core: diagnose overrider and virtual_ptr mistakes where they are made

[jll63]

(Written by Claude Code, on behalf of @jll63.)

Six mistakes used to fail deep inside the library: in the overrider thunk, the type traits, or at dispatch. The user's own declaration showed up only in an instantiation note, or not at all. Each now fails with a static_assert that names the problem, and that message is the first error on gcc, clang and MSVC.

Mistake	Before	Now
Overrider drops const: virtual_ptr<const Node> → virtual_ptr<Plus>, virtual_<const Node&> → Plus&	dynamic_cast ... casts away qualifiers in std_rtti.hpp	overrider parameter discards const qualifier
Overrider return type does not convert to the method's	no viable conversion from returned value in the thunk	overrider return type is not convertible to method return type
Plain virtual_ptr in the method, shared_virtual_ptr in the overrider	no viable conversion from 'virtual_ptr<Plus>' to 'virtual_ptr<std::shared_ptr<Plus>>' in the thunk	different virtual_ptr<> pointer kinds
Method takes virtual_ptr<Node>&	compiled; initialize then aborted with unknown class virtual_ptr<Node, ...>	virtual_ptr<> must be passed by value or by const reference
Method takes virtual_ptr<Node>&&	method has no virtual parameters, or an ambiguous validate_overrider_parameter instantiation	same as above
virtual_ptr<Node*>	declaration accepted; every call then fails with no matching function	virtual_ptr<> takes a class or a smart pointer: virtual_ptr<Node>, not virtual_ptr<Node*>
Incomplete class in BOOST_OPENMETHOD_CLASSES	5 errors inside <type_traits> and typeid	use_classes: a class is incomplete

Notes:

• The const check is limited to classes of one hierarchy. Interop parameters such as virtual_<const std::any&> keep their own constraint and their existing diagnostics.
• Rejecting virtual_ptr& and virtual_ptr&& breaks nothing that worked. Both shapes were checked as non-virtual parameters too. & compiled but was dispatched as if virtual_ptr were the class of the object. && did not compile once an overrider existed.
• The opposite of two mismatches is already reported, by cannot find '...' method. That covers a const overrider for a non-const method, and a smart virtual_ptr in the method with a plain one in the overrider. In both cases the overrider's arguments do not convert to the method's parameters, so the macro cannot find the method.

Found and left alone: virtual_<std::shared_ptr<Node>&&> fails with virtual_<> parameter is not a polymorphic class, before and after this change. std_shared_ptr.hpp has a shared_ptr_cast_traits<std::shared_ptr<T>&&> specialization, so the shape may be meant to work. That looks like a separate bug rather than a diagnostic problem.

Tests: one test/compile_fail_*.cpp per check, eight in all, each with its expected-error marker. Each was confirmed to compile once the mistake is fixed. Local runs:

• clang 22 and gcc 16: 206/206.
• g++-16 with BOOST_OPENMETHOD_ENABLE_REFLECTION=ON: 207/207.
• b2 toolset=gcc: exit 0, no warnings.
• MSVC v18 (/W4 /permissive-): every new test leads with its message, and positive examples compile cleanly.

The troubleshooting page that documents these messages is in a follow-up PR stacked on this branch.

🤖 Generated with Claude Code

https://claude.ai/code/session_016NKQZ2ktRZ1AjCgSLLeXma

[cppalliance-bot]

An automated preview of the documentation is available at https://132.openmethod.prtest3.cppalliance.org/libs/openmethod/doc/html/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-10-18 15:16:20 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.34%. Comparing base (5356efb) to head (63dc301).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #132   +/-   ##
========================================
  Coverage    92.34%   92.34%           
========================================
  Files           27       27           
  Lines         2234     2234           
  Branches       677      677           
========================================
  Hits          2063     2063           
  Misses         112      112           
  Partials        59       59           

Files with missing lines	Coverage Δ
include/boost/openmethod/core.hpp	91.80% <ø> (ø)

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