This is a hard coded extra case. It is not an instance of has_coe.
Even if we change has_coe to accomodate this case, it will not be a
satisfactory solution because this coercion depends on the element and
not the type, and the element usually contains metavariables.
We should eventually write a tactic for synthesizing coercions.
@Kha, we now support variable/constant shadowing in patterns.
A constant may occur in a pattern if it is a constructor or tagged with
the new [pattern] attribute. In the standard library, I have tagged
'add', 'zero', 'one', 'bit0', 'bit1' and 'rfl' with this new attribute.
BTW, arbitrary constants and variables may occur nested in type ascriptions and
inaccessible terms.
Here is an example:
meta_definition tactic_result_to_string {A : Type} : tactic_result A → string
| (success a s) := to_string a
| (exception ⌞A⌟ e s) := "Exception: " ++ to_string (e ())
I had to use the inaccessible ⌞A⌟ in the example above, otherwise, we would be shadowing the parameter
{A : Type}, and we would get a type error.
The new validation is performed at to_pattern_fn (parser.cpp).
