P1102R2
Down with ()!

1. Revision History

EWG discussed this change as [EWG135] in Lenexa and voted 15 to 1 on forwarding to core. It became [CWG2121], discussed in Kona and needed someone to volunteer wording. This paper presents text that implements that decision, for consideration.

[P1102R0] was published in June 2018. It was discussed on the EWG reflector in June 2018, Nina Ranns provided feedback, and EWG chair agreed that the paper should move to CWG directly given previous polls.

[P1102R1] responded to feedback about ambiguous requires-clauses from Hubert Tong and was discussed in a CWG teleconference on 2020-12-07.

P1102R2 incorporates the feedback given by CWG during that teleconference.

2. Introduction and motivation

Currently, C++ lambdas with no parameters do not require a parameter declaration clause. The specification even contains this language in [expr.prim.lambda] section 8.4.5 ❡4:

If a lambda-expression does not include a lambda-declarator, it is as if the lambda-declarator were ().

This allows us to omit the unused () in simple lambdas such as this:

std::string s1 = "abc";
auto withParen = [s1 = std::move(s1)] () {
  std::cout << s1 << '\n'; 
};

std::string s2 = "abc";
auto noSean = [s2 = std::move(s2)] { // Note no syntax error.
  std::cout << s2 << '\n'; 
};

These particular lambdas have ownership of the strings, so they ought to be able to mutate it, but s1 and s2 are const (because the const operator is declared const by default) so we need to add the mutable keyword:

std::string s1 = "abc";
auto withParen = [s1 = std::move(s1)] () mutable {
  s1 += "d";
  std::cout << s1 << '\n'; 
};

std::string s2 = "abc";
auto noSean = [s2 = std::move(s2)] mutable { // Currently a syntax error.
  s2 += "d";
  std::cout << s2 << '\n'; 
};

Confusingly, the current Standard requires the empty parens when using the mutable keyword. This rule is unintuitive, causes common syntax errors, and clutters our code. When compiling with clang, we even get a syntax error that indicates the compiler knows exactly what is going on:

example.cpp:11:54: error: lambda requires '()' before 'mutable'
auto noSean = [s2 = std::move(s2)] mutable { // Currently a syntax error.
                                   ^
                                   () 
1 error generated.

This proposal would make these parentheses unnecessary like they were before we added mutable. This will apply to:

• lambda template parameters

• constexpr

• mutable

• consteval

• Exception specifications and noexcept

• attributes

• trailing return types

• requires

3. Impact

This change will not break existing code.

4. Wording

Modify Lambda expressions [expr.prim.lambda] as follows:

lambda-expression :
lambda-introducer lambda-declarator_opt compound-statement
lambda-introducer < template-parameter-list > requires-clause_opt
lambda-declarator_opt compound-statement
lambda-introducer :
[ lambda-capture_opt ]
lambda-declarator:
lambda-specifiers
( parameter-declaration-clause ) decl-specifier-seq_opt lambda-specifiers
noexcept-specifier_opt attribute-specifier-seq_opt trailing-return-type_opt requires-clause_opt
lambda-specifiers:
decl-specifier-seq_optnoexcept-specifier_opt attribute-specifier-seq_opt trailing-return-type_opt

The _opt after lambda-declarator is removed because lambda-declarator can now be empty and the _opt would be redundant. lambda-specifiers is given a name to reduce redunancy in the grammar.

Modify ❡4 to clarify that all lambda-expressions now have a lambda-declarator but it may be empty:

If a lambda-expression does not include a includes an empty lambda-declarator, it is as if the lambda-declarator were (). [...]