Proposed resolution (approved by CWG 2022-11-08):

Change in 9.3.4.6 [dcl.fct] paragraph 3 as follows:

parameter-declaration:
    attribute-specifier-seqopt thisopt decl-specifier-seq declarator
    attribute-specifier-seqopt thisopt decl-specifier-seq declarator = initializer-clause
    attribute-specifier-seqopt thisopt decl-specifier-seq abstract-declaratoropt
    attribute-specifier-seqopt thisopt decl-specifier-seq abstract-declaratoropt = initializer-clause

[Accepted at the November, 2022 meeting.]

The syntax and semantics appear to allow:

  struct S { void f(this const S& = S{}); };