Rule out more implicit based on bounds of type parameters

Before:

```
implicitly[Foo[String]] BYVALmode-EXPRmode (site: value <local Test> in Test)
 |-- implicitly BYVALmode-EXPRmode-FUNmode-POLYmode-TAPPmode (site: value <local Test> in Test)
 |    \-> [T](implicit e: T)T
 |-- Foo[String] TYPEmode (site: value <local Test> in Test)
 |    |-- String TYPEmode (site: value <local Test> in Test)
 |    |    [adapt] String is now a TypeTree(String)
 |    |    \-> String
 |    \-> Foo[String]
 [search #1] start `[T](implicit e: T)T`, searching for adaptation to pt=Foo[String] (silent: value <local Test> in Test) implicits disabled
 [search #1] considering Foo.javaEnumFoo
 solving for (T: ?T)
 [adapt] [T]=> Foo[T] adapted to [T]=> Foo[T] based on pt Foo[String]
 [search #1] success inferred value of type Foo[String] is SearchResult(Foo.javaEnumFoo[String], )
 |-- [T](implicit e: T)T BYVALmode-EXPRmode (site: value <local Test> in Test)
 |    \-> Foo[String]
 [adapt] [T](implicit e: T)T adapted to [T](implicit e: T)T
```

After:

```
implicitly[Foo[String]] BYVALmode-EXPRmode (site: value <local Test> in Test)
 |-- implicitly BYVALmode-EXPRmode-FUNmode-POLYmode-TAPPmode (site: value <local Test> in Test)
 |    \-> [T](implicit e: T)T
 |-- Foo[String] TYPEmode (site: value <local Test> in Test)
 |    |-- String TYPEmode (site: value <local Test> in Test)
 |    |    [adapt] String is now a TypeTree(String)
 |    |    \-> String
 |    \-> Foo[String]
 [search #1] start `[T](implicit e: T)T`, searching for adaptation to pt=Foo[String] (silent: value <local Test> in Test) implicits disabled
 [search #1] considering Foo.stringFoo
 [search #1] success inferred value of type Foo[String] is SearchResult(Foo.stringFoo, )
 |-- [T](implicit e: T)T BYVALmode-EXPRmode (site: value <local Test> in Test)
 |    \-> Foo[String]
 [adapt] [T](implicit e: T)T adapted to [T](implicit e: T)T
 \-> Foo[String]

```

(cherry picked from commit 68799b9)

Author: retronym

src/compiler/scala/tools/nsc/typechecker/Implicits.scala

@@ -617,17 +617,31 @@ trait Implicits {
     /** This expresses more cleanly in the negative: there's a linear path
      *  to a final true or false.
      */
-    private def isPlausiblySubType(tp1: Type, tp2: Type) = !isImpossibleSubType(tp1, tp2)
-    private def isImpossibleSubType(tp1: Type, tp2: Type) = tp1.dealiasWiden match {
+    private def isPlausiblySubType(tp1: Type, tp2: Type): Boolean = !isImpossibleSubType(tp1, tp2)
+    private def isImpossibleSubType(tp1: Type, tp2: Type): Boolean = tp1.dealiasWiden match {
       // We can only rule out a subtype relationship if the left hand
       // side is a class, else we may not know enough.
-      case tr1 @ TypeRef(_, sym1, _) if sym1.isClass =>
+      case tr1 @ TypeRef(_, sym1, args1) if sym1.isClass =>
         def typeRefHasMember(tp: TypeRef, name: Name) = {
           tp.baseClasses.exists(_.info.decls.lookupEntry(name) != null)
         }
-        tp2.dealiasWiden match {
-          case TypeRef(_, sym2, _)         => ((sym1 eq ByNameParamClass) != (sym2 eq ByNameParamClass)) || (sym2.isClass && !(sym1 isWeakSubClass sym2))
-          case RefinedType(parents, decls) => decls.nonEmpty && !typeRefHasMember(tr1, decls.head.name) // opt avoid full call to .member
+
+        def existentialUnderlying(t: Type) = t match {
+          case et: ExistentialType => et.underlying
+          case tp => tp
+        }
+        val tp2Bounds = existentialUnderlying(tp2.dealiasWiden.bounds.hi)
+        tp2Bounds match {
+          case TypeRef(_, sym2, args2) if sym2 ne SingletonClass =>
+            val impossible = if ((sym1 eq sym2) && (args1 ne Nil)) !corresponds3(sym1.typeParams, args1, args2) {(tparam, arg1, arg2) =>
+              if (tparam.isCovariant) isPlausiblySubType(arg1, arg2) else isPlausiblySubType(arg2, arg1)
+            } else {
+              ((sym1 eq ByNameParamClass) != (sym2 eq ByNameParamClass)) || (sym2.isClass && !(sym1 isWeakSubClass sym2))
+            }
+            impossible
+          case RefinedType(parents, decls) =>
+            val impossible = decls.nonEmpty && !typeRefHasMember(tr1, decls.head.name) // opt avoid full call to .member
+            impossible
           case _                           => false
         }
       case _ => false

test/files/pos/implicit-implausible.scala

@@ -0,0 +1,12 @@
+trait Foo[T]
+object Foo {
+	implicit def javaEnumFoo[T <: java.lang.Enum[_]]: Foo[T] = ???
+	implicit def stringFoo: Foo[String] = ???
+}
+
+object Test {
+	// -Ytyper-debug output shows whether or not `javaEnumFoo` is considered
+	// By making `isImpossibleSubtype` a little smarter, we can exclude it
+	// on the grounds that `String` can't be a subtpe of the bounds ot `Enum[_]`.
+	implicitly[Foo[String]]
+}

test/files/pos/sip23-singleton-view.scala

@@ -0,0 +1,6 @@
+import language.implicitConversions
+
+class Test {
+  implicit def singletonToString(c: Singleton): String = ""
+  def foo(a: 1): String = a // implicit was being ruled out because Int(1).widen was not a subclass of Singletom
+}