Don't generate specialized overrides in traits

The term "specialized override" is used to describe a method
in a synthetic specialized subclass that generically substitutes
the specialized type args into the siganture of a generic method.

For example, `trait T[@SPEC A] { def t(a: A) }` gives rise to
`def t(a: Int)` under the type environment `A=Int`.

This commit avoids doing this for specialized traits, only classes
have these overrides now. The motivation is to make it simpler to
use specialized interfaces (like `T$mcI$sp` from the example above)
as Java functional interfaces.

Author: retronym

src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala

@@ -697,19 +697,19 @@ abstract class SpecializeTypes extends InfoTransform with TypingTransformers {
           else
             debuglog("conflicting env for " + m + " env: " + env)
         }
-        else if (m.isDeferred) { // abstract methods
+        else if (m.isDeferred && m.isSpecialized) { // abstract methods
           val specMember = enterMember(cloneInSpecializedClass(m, _ | DEFERRED))
           // debuglog("deferred " + specMember.fullName + " remains abstract")
 
           info(specMember) = new Abstract(specMember)
           // was: new Forward(specMember) {
           //   override def target = m.owner.info.member(specializedName(m, env))
           // }
-        } else if (m.isMethod && !m.hasAccessorFlag) { // other concrete methods
+        } else if (!sClass.isTrait && m.isMethod && !m.hasAccessorFlag) { // other concrete methods
           // log("other concrete " + m)
           forwardToOverload(m)
 
-        } else if (m.isMethod && m.hasFlag(LAZY)) {
+        } else if (!sClass.isTrait && m.isMethod && m.hasFlag(LAZY)) {
           forwardToOverload(m)
 
         } else if (m.isValue && !m.isMethod && !m.hasFlag(LAZY)) { // concrete value definition

test/files/run/trait-default-specialize.check

@@ -1,4 +1,3 @@
-public abstract void T$mcI$sp.t(int)
 public abstract void T.t(java.lang.Object)
 0
 0

test/files/run/trait-default-specialize.scala

@@ -1,4 +1,4 @@
-trait T[@specialized A] {
+trait T[@specialized(Int) A] {
   def t(a: A): Unit
 }