Users may add the expensive instances if they want.
The goal is to make sure the default configuration is solid, and
covers all examples we really want to support.
cc @kha @dselsam
The main issue is nontermination for problems that do not have
solution. When using dependent coercions, we keep creating goals of
the form `CoeSort ... (coe (coe (coe ...))) ...`. Same for `CoeFun`.
I am considering simplifying it even further, and making sure
`CoeDep` can be used at most once in a sequence of coercions `CoeTC`.
Another option is to use a very small amount of fuel to
guarantee termination when solving coercion TC problems.
cc @Kha @dselsam
@Kha I didn't find any workaround for the instance
```lean
instance coeFunTrans {α : Sort u} {β : Sort v} {γ : Sort w} (a : α) [CoeT α a β] [CoeFun β (coe a) γ] : CoeFun α a γ :=
```
`γ` is an output param. Thus, it is just a metavariable, and if we use
the default order, the subgoal `CoeFun ?β (coe ?a) ?γ` is really bad.
We need to go from left to right here.
The macOS CI build rarely, randomly fails with a "failed to lock file" message.
This commit will not fix this error, but at least should make it clearer whether
the file actually cannot be locked or is just missing.
/cc @leodemoura
@Kha Patterns with nested "choice" nodes produce counterintuitive
behavior, and hard to understand errors. They only work when we have
the exact same overloads at macro definition and application time.
Here is a funky example
```
syntax [myAdd] term "++":65 term:65 : term -- overloads "++"
/- The following `macro_rules` manages to eliminate "choice" node by using the
explicitly provided node kind. -/
macro_rules [myAdd] `($a ++ $b) => `($a + $b)
check (1:Nat) ++ 2 -- works as expected
syntax "FOO!" term : term
/- Before this commit, the following pattern was accepted,
and it contained a "choice" node for `++`. It is a `myAdd` or
`HasAppend.append`.
Now, this kind of pattern is rejected since it contains a nested
"choice" -/
macro_rules `(FOO! $a ++ $b) => `($a ++ $b)
/- Before this commit, the following command worked because
it contained a "choice" node similar to the one at macro definition
time. -/
check FOO! [1, 2] ++ [2, 3]
-- Now, we have 3 overloads for "++".
syntax [myAppend] term "++":65 term:65 : term
-- The `macro_rules` fails here.
```
This commit fixes this weird behavior by disallowing "choice" nodes in
patterns.
