[PATCH] D97990: [clang] Always provide relevant subobject for 'no viable comparison'

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added a comment.

I don't think this change is right: for

  struct A {
    int &r;
    bool operator==(const A&) const = default;
  };

we should warn about the reference member (as we do), but for

  struct A {
    int &r;
    bool operator<(const A&) const = default;
  };

... we shouldn't recurse to subobjects because they make no difference. The problem in that example is that there's no `operator<=>` for `A`; the fact that there's a reference member is irrelevant. I think it would be useful to change the diagnostic from

  <stdin>:3:8: warning: explicitly defaulted relational comparison operator is implicitly deleted [-Wdefaulted-function-deleted]
    bool operator<(const A &) const = default;
         ^
  <stdin>:3:8: note: defaulted 'operator<' is implicitly deleted because there is no viable comparison function

to something more verbose, such as

  <stdin>:3:8: warning: explicitly defaulted relational comparison operator is implicitly deleted [-Wdefaulted-function-deleted]
    bool operator<(const A &) const = default;
         ^
  <stdin>:3:8: note: defaulted 'operator<' is implicitly deleted because there is no viable three-way comparison function for 'A'


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https://reviews.llvm.org/D97990