


Can C 20 Class Template Constructors Omit Redundant Template Parameter Lists?
Nov 19, 2024 pm 03:12 PMCan 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
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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