coqc -beautify outputs syntactically and semantically invalid code

[Samuel Gruetter]

$ cat toBeautify.v

Inductive Foo := Foo1 | Foo2 | Foo3.

Definition isFoo12(f: Foo): bool :=
  match f with
  | Foo1 => true
  | Foo2 => true
  | _ => false
  end.

$ coqc -beautify ./toBeautify.v

$ cat toBeautify.v.beautified

Inductive Foo :=
  | Foo1 : _
  | Foo2 : _
  | Foo3 : _.Definition isFoo12 (f : Foo) :

  bool := match f with
	  | _ => false
	  | _ => true
	  end.

$ mv toBeautify.v.beautified toBeautify2.v

$ coqc toBeautify2.v

File "./toBeautify2.v", line 4, characters 12-23:
Error: Syntax error: '.' expected after [vernac:gallina] (in [vernac_aux]).

(edit ./toBeautify2.v to fix the syntax error)

$ coqc toBeautify2.v

File "./toBeautify2.v", line 9, characters 12-21:
Error: Pattern "_" is redundant in this clause.

$ coqc --version

The Coq Proof Assistant, version 8.11+alpha (November 2019)
compiled on Nov 4 2019 14:19:52 with OCaml 4.09.0

[Gaëtan Gilbert]

When beautifying we pass terms through intern/extern (not sure why).
When externing the match, we select Foo1 | Foo2 => true as the default clause (because 2 cases becoming an underscore is better than 1), but this causes ambiguity with the already present default clause.
Usually we extern glob_constr which come from constr, such that they have no default clause, so this isn't an issue.
I guess we could add a check in select_default_clause that there isn't already one. Not sure if this would leave related issues.