IDE: Don't display parameter lists made of CanBuildFrom in signatures

It's a hack, but this should be less confusing for students, and it's
only temporary until 2.13 is out.

Author: smarter

compiler/src/dotty/tools/dotc/util/Signatures.scala

@@ -84,16 +84,25 @@ object Signatures {
     def toParamss(tp: MethodType)(implicit ctx: Context): List[List[Param]] = {
       val rest = tp.resType match {
         case res: MethodType =>
-          toParamss(res)
+          // HACK: Hide parameter lists consisting only of CanBuildFrom,
+          // remove this once we switch to the 2.13 standard library.
+          if (res.resultType.isParameterless &&
+              res.isImplicitMethod &&
+              res.paramInfos.forall(_.classSymbol.fullName.toString == "scala.collection.generic.CanBuildFrom"))
+            Nil
+          else
+            toParamss(res)
         case _ =>
           Nil
       }
-      tp.paramNames.zip(tp.paramInfos).map { case (name, info) =>
+      val params = tp.paramNames.zip(tp.paramInfos).map { case (name, info) =>
         Signatures.Param(name.show,
           info.widenTermRefExpr.show,
           docComment.flatMap(_.paramDoc(name)),
           isImplicit = tp.isImplicitMethod)
-      } :: rest
+      }
+
+      params :: rest
     }
 
     denot.info.stripPoly match {

language-server/test/dotty/tools/languageserver/SignatureHelpTest.scala

@@ -8,7 +8,7 @@ import dotty.tools.dotc.util.Signatures.{Param => P, Signature => S}
 
 class SignatureHelpTest {
 
-  @Test def javaParam: Unit = {
+  @Test def fromJava: Unit = {
     val signature =
       S("codePointAt", Nil, List(List(P("x$0", "Int"))), Some("Int"))
     code"""object O {
@@ -17,13 +17,17 @@ class SignatureHelpTest {
       .signatureHelp(m1, List(signature), Some(0), 0)
   }
 
-  @Test def scala2Param: Unit = {
-    val signature =
+  @Test def fromScala2: Unit = {
+    val applySig =
       S("apply[A]", Nil, List(List(P("xs", "A*"))), Some("List[A]"))
+    val mapSig =
+      S("map[B, That]", Nil, List(List(P("f", "A => B"))), Some("That"))
     code"""object O {
              List($m1)
+             List(1, 2, 3).map($m2)
            }""".withSource
-      .signatureHelp(m1, List(signature), Some(0), 0)
+      .signatureHelp(m1, List(applySig), Some(0), 0)
+      .signatureHelp(m2, List(mapSig), Some(0), 0)
   }
 
   @Test def singleParam: Unit = {