[clang][reflection] Complete reflected specializations with implicit-instantiation semantics

[Cfretz244]

Fixes #296.

Problem

SemaMetaActions::EnsureInstantiated — the single completion funnel for every type-completing metafunction (members_of, bases_of, size_of, is_complete_type, ...) — completed class template specializations with TSK_ExplicitInstantiationDefinition plus InstantiateClassTemplateSpecializationMembers, eagerly instantiating every member definition. A specialization whose never-odr-used member bodies are ill-formed (the "specialized storage base" idiom — valid C++; tl::expected<void, E> is the field shape) was wrong-rejected, order-dependently: a preceding ordinary use completed the class lazily first and the identical members_of loop then compiled clean (see #296 for the reproducer + -DPREINSTANTIATE control). The eager kind also marked the specialization as a user-written explicit instantiation definition — weak-ODR emission of every member in the TU, and collisions with genuine template struct ...; declarations.

Fix

clang/lib/Sema/SemaReflect.cpp, EnsureInstantiated: mirror Sema::RequireCompleteTypeImpl —

• InstantiateClassTemplateSpecialization(..., TSK_ImplicitInstantiation, /*Complain=*/false, CTSD->hasStrictPackMatch());
• drop the InstantiateClassTemplateSpecializationMembers sweep entirely (a reflection query needs completeness — layout, bases, declared members — never member bodies);
• add the member-class-of-a-template branch (InstantiateClass on getInstantiatedFromMemberClass, TSK_ImplicitInstantiation) that the eager member sweep used to cover as a side effect, so members_of(^^Outer<T>::Inner) keeps working when reflection completes Inner on demand.

Body-needing consumers (extract, reflect_invoke, substitute of deduced-return functions) re-enter EnsureInstantiated with the specific FunctionDecl/VarDecl; those branches are untouched, so constant-evaluating a member of an implicitly-instantiated specialization still instantiates its body on demand.

Test

libcxx/test/std/experimental/reflection/members-of-lazily-ill-formed-bodies.pass.cpp (new): the specialized-storage-base idiom with five distinct templates so each metafunction (members_of, nonstatic_data_members_of, bases_of, is_complete_type, size_of) is the first instantiation trigger for its specialization; an order-independence control (ordinary use first → identical enumeration); and a runtime check that odr-used bodies still instantiate lazily.

Validation

• Base: 837da39eb88c (current p2996 tip; applies cleanly, builds standalone).
• Without the fix: the members_of instantiates member function definitions when it triggers the class template specialization's instantiation — valid types with lazily-ill-formed member bodies are wrong-rejected (order-dependent) #296 reproducer errors (no member named 'm_val' in 'Exp<void>' from inside the members_of iteration) and is order-dependent; the new test wrong-rejects.
• With the fix: reproducer clean in both orderings; new test passes.
• clang/test/Reflection: 16/16 with the fix on this machine — identical to base. Body-needing suite spot-checks green: reflect-invoke.pass.cpp, to-and-from-values.pass.cpp, consteval-reentrant-instantiation.pass.cpp, members-and-subobjects.pass.cpp.
• Downstream soak (reflection-driven nanobind binding generator): 53-test binder suite green; TartanLlama/expected v1.3.1 corpus run with tl::expected<void, std::string> in the bind set passes its full differential suite (previously: nine hard errors from tl's implementation details on bare enumeration).

🤖 Generated with Claude Code

[zebullax]

good, thanks