SI-9131 Fix use of apply syntactic sugar with by-name param

After typechecking a tree, the typer adapts it to the current
mode and expected type. If we are in FUNmode (typechecking the
qualifier of a value- or type-application), and the tree does not
already have a MethodType or PolyType, it is reinterepreted as
`qual.apply`.

In doing so, `insertApply` stabilizes the type of `qual`, e.g.
replacing `Ident(foo).setType(typeOf[Int])` with
`Ident(foo).setType(typeOf[foo.type])`.

However, this does not check for by-name parameters, which cannot
form the basis for a singleton type, as we can see by trying that
directly:

```
scala> def foo(a: => String) { type T = a.type }
<console>:7: error: stable identifier required, but a.type found.
       def foo(a: => String) { type T = a.type }
                                         ^
```

When I last touched this code in SI-6206 / 267650cf9, I noted:

    // TODO reconcile the overlap between Typers#stablize and TreeGen.stabilize

I didn't get around to that, but Adriaan gave that code a thorough
cleanup in fada1ef6b.

It appears that on the back of his work, we can now remove the local
stabilization logic in `insertApply` in favour of `TreeGen.stabilize`.
We then avoid the ill-formed singleton type, and the spurious
"apply is not a member" type error.

I did have to modify `isStableIdent` to check the symbol's info
in addition to the tree's type for by-name-ness.

2 files changed, 2 insertions, 18 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 2dd79075ee..e883bacc88 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -918,24 +918,7 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
       def insertApply(): Tree = {
         assert(!context.inTypeConstructorAllowed, mode) //@M
         val adapted = adaptToName(tree, nme.apply)
-        def stabilize0(pre: Type): Tree = stabilize(adapted, pre, MonoQualifierModes, WildcardType)
-
-        // TODO reconcile the overlap between Typers#stablize and TreeGen.stabilize
-        val qual = adapted match {
-          case This(_) =>
-            gen.stabilize(adapted)
-          case Ident(_) =>
-            val owner = adapted.symbol.owner
-            val pre =
-              if (owner.isPackageClass) owner.thisType
-              else if (owner.isClass) context.enclosingSubClassContext(owner).prefix
-              else NoPrefix
-            stabilize0(pre)
-          case Select(qualqual, _) =>
-            stabilize0(qualqual.tpe)
-          case other =>
-            other
-        }
+        val qual = gen.stabilize(adapted)
         typedPos(tree.pos, mode, pt) {
           Select(qual setPos tree.pos.makeTransparent, nme.apply)
         }
diff --git a/src/reflect/scala/reflect/internal/TreeInfo.scala b/src/reflect/scala/reflect/internal/TreeInfo.scala
index 4657fa0000..7ad5fdf096 100644
--- a/src/reflect/scala/reflect/internal/TreeInfo.scala
+++ b/src/reflect/scala/reflect/internal/TreeInfo.scala
@@ -128,6 +128,7 @@ abstract class TreeInfo {
        symOk(tree.symbol)
     && tree.symbol.isStable
     && !definitions.isByNameParamType(tree.tpe)
+    && !definitions.isByName(tree.symbol)
     && (allowVolatile || !tree.symbol.hasVolatileType) // TODO SPEC: not required by spec
   )