SI-6555 Better parameter name retention

We were losing track of parameter names in two places:

  1. Uncurry was using fresh names for the apply method
     parameters during Function expansion. (The parameter names
     in the tree were actually correct, they just had synthetic
     symbols with "x$1" etc.)
  2. When adding specialized overrides, the parameter names
     of the overriden method were used, rather than the parameter
     names from the overriding method in the class to which we are
     adding methods.

The upshot of this is that when you're stopped in the debugger in
the body of, say, `(i: Int) => i * i`, you see `v1` rather than `i`.

This commit changes Uncurry and SpecializeTypes to remedy this.

5 files changed, 48 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala b/src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala
index 2f79472cfb..bbab545d9e 100644
--- a/src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala
+++ b/src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala
@@ -965,6 +965,11 @@ abstract class SpecializeTypes extends InfoTransform with TypingTransformers {
         case (NoSymbol, _)     => None
         case (overridden, env) =>
           val om = specializedOverload(clazz, overridden, env)
+          foreachWithIndex(om.paramss) { (params, i) =>
+            foreachWithIndex(params) { (param, j) =>
+              param.name = overriding.paramss(i)(j).name // SI-6555 Retain the parameter names from the subclass.
+            }
+          }
           debuglog("specialized overload %s for %s in %s: %s".format(om, overriding.name.decode, pp(env), om.info))
           typeEnv(om) = env
           addConcreteSpecMethod(overriding)
diff --git a/src/compiler/scala/tools/nsc/transform/UnCurry.scala b/src/compiler/scala/tools/nsc/transform/UnCurry.scala
index 9829fd0e57..838ea7d5a0 100644
--- a/src/compiler/scala/tools/nsc/transform/UnCurry.scala
+++ b/src/compiler/scala/tools/nsc/transform/UnCurry.scala
@@ -251,7 +251,10 @@ abstract class UnCurry extends InfoTransform
 
           val applyMethodDef = {
             val methSym = anonClass.newMethod(nme.apply, fun.pos, FINAL)
-            methSym setInfoAndEnter MethodType(methSym newSyntheticValueParams formals, restpe)
+            val paramSyms = map2(formals, fun.vparams) {
+              (tp, param) => methSym.newSyntheticValueParam(tp, param.name)
+            }
+            methSym setInfoAndEnter MethodType(paramSyms, restpe)
 
             fun.vparams foreach  (_.symbol.owner =  methSym)
             fun.body changeOwner (fun.symbol     -> methSym)
diff --git a/test/files/neg/t6260.check b/test/files/neg/t6260.check
index 2b7f1a8bfb..46e9bd1dfc 100644
--- a/test/files/neg/t6260.check
+++ b/test/files/neg/t6260.check
@@ -1,10 +1,10 @@
-t6260.scala:3: error: bridge generated for member method apply: (x$1: Box[X])Box[Y] in anonymous class $anonfun
+t6260.scala:3: error: bridge generated for member method apply: (bx: Box[X])Box[Y] in anonymous class $anonfun
 which overrides method apply: (v1: T1)R in trait Function1
 clashes with definition of the member itself;
 both have erased type (v1: Object)Object
     ((bx: Box[X]) => new Box(f(bx.x)))(this)
                   ^
-t6260.scala:8: error: bridge generated for member method apply: (x$1: Box[X])Box[Y] in anonymous class $anonfun
+t6260.scala:8: error: bridge generated for member method apply: (bx: Box[X])Box[Y] in anonymous class $anonfun
 which overrides method apply: (v1: T1)R in trait Function1
 clashes with definition of the member itself;
 both have erased type (v1: Object)Object
diff --git a/test/files/run/t6555.check b/test/files/run/t6555.check
new file mode 100644
index 0000000000..04117b7c2f
--- /dev/null
+++ b/test/files/run/t6555.check
@@ -0,0 +1,22 @@
+[[syntax trees at end of                specialize]] // newSource1
+package <empty> {
+  class Foo extends Object {
+    def <init>(): Foo = {
+      Foo.super.<init>();
+      ()
+    };
+    private[this] val f: Int => Int = {
+      @SerialVersionUID(0) final <synthetic> class $anonfun extends scala.runtime.AbstractFunction1$mcII$sp with Serializable {
+        def <init>(): anonymous class $anonfun = {
+          $anonfun.super.<init>();
+          ()
+        };
+        final def apply(param: Int): Int = $anonfun.this.apply$mcII$sp(param);
+        <specialized> def apply$mcII$sp(param: Int): Int = param
+      };
+      (new anonymous class $anonfun(): Int => Int)
+    };
+    <stable> <accessor> def f(): Int => Int = Foo.this.f
+  }
+}
+
diff --git a/test/files/run/t6555.scala b/test/files/run/t6555.scala
new file mode 100644
index 0000000000..b1a6137786
--- /dev/null
+++ b/test/files/run/t6555.scala
@@ -0,0 +1,15 @@
+import scala.tools.partest._
+import java.io.{Console => _, _}
+
+object Test extends DirectTest {
+
+  override def extraSettings: String = "-usejavacp -Xprint:specialize -d " + testOutput.path
+
+  override def code = "class Foo { val f = (param: Int) => param } "
+
+  override def show(): Unit = {
+    Console.withErr(System.out) {
+      compile()
+    }
+  }
+}