Track Function's SAM symbol & target type using an attachment

We cannot use the expected type to track whether a Function node
targets a SAM type, as the expected type may be erased (see test
for an example).

Thus, the type checker attaches a SAMFunction attachment to a
Function node when SAM conversion is performed in adapt. Ideally,
we'd move to Dotty's Closure AST, but that will need a
deprecation cycle.

Thanks to Jason for catching my mistake, suggesting the fix and
providing the test.

Both the sam method symbol and sam target type must be tracked,
as their relationship can be complicated (due to inheritance).

For example, the sam method could be defined in a superclass (T)
of the Function's target type (U).

```
trait T { def foo(a: Any): Any }
trait U extends T { def apply = ??? }

(((x: Any) => x) : U).foo("")
```

This removes some of the duplication in deriving the sam method
from the expected type, but some grossness (see TODO) remains.

2 files changed, 14 insertions, 0 deletions

diff --git a/src/reflect/scala/reflect/internal/StdAttachments.scala b/src/reflect/scala/reflect/internal/StdAttachments.scala
index 8358c1295c..0243dd48d2 100644
--- a/src/reflect/scala/reflect/internal/StdAttachments.scala
+++ b/src/reflect/scala/reflect/internal/StdAttachments.scala
@@ -38,6 +38,19 @@ trait StdAttachments {
    */
   case class CompoundTypeTreeOriginalAttachment(parents: List[Tree], stats: List[Tree])
 
+  /** Attached to a Function node during type checking when the expected type is a SAM type (and not a built-in FunctionN).
+    *
+    * Ideally, we'd move to Dotty's Closure AST, which tracks the environment,
+    * the lifted method that has the implementation, and the target type.
+    * For backwards compatibility, an attachment is the best we can do right now.
+    *
+    * @param samTp the expected type that triggered sam conversion (may be a subtype of the type corresponding to sam's owner)
+    * @param sam the single abstract method implemented by the Function we're attaching this to
+    *
+    * @since 2.12.0-M4
+    */
+  case class SAMFunction(samTp: Type, sam: Symbol) extends PlainAttachment
+
   /** When present, indicates that the host `Ident` has been created from a backquoted identifier.
    */
   case object BackquotedIdentifierAttachment extends PlainAttachment
diff --git a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
index 13874916cc..4630597668 100644
--- a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
+++ b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
@@ -37,6 +37,7 @@ trait JavaUniverseForce { self: runtime.JavaUniverse  =>
     this.FixedMirrorTreeCreator
     this.FixedMirrorTypeCreator
     this.CompoundTypeTreeOriginalAttachment
+    this.SAMFunction
     this.BackquotedIdentifierAttachment
     this.ForAttachment
     this.SyntheticUnitAttachment