Why Are Redundant Template Parameter Lists in C 20 Class Template Constructors Now an Error?

Redundant Template Parameter Lists in Class Template Constructors in C 20

In C , the syntax for declaring constructors in class templates has undergone a change in C 20. Previously, code of the following form was considered well-formed:

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

Despite the redundancy of the template parameter in the constructor declaration, this code compiled successfully. However, in C 20 with the -std=c 20 flag enabled, GCC trunk now reports an error for this code:

error: expected unqualified-id before ')' token
3 |     S<T>();
^

While this error is not encountered in GCC10.2 or Clang with -std=c 20, it stems from a fundamental change introduced in C 20.

According to the C 20 compatibility section, the use of a simple-template-id as the declarator-id for a constructor or destructor is no longer valid. This move aims to eliminate potential error-prone redundancy.

The new wording in C 20's [class.ctor] section specifies that for constructors in member declarations within class templates, the injected-class-name should be used instead:

template<typename T>
struct S {
    S();  // Use injected-class-name
};

In this example, S represents the injected-class-name for the inner class template. Consequently, the redundant template parameter is removed from the constructor declaration.

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