Proposed resolution (December, 2021):

1. Change the grammar in 7.5.6.1 [expr.prim.lambda.general] as follows:

lambda-specifiers:
decllambda-specifier-seq_opt noexcept-specifier_opt attribute-specifier-seq_opt trailing-return-type_opt
lambda-specifier:
consteval
constexpr
mutable
lambda-specifier-seq:
lambda-specifier
lambda-specifier lambda-specifier-seq

2. Change 7.5.6.1 [expr.prim.lambda.general] paragrap 3 as follows:

In the decl-specifier-seq of the lambda-declarator, each decl-specifier shall be one of mutable, constexpr, or consteval. A lambda-specifier-seq shall contain at most one of each lambda-specifier and shall not contain both constexpr and consteval. If the lambda-declarator contains an explicit object parameter (9.3.4.6 [dcl.fct]), then no decllambda-specifier in the decllambda-specifier-seq shall be mutable.

[Accepted at the February, 2022 meeting.]

(From editorial issue 2338.)

Use of decl-specifier-seq in the production for lambda-specifiers is too general and should be restricted.