warning: a pure expression does nothing in statement position

[vlovgr]

The following markdown document:

```scala mdoc
final case class Test(value: Int)

val test = Test(123)

test.value
```

```scala mdoc:fail
val x: Int = "123"
```

yields the following output with mdoc:

```scala
final case class Test(value: Int)

val test = Test(123)
// test: Test = Test(123)

test.value
// res0: Int = 123
```

```scala
val x: Int = "123"
// warning: a pure expression does nothing in statement position; multiline     expressions may require enclosing parentheses
// test.value
// ^^^^^^^^^^
// error: type mismatch;
//  found   : String("123")
//  required: Int
// val x: Int = "123"
//              ^^^^^
```

where the following warning is unexpected.

warning: a pure expression does nothing in statement position; multiline     expressions may require enclosing parentheses
test.value
^^^^^^^^^^

[olafurpg]

Thanks for reporting! This issue is caused by the different compilation encoding used for :fail fences and default fences. In default fences, every statement is bound to a variable like

val res0 = test.value

avoiding the error. We should do the same transformation for non-:fail blocks.

[olafurpg]

Alternatively, we could filter out diagnostics that don't point to the :fail block, that might be simpler but risks hiding useful errors.

[olafurpg]

Pending fix in #154

[olafurpg]

We still report errors from non-fail blocks but ignore warnings unless they origin from the fail block.