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Regressed in SI-7915 / 3009a525b5
We should be deriving the position of the synthetic `Select`
from `basefun1`, rather than `basefun`. In the new, enclosed
test, the difference amounts to:
new Container().typeParamAndDefaultArg[Any]()
`------------ basefun1 --------------'
`----------------- basefun ---------------'
For monomorphic methods, these are one and the same, which is
why `presentation/t7915` was working. I've extended that test
to a polymorphic method to check that hyperlink resolution works.
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The tree created during expansion of default arguments contained trees with the
wrong type of positions. Let's discuss this with an example. Below is the tree
generated for the `foo` method in the test class included in this commit.
Before this commit:
```
[54:94]def foo(): [58]Unit = <70:90>{
[70:79]<artifact> val qual$1: [70]Bar = [70:79][70:79][70:79]new [74:77]Bar();
[80]<artifact> val x$1: [80]Int = [80]qual$1.bar$default$1;
<70:90><70:83>qual$1.bar([80]x$1)
}
```
Now:
```
[54:99]def foo(): [58]Unit = <70:95>{
<70:84><artifact> val qual$1: [70]Bar = [70:84][70:84][70:84]new [74:77]Bar();
[85]<artifact> val x$1: [85]Int = [85]qual$1.bar$default$1;
<70:95>[84:88]qual$1.bar([85]x$1)
}
```
Here are the list of changes:
* The synthetic `qual$1` has a transparent position, instead of a range position.
* The new Select tree (i.e., `qual$1.bar`) should always have a range position,
because `selected` (i.e., the called method) is always visible in the source
(in fact, this is the whole point of the fix, we need a range position or
hyperlinking request from the Scala IDE won't work).
* The Block that contains the expanded default arguments is forced to have a
transparent position, as it never exist in the original source.
The tricky part of the fix is the position assigned to the new Select tree,
which needs to respect the range position's invariants. In the specific case,
we ought to make sure that range positions don't overlap. Therefore, the position
assigned to the new Select tree is computed by intersecting the original Select
position (i.e., `baseFun`'s position) and the original qualifier's position
(i.e., `qual`'s position).
If you take a closer look at the range positions assigned in the tree after this
commit, you'll notice that the range position of the `qual$1`'s rhs (i.e.,
[70:84]), and `qual$1.bar` (i.e., [84:88]) might seem to overlap, because the
former ends where the latter begins. However, this not the case because of the
range position's invariant 2, which states:
> Invariant 2: in a range position, start <= point < end
Hence, the above two positions aren't overlapping as far as the compiler is
concerned.
One additional aspect (that may look like a detail) is that we make sure to
never generate a position such that its start is after its end. This is why we
take the position with the smallest end point.
Furthermore, calling `withStart` would turn any position in a range position,
which isn't desiderable in general (and, even worse, this can lead to
generation of invalid positions - bad offsets - if the computation is performed
on offset positions). Hence, the position's computation is only performed when
both `baseFun` and `qual` positions are range positions. Indeed, I expect this to
be always the case if the compiler is started with -Yrangepos.
(cherry picked from commit 3009a525b58a4c7865ff524899b85518884ee5f7)
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