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.
We don't use the following hack anymore:
- /- HACK: `fvarId` is not in the scope of `mvarId`
- If this generates problems in the future, we should update the metavariable declarations. -/
- assignExprMVar mvarId (mkFVar fvarId)
This hack was corrupting the `InfoTree`.
The unifier used to implement the `cases` tactic should not discard
equations of the form `x = t` and `t = x` using proof irrelanvance.
The new test demonstrates the issue. The unifier was reaching the
state
```
x : Conw Con.nil
|- x = Conw.nilw -> x = Conw.nilw
```
and discarding the equality instead of substituting `x`
because `x` and `Conw.nilw` are definionally equal due to
proof irrelevance.
@javra Do you have more complicated examples that were being
affected by this issue?
@Kha We will probably have to refine the heuristic for hiding the
inaccessible names, but the first version is already useful.
Here is the error message for a `_` before this commit
```
error: don't know how to synthesize placeholder
context:
x✝⁴ : List Nat
x✝³ : Nat
x✝² : x✝⁴ ≠ []
a b x✝¹ : Nat
x✝ : [a, b] ≠ []
⊢ Nat
```
After
```
error: don't know how to synthesize placeholder
a b : Nat
: [a, b] ≠ []
⊢ Nat
```
