SI-7788 Avoid accidental shadowing of Predef.conforms

Rename `conforms` to `$conforms` and put in a minimal backstop: pos/t7788.scala

TODO: predicate the backwards compatibility shim for `Predef_conforms`
on `-Xsource:2.10`

4 files changed, 11 insertions, 6 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
index 8f5778862d..42687fa7b1 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
@@ -807,7 +807,7 @@ trait Implicits {
 
       private def isIneligible(info: ImplicitInfo) = (
            info.isCyclicOrErroneous
-        || isView && (info.sym eq Predef_conforms)
+        || isView && (info.sym eq Predef_conforms) // as an implicit conversion, Predef.$conforms is a no-op, so exclude it
         || (!context.macrosEnabled && info.sym.isTermMacro)
       )
 
diff --git a/src/library/scala/Predef.scala b/src/library/scala/Predef.scala
index 2bde6795e0..faeb1dcbe2 100644
--- a/src/library/scala/Predef.scala
+++ b/src/library/scala/Predef.scala
@@ -382,9 +382,13 @@ object Predef extends LowPriorityImplicits with DeprecatedPredef {
   @implicitNotFound(msg = "Cannot prove that ${From} <:< ${To}.")
   sealed abstract class <:<[-From, +To] extends (From => To) with Serializable
   private[this] final val singleton_<:< = new <:<[Any,Any] { def apply(x: Any): Any = x }
-  // not in the <:< companion object because it is also
-  // intended to subsume identity (which is no longer implicit)
-  implicit def conforms[A]: A <:< A = singleton_<:<.asInstanceOf[A <:< A]
+  // The dollar prefix is to dodge accidental shadowing of this method
+  // by a user-defined method of the same name (SI-7788).
+  // The collections rely on this method.
+  implicit def $conforms[A]: A <:< A = singleton_<:<.asInstanceOf[A <:< A]
+
+  @deprecated("Use `implicitly[T <:< U]` or `identity` instead.", "2.11.0")
+  def conforms[A]: A <:< A = $conforms[A]
 
   /** An instance of `A =:= B` witnesses that the types `A` and `B` are equal.
    *
diff --git a/src/reflect/scala/reflect/internal/Definitions.scala b/src/reflect/scala/reflect/internal/Definitions.scala
index 645d6aa4ff..73f344345b 100644
--- a/src/reflect/scala/reflect/internal/Definitions.scala
+++ b/src/reflect/scala/reflect/internal/Definitions.scala
@@ -1429,7 +1429,8 @@ trait Definitions extends api.StandardDefinitions {
         TypeTagClass     -> materializeTypeTag
       )
       lazy val TagSymbols = TagMaterializers.keySet
-      lazy val Predef_conforms     = getMemberMethod(PredefModule, nme.conforms)
+      lazy val Predef_conforms     = (getMemberIfDefined(PredefModule, nme.conforms)
+                               orElse getMemberMethod(PredefModule, "conforms": TermName)) // TODO: predicate on -Xsource:2.10 (for now, needed for transition from M8 -> RC1)
       lazy val Predef_classOf      = getMemberMethod(PredefModule, nme.classOf)
       lazy val Predef_implicitly   = getMemberMethod(PredefModule, nme.implicitly)
       lazy val Predef_wrapRefArray = getMemberMethod(PredefModule, nme.wrapRefArray)
diff --git a/src/reflect/scala/reflect/internal/StdNames.scala b/src/reflect/scala/reflect/internal/StdNames.scala
index aad5f32b5f..92245790cf 100644
--- a/src/reflect/scala/reflect/internal/StdNames.scala
+++ b/src/reflect/scala/reflect/internal/StdNames.scala
@@ -661,7 +661,7 @@ trait StdNames {
     val classOf: NameType              = "classOf"
     val clone_ : NameType              = "clone"
     val collection: NameType           = "collection"
-    val conforms: NameType             = "conforms"
+    val conforms: NameType             = "$conforms" // dollar prefix to avoid accidental shadowing
     val copy: NameType                 = "copy"
     val create: NameType               = "create"
     val currentMirror: NameType        = "currentMirror"