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, 16 insertions, 0 deletions

diff --git a/test/files/pos/t6815_import.scala b/test/files/pos/t6815_import.scala
new file mode 100644
index 0000000000..56f4358d59
--- /dev/null
+++ b/test/files/pos/t6815_import.scala
@@ -0,0 +1,16 @@
+trait U {
+  trait ValOrDefDefApi {
+    def name: Any
+  }
+  type ValOrDefDef <: ValOrDefDefApi
+  type ValDef <: ValOrDefDef with ValDefApi
+  trait ValDefApi extends ValOrDefDefApi { this: ValDef => }
+  val emptyValDef: ValDef // the result type is volatile
+}
+
+object Test {
+  val u: U = ???
+
+  // but we shouldn't let that stop us from treating it as a stable identifier for import
+  import u.emptyValDef.name
+}