Subclause 12.2.2.2.3 [over.call.object] paragraph 2 considers only those conversion funtions matching a particular grammar pattern. This unintendedly excludes conversion functions with an explicit object parameter (and, as a pre-existing defect, noexcept conversion functions):

In addition, for each non-explicit conversion function declared in T of the form

operator conversion-type-id ( ) cv-qualifier-seqopt ref-qualifieropt noexcept-specifieropt attribute-specifier-seqopt ;

where the optional cv-qualifier-seq is the same cv-qualification as, or a greater cv-qualification than, cv, and where conversion-type-id denotes the type “pointer to function of (P1 , . . . , Pn ) returning R”, or the type “reference to pointer to function of (P1 , . . . , Pn ) returning R”, or the type “reference to function of (P1 , . . . , Pn ) returning R”, a surrogate call function with the unique name call-function and having the form

R call-function ( conversion-type-id F, P1 a1 , ... , Pn an ) { return F (a1 , . . . , an ); }

is also considered as a candidate function. Similarly, surrogate call functions are added to the set of candidate functions for each non-explicit conversion function declared in a base class of T provided the function is not hidden within T by another intervening declaration. [ Footnote: ...]

For example, there is implementation divergence in handling this example:

  using fn_t = void();
  struct C {
    operator fn_t * (this C const &);
  };

  void foo(C c) {
    c();
  }