Checks that constructors callable with a single argument and conversion operators are marked explicit to avoid the risk of unintentional implicit conversions.

Consider this example:

struct S {
  int x;
  operator bool() const { return true; }

bool f() {
  S a{1};
  S b{2};
  return a == b;

The function will return true, since the objects are implicitly converted to bool before comparison, which is unlikely to be the intent.

The check will suggest inserting explicit before the constructor or conversion operator declaration. However, copy and move constructors should not be explicit, as well as constructors taking a single initializer_list argument.

This code:

struct S {
  S(int a);
  explicit S(const S&);
  operator bool() const;

will become

struct S {
  explicit S(int a);
  S(const S&);
  explicit operator bool() const;