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.

4 files changed, 41 insertions, 0 deletions

diff --git a/test/files/neg/t6815.check b/test/files/neg/t6815.check
new file mode 100644
index 0000000000..fae3819be1
--- /dev/null
+++ b/test/files/neg/t6815.check
@@ -0,0 +1,5 @@
+t6815.scala:15: error: stable identifier required, but Test.this.u.emptyValDef found.
+ Note that value emptyValDef is not stable because its type, Test.u.ValDef, is volatile.
+    case _: u.emptyValDef.T => // and, unlike in pos/t6185.scala, we shouldn't allow this.
+              ^
+one error found
diff --git a/test/files/neg/t6815.scala b/test/files/neg/t6815.scala
new file mode 100644
index 0000000000..ff973a7437
--- /dev/null
+++ b/test/files/neg/t6815.scala
@@ -0,0 +1,17 @@
+trait U {
+  trait ValOrDefDefApi {
+    def name: Any
+  }
+  type ValOrDefDef <: ValOrDefDefApi
+  type ValDef <: ValOrDefDef with ValDefApi { type T }
+  trait ValDefApi extends ValOrDefDefApi { this: ValDef => }
+  val emptyValDef: ValDef // the result type is volatile
+}
+
+object Test {
+  val u: U = ???
+
+  (null: Any) match {
+    case _: u.emptyValDef.T => // and, unlike in pos/t6185.scala, we shouldn't allow this.
+  }
+}
diff --git a/test/files/neg/volatile_no_override.check b/test/files/neg/volatile_no_override.check
new file mode 100644
index 0000000000..a9a60ab697
--- /dev/null
+++ b/test/files/neg/volatile_no_override.check
@@ -0,0 +1,5 @@
+volatile_no_override.scala:13: error: overriding value x in class A of type Volatile.this.D;
+ value x has a volatile type; cannot override a member with non-volatile type
+  val x: A with D = null
+      ^
+one error found
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
+}