Can attributes be applied to templates?

Question

Does the C++ standard allow setting attributes on template declarations? For example:

[[attr1]] template <typename T [[attr2]] > [[attr3]]
class [[attr4]] C {};

I believe attr4 would be considered a valid attribute for the template instances (i.e., it would be an attribute on T<int> and T<char>, for example).

However would either attr1 or attr3 be considered valid by the compiler (even if it would be ignored)?

Would attr2 also be valid? I think not, but I'm not sure. The issue would be if it "adds" an attribute to a given type used in instantiation, but IIRC currently template parameters have all attributes ignored.

My interest in attr1 and/or attr3 is for a custom test DSL, where a given attribute on a template would automatically force instantiation of the class with a given set of types.

Answer 1

I digged into C++ grammar and here are relevant parts:

declaration:
    block-declaration
    template-declaration

template-declaration:
    template < template-parameter-list > declaration

That forbids [[attr1]] and [[attr3]], if you believe me, that declaration may not begin with attribute-specifier-seq.

---

template-parameter-list:
    template-parameter

template-parameter:
    type-parameter

type-parameter:
    type-parameter-key ..._opt identifier_opt

type-parameter-key:
    class
    typename

That forbids [[attr2]], identifier in regex is [a-zA-Z_][a-zA-Z_0-9]*, with universal-character-name and other implementation-defined characters.

---

declaration:
    block-declaration

simple-declaration:
    decl-specifier-seq_opt init-declarator-list_opt ;

decl-specifier-seq:
    decl-specifier attribute-specifier-seq_opt

decl-specifier:
    type-specifier

type-specifier:
    class-specifier

class-specifier:
    class-head { member-specification_opt }

class-head:
    class-key attribute-specifier-seq_opt class-head-name class-virt-specifier_opt base-clause_opt

That allows [[attr4]]