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How does the ODR interact with "unique" types that use a non-type template defaulted to a lambda expression?

  • Thread starter Thread starter Joshua Maiche
  • Start date Start date

Joshua Maiche

If a class template uses a lambda a default template argument, every default usage of that class type will be a new type.

template <auto unique = [] {}>
struct UniqueStruct {};
static_assert(!std::is_same_v<UniqueStruct<>, UniqueStruct<>>);

However, in a single translation unit, both deriving and aliasing the class keeps that derivation / alias to a single type in all the modern compilers I tested.

using UniqueAlias = UniqueStruct<>;
static_assert(std::is_same_v<UniqueAlias, UniqueAlias>);

struct UniqueDerived : UniqueStruct<> {};
static_assert(std::is_same_v<UniqueDerived, UniqueDerived>);

Will this behavior safely hold across TUs, or is there a possibility that different TUs will come up with a different lambda type for UniqueAlias / UniqueDerived? If so, would that make those types unsafe to use across multiple TUs, leaving UniqueStruct as the only safe type to use across TUs?
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