This PR improves the type-as-hole error message. Type-as-hole error for
theorem declarations should not admit the possibility of omitting the
type entirely.
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Co-authored-by: Joachim Breitner <mail@joachim-breitner.de>
When an implicit argument cannot be inferred, the error should show the
name of the argument.
Showing the argument name in the error message for an uninstantiated
metavariable was introduced in da33f498f5,
but this implementation causes some argument names to get lost.
Also consistently use binders as reference position for an elided binder type.
Before, type errors were always given extent 1, the length of the
synthetic `_` token.
