I believe I've understood the issue, but if I've misunderstood then I feel extra need to comment, that my misunderstanding might be clarified.
From my first reading of what you'd written, tome seemed spot on.
On careful re-examination (after reading this comment), I think when you said "Things you don't receive warnings for:" you did not mean to say that you can write that code and receive no warnings (a reasonable interpretation), but that the warnings would not read as directed at the points you raise (also a reasonable interpretation) - which (you note) are substantially more important than "this is generally a good idea."
Firstly, one tends to believe that one understands before one comments even if one actually doesn't, so admonishment to understand before commenting will typically be redundant. Secondly, if one doesn't understand, blame can often lie on both sides of the conversation.
I claim that if you never have orphan instance warnings then your code has "the guarantees Kmett keeps boasting about".
In particular you can never "define the same instance twice", and this of course implies that you cannot "have code which uses both instances".
What exactly is your complaint and why does my claim not address it?
I believe I've understood the issue, but if I've misunderstood then I feel extra need to comment, that my misunderstanding might be clarified.
From my first reading of what you'd written, tome seemed spot on.
On careful re-examination (after reading this comment), I think when you said "Things you don't receive warnings for:" you did not mean to say that you can write that code and receive no warnings (a reasonable interpretation), but that the warnings would not read as directed at the points you raise (also a reasonable interpretation) - which (you note) are substantially more important than "this is generally a good idea."