Add constr_fail and constr_fail_with

Rather than taking the convention that failures during constr
construction emit a type error from `I : I` with the actual error
message `idtac`d above them (because Coq has no way to emit multiple
things on stderr), we instead factor everything through a new
`constr_fail` or `constr_fail_with msg_tac`, which emit more helpful
messages instructing the user to look in `*coq*` or to use `Fail`/`try`
to see the more informative error message.  When we can make our own
version that does both `idtac` and `fail` (c.f.
https://github.com/coq/coq/issues/3913), then we can do something a bit
more sane, hopefully.

1 files changed, 9 insertions, 0 deletions

diff --git a/src/Util/Tactics/ConstrFail.v b/src/Util/Tactics/ConstrFail.v
new file mode 100644
index 000000000..fe8a05630
--- /dev/null
+++ b/src/Util/Tactics/ConstrFail.v
@@ -0,0 +1,9 @@
+(** A tactic that executes immediately (during expression evaluation / constr-construction) and fails.  Ideally we can eventually give it a nicer error message.  COQBUG(3913) *)
+
+Ltac constr_fail :=
+  let __ := match goal with _ => fail 1 "Constr construction failed.  Please look at the message log (*coq*, or run your tactic again inside Fail or try) to see more details" end in
+  ().
+
+Ltac constr_fail_with msg_tac :=
+  let __ := match goal with _ => msg_tac () end in
+  constr_fail.