SI-6815 untangle isStable and hasVolatileType

`Symbol::isStable` is now independent of `Symbol::hasVolatileType`,
so that we can allow stable identifiers that are volatile in ident patterns.

This split is validated by SI-6815 and the old logic in RefChecks,
which seems to assume this independence, and thus I don't think ever worked:

```
if (member.isStable && !otherTp.isVolatile) {
  if (memberTp.isVolatile)
    overrideError("has a volatile type; cannot override a member with non-volatile type")
```

Introduces `admitsTypeSelection` and `isStableIdentifierPattern` in treeInfo,
and uses them instead of duplicating that logic all over the place.

Since volatility only matters in the context of type application,
`isStableIdentifierPattern` is used to check patterns (resulting in `==` checks)
and imports.

1 files changed, 14 insertions, 0 deletions

diff --git a/test/files/neg/volatile_no_override.scala b/test/files/neg/volatile_no_override.scala
new file mode 100644
index 0000000000..9fad082a90
--- /dev/null
+++ b/test/files/neg/volatile_no_override.scala
@@ -0,0 +1,14 @@
+class B
+class C(x: String) extends B
+
+abstract class A {
+  class D { type T >: C <: B }
+  val x: D
+  var y: x.T = new C("abc")
+}
+
+class Volatile extends A {
+  type A >: Null
+  // test (1.4), pt 2 in RefChecks
+  val x: A with D = null
+}