(As raised on the core reflector.)

We can simplify the lambda-expression grammar by

• moving the requires-clause[opt] following the lambda-declarator into the lambda-declarator production,
• making the lambda-declarator optional, and
• deleting the (now-redundant) productions that do not contain an optional lambda-declarator

(We could reduce to a single production by moving the template parameter list to its own production, but it's unclear that that's worthwhile.) That should leave:

lambda-expression:
    lambda-introducer lambda-declarator[opt] compound-statement
    lambda-introducer < template-parameter-list > requires-clause[opt] lambda-declarator[opt] compound-statement

lambda-declarator:
    ( parameter-declaration-clause ) decl-specifier-seq[opt] noexcept-specifier[opt]
attribute-specifier-seq[opt] trailing-return-type[opt] requires-clause[opt]

We will also need to update [expr.prim.lambda.closure]/3, changing "the requires-clause following the lambda-declarator" to "the requires-clause of the lambda-declarator".