even when rewriting the type of `h` becuase there is no expected type.
(When there is an expected type, it already tried both orientations.)
Also feeble attempt to include this information in the docstring without
writing half a manual chapter.
The subst notation substitues in the expected type, if present, or in
the type of the argument, if no expected type is known.
If there is an expected type it already fails if it cannot find the
equations' left hand side or right hand side. But if the expected type
is not known and the equation's lhs is not present in the second
argument's type, it will happily do a no-op-substitution.
This is inconsistent and unlikely what the user intended to do, so we
now print an error message now.
This still only looks for the lhs; search for the rhs as well seems
prudent, but I’ll leave that for a separate PR, to better diagnose the
impact on mathlib.
This triggers a small number of pointless uses of subst in mathlib, see
https://github.com/leanprover-community/mathlib4/pull/12451
