Proposed resolution:

lambda-expression:
      lambda-introducer lambda-declaratoropt compound-statement
...

lambda-declarator:
      ( parameter-declaration-clause ) mutableopt
            exception-specificationopt attribute-specifier-seqopt trailing-return-typeopt
      mutableopt exception-specificationopt attribute-specifier-seqopt trailing-return-typeopt

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

This function call operator or operator template is declared const (9.3.1) if and only if the lambda-expression's parameter-declaration-clause is not followed by lambda-declarator does not contain the keyword mutable.

It has been reported that various people have noticed that it's possible to write

auto lambda = []{};

but not

auto lambda2 = [] mutable {};

In the mutable case, parentheses are required, thus:

auto lambda3 = []() mutable {};

The proposed consistency fix is to change the grammar to allow omitting the parentheses for mutable lambdas as well.

An additional question EWG needs to decide is whether the empty parentheses could be omitted for other cases besides mutable, such as

[] -> float {return 42;};
[] noexcept {foo();};