit by argument type scored a const T* parameter and its T* twin alike, and the one declared first won. Protobuf's DynamicCastMessage<T>(ptr.get()) on a std::shared_ptr<Message> took const T* DynamicCastMessage(const MessageLite*), where C++ picks T* DynamicCastMessage(MessageLite*).

The fix.

• cppConstTwinChoice (new). Of the best-fitting overload's twins, alike but for the const of a pointer or reference parameter, it takes the one that adds const to the fewest arguments. It is a tie-break among twins only: an earlier version penalized every const T& parameter, and that moved fmt's to_string(bigint) off its only viable const T& overload.

• cppWrittenTypeFit's hard rules. These all score -3:

  • a const object given to a pointer or reference to a non-const type;
  • a value a call returns given to a non-const reference (cppNamesObject tells a named object from a call's result);
  • an object given to a pointer;
  • a pointer to characters given to a pointer to another type, and the reverse.

  Two exceptions:

  • an argument of a template parameter's type is exempt from all of them (fmt's const S& fmt could be a pointer);
  • a parameter the function's own template parameter types (Src* x) is exempt from the pointer and const rules, because it deduces the const (rocksdb's static_cast_with_check).

• Argument types.

  • A leading const/constexpr that the declarator regex leaves out is kept (const Buffer* b).
  • An array is a pointer to its element (fmt's format_base2e(4, buf, …)).
  • &ref is a pointer to the referee.
  • cppLibraryElementOf keeps a const inside a holder's template argument (std::shared_ptr<const Message>), which fix(cpp): a call on what a container's front()/back()/at() or a holder's get()/value() returns, like rocksdb's edits.back()->MarkAtomicGroup(…), calls the element's method #2616's version stripped.

• Template headers are read up to 8 lines above a declaration, not 3. fmt's join spends four lines on its parameters, so its S&& sep wasn't recognized as dependent.

A/B on the C/C++ kit (main 0893606e vs this branch)

repo	retargeted
leveldb, redis	0
fmt	14
rocksdb	25
protobuf	194

No call was added or removed. Every retarget was compared against C++'s choice, overload pair by overload pair:

• Const twins. protobuf's DynamicCastMessage/DownCastMessage move to the pointer and reference twins matching their argument's constness (48 calls), including the static_cast<const MessageLite*>(from) forwarders. RefAt(base, …) on a const void* base takes const T& RefAt(const void*, …) (16), and VisitDescriptors(file, proto, …) on a mutable proto takes the mutable overload (5). In rocksdb, GetOffset, GetOffsetAs and AsRawPointer on a void* take their non-const twins (12). The GetExtensionSet(m) calls on a const m take its const overload (3).
• Objects off pointer overloads.
  • protobuf: WithVars(EnumVars(…)) reaches WithVars(Map&&), not const Map* (110), plus GetMiniTable(message) on a PtrOrRaw<T> (3) and MicroString(*new auto(…)) (1).
  • fmt: the appender/iterator arguments of format_decimal, format_base2e, fill_n and write_significand, and print(wos, …) on a std::wostringstream.
  • rocksdb: toku_unsafe_fetch(m_sto_txnid) → T&, and GetAndRefSuperVersion(cf) on a ColumnFamilyId → uint32_t.
• Arrays as pointers. fmt's Char buffer[…] and char xdigits[…] arguments now reach the Char* overloads. rocksdb's Add(level, sst, smallest, largest, …) on constexpr char smallest[] reaches the const char* overload. protobuf's python generator's printer_->Print(m, template) reaches Print(const Map&, string_view), because a constexpr char template[] no longer counts against string_view.
• Others. rocksdb's MultiGet(ReadOptions(), keys.size(), …) reaches the num_keys overload.

Intermediate arms caught three regressions before this version:

• the const T& penalty that moved to_string(bigint);
• the template-parameter cases: rocksdb's static_cast_with_check(&other) moved to its shared_ptr overload, and fmt's to_string_view(sep) was a coin flip;
• char arrays no longer counting against ColumnFamilyHandle*, which moved rocksdb's db_->Put(opts, key, ts, value).

All are fixed, and the final arm shows none of those changes.

Tests

• New file __tests__/cpp-overload-constness.test.ts (5 tests, all failing on 0893606e, checked on a detached checkout). It covers protobuf's cast helpers with pointer and reference twins, const and non-const holders, variables, references and a temporary, and class methods with either twin first. It also covers a T* const argument, overloads that differ by more than a const, an array, a deduced Src*, arguments of a template parameter's type (a one-line and a four-line header), char arrays against a class pointer and against a deduced T*, a pointer against const char*, and a constexpr array.
• The existing overload suites still pass: argument types, choice, nullptr/deleted, free functions, container accessors, member-template returns, specialization names.
• Mutations: all 19 killed. They cover each rule, the dependent/deduced exemptions, the twin shape, the fewest-added choice, leading const/constexpr, array decay, the holder's template-argument const, both header windows and cppNamesObject.
• Full suite: 7210 passed. One mcp-status-freshness.test.ts test hit an EBUSY teardown under load; the file passes 4/4 when rerun on its own.

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