SI-7584 Fix typer regression with by-name parameter types

It regressed in fada1ef6b#L4L614. Partially reverting just
this change restores the correct behaviour:

```
-        if (sym.isStable && pre.isStable && !isByNameParamType(tree.tpe) &&
+        if (treeInfo.admitsTypeSelection(tree) &&
```

This patch embeds the check for by-name parameter types into
`TreeInfo.isStableIdentifier`. That code already checks for
`Symbol#isStable`, which exludes direct references to by-name
parameters. But the additional check is required to deal with
by-name parameters in function types, e.g `(=> Int) => Any`.

Open question: should we go further and embed this check in `isStable`?

Currently:

    final def isStable        = isTerm && !isMutable && !(hasFlag(BYNAMEPARAM)) && (!isMethod || hasStableFlag)

Such function types are an underspecified corner of the language,
albeit one that is pretty useful writing, for example, in the
signature of a lazy foldRight that can operate over infinite
structures:

    def foldRight[A, B](fa: F[A], z: => B)(f: (A, => B) => B): B

The next commit subjects them to a little testing.

2 files changed, 19 insertions, 1 deletions

diff --git a/src/reflect/scala/reflect/internal/TreeInfo.scala b/src/reflect/scala/reflect/internal/TreeInfo.scala
index 3a8d3fd460..a5cf46071f 100644
--- a/src/reflect/scala/reflect/internal/TreeInfo.scala
+++ b/src/reflect/scala/reflect/internal/TreeInfo.scala
@@ -98,7 +98,7 @@ abstract class TreeInfo {
    */
   def isStableIdentifier(tree: Tree, allowVolatile: Boolean): Boolean =
     tree match {
-      case Ident(_)        => symOk(tree.symbol) && tree.symbol.isStable && !tree.symbol.hasVolatileType // TODO SPEC: not required by spec
+      case i @ Ident(_)    => isStableIdent(i)
       case Select(qual, _) => isStableMemberOf(tree.symbol, qual, allowVolatile) && isPath(qual, allowVolatile)
       case Apply(Select(free @ Ident(_), nme.apply), _) if free.symbol.name endsWith nme.REIFY_FREE_VALUE_SUFFIX =>
         // see a detailed explanation of this trick in `GenSymbols.reifyFreeTerm`
@@ -119,6 +119,13 @@ abstract class TreeInfo {
     typeOk(tree.tpe) && (allowVolatile || !hasVolatileType(tree)) && !definitions.isByNameParamType(tree.tpe)
   )
 
+  private def isStableIdent(tree: Ident): Boolean = (
+       symOk(tree.symbol)
+    && tree.symbol.isStable
+    && !definitions.isByNameParamType(tree.tpe)
+    && !tree.symbol.hasVolatileType // TODO SPEC: not required by spec
+  )
+
   /** Is `tree`'s type volatile? (Ignored if its symbol has the @uncheckedStable annotation.)
    */
   def hasVolatileType(tree: Tree): Boolean =
diff --git a/test/files/pos/t7584.scala b/test/files/pos/t7584.scala
new file mode 100644
index 0000000000..52d127ecb9
--- /dev/null
+++ b/test/files/pos/t7584.scala
@@ -0,0 +1,11 @@
+object Test {
+  def fold[A, B](f: (A, => B) => B) = ???
+  def f[A, B](x: A, y: B): B = ???
+  def bip[A, B] = fold[A, B]((x, y) => f(x, y))
+  def bop[A, B] = fold[A, B](f)
+
+  // these work:
+  fold[Int, Int]((x, y) => f(x, y))
+  fold[Int, Int](f)
+}
+