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, 4 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
index 290ad76a1d..bbba786c0c 100644
--- a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
@@ -511,7 +511,10 @@ abstract class RefChecks extends InfoTransform with scala.reflect.internal.trans
             }
 
             if (member.isStable && !otherTp.isVolatile) {
-	            if (memberTp.isVolatile)
+              // (1.4), pt 2 -- member.isStable && memberTp.isVolatile started being possible after SI-6815
+              // (before SI-6815, !symbol.tpe.isVolatile was implied by symbol.isStable)
+              // TODO: allow overriding when @uncheckedStable?
+              if (memberTp.isVolatile)
                 overrideError("has a volatile type; cannot override a member with non-volatile type")
               else memberTp.dealiasWiden.resultType match {
                 case rt: RefinedType if !(rt =:= otherTp) && !(checkedCombinations contains rt.parents) =>