Can C 20 Class Template Constructors Omit Redundant Template Parameter Lists?

Can Class Template Constructors Have a Redundant Template Parameter List in C 20?

In C 17, it was permissible to declare class template constructors with a redundant template parameter list, as exemplified by the following code:

template<typename T>
struct S {
    S<T>();
};

However, with the advent of C 20 and the implementation of compatibility changes, the aforementioned code now raises an error on GCC trunk (for -std=c 20). Clang trunk, on the other hand, compiles the code without issue. This inconsistency begs the question: is this a bug or an intended breaking change that has yet to be implemented fully across compilers?

The Answer

The answer lies in a subtle change in the C 20 specification. Specifically, the following section has been revised:

[class.ctor]

"A constructor is introduced by a declaration whose declarator is a function declarator of the form:

ptr-declarator ( parameter-declaration-clause ) noexcept-specifier attribute-specifier-seq

Where the ptr-declarator consists solely of an id-expression, an optional attribute-specifier-seq, and optional surrounding parentheses, and the id-expression has one of the following forms:"

In the case of a member-declaration within a class template, the id-expression must now be the injected class name of the immediately-enclosing entity.

Hence, while C 17 allowed S() as a constructor declaration within the template S, C 20 requires S() instead. This change effectively removes the redundancy and aligns with CWG 2237, which addresses potential error-prone practices.

Therefore, the error encountered on GCC trunk with -std=c 20 is not a bug but an indication of the breaking change introduced in C 20. Compilers that have not yet implemented this change will continue to compile the old code successfully, while those that have implemented it will adhere to the new requirement.

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