Fix #2404: Fixes to make collections strawman compile, take 3

compiler/src/dotty/tools/dotc/core/Denotations.scala

@@ -310,12 +310,28 @@ object Denotations {
             }
           case tp1: MethodOrPoly =>
             tp2 match {
-              case tp2: MethodOrPoly
-              if ctx.typeComparer.matchingParams(tp1, tp2) &&
-                 tp1.isImplicit == tp2.isImplicit =>
-                tp1.derivedLambdaType(
-                  mergeParamNames(tp1, tp2), tp1.paramInfos,
-                  infoMeet(tp1.resultType, tp2.resultType.subst(tp2, tp1)))
+              case tp2: MethodOrPoly =>
+                // Two remedial strategies:
+                //
+                //  1. Prefer method types over poly types. This is necessary to handle
+                //     overloaded definitions like the following
+                //
+                //        def ++ [B >: A](xs: C[B]): D[B]
+                //        def ++ (xs: C[A]): D[A]
+                //
+                //     (Code like this is found in the collection strawman)
+                //
+                // 2. In the case of two method types or two polytypes with matching
+                //    parameters and implicit status, merge corresppnding parameter
+                //    and result types.
+                if (tp1.isInstanceOf[PolyType] && tp2.isInstanceOf[MethodType]) tp2
+                else if (tp2.isInstanceOf[PolyType] && tp1.isInstanceOf[MethodType]) tp1
+                else if (ctx.typeComparer.matchingParams(tp1, tp2) &&
+                         tp1.isImplicit == tp2.isImplicit)
+                  tp1.derivedLambdaType(
+                    mergeParamNames(tp1, tp2), tp1.paramInfos,
+                    infoMeet(tp1.resultType, tp2.resultType.subst(tp2, tp1)))
+                else mergeConflict(tp1, tp2)
               case _ =>
                 mergeConflict(tp1, tp2)
             }
@@ -553,7 +569,7 @@ object Denotations {
       if (sd1.exists)
         if (sd2.exists)
           if (isDoubleDef(denot1.symbol, denot2.symbol)) doubleDefError(denot1, denot2)
-          else throw new TypeError(s"failure to disambiguate overloaded reference $this")
+          else throw new TypeError(i"failure to disambiguate overloaded reference at $this")
         else sd1
       else sd2
     }

compiler/src/dotty/tools/dotc/core/Signature.scala

@@ -1,7 +1,7 @@
 package dotty.tools.dotc
 package core
 
-import Names._, Types._, Contexts._, StdNames._
+import Names._, Types._, Contexts._, StdNames._, Decorators._
 import TypeErasure.sigName
 
 import scala.annotation.tailrec
@@ -49,6 +49,20 @@ case class Signature(paramsSig: List[TypeName], resSig: TypeName) {
     loop(this.paramsSig, that.paramsSig)
   }
 
+  /** `that` signature, but keeping all corresponding parts of `this` signature. */
+  final def updateWith(that: Signature): Signature = {
+    def update(name1: TypeName, name2: TypeName): TypeName =
+      if (consistent(name1, name2)) name1 else name2
+    if (this == that) this
+    else if (!this.paramsSig.hasSameLengthAs(that.paramsSig)) that
+    else {
+      val mapped = Signature(
+          this.paramsSig.zipWithConserve(that.paramsSig)(update),
+          update(this.resSig, that.resSig))
+      if (mapped == this) this else mapped
+    }
+  }
+
   /** The degree to which this signature matches `that`.
    *  If parameter names are consistent and result types names match (i.e. they are the same
    *  or one is a wildcard), the result is `FullMatch`.

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1867,8 +1867,18 @@ object Types {
       }
       else candidate
 
-    override def newLikeThis(prefix: Type)(implicit ctx: Context): TermRef =
-      fixDenot(TermRef.withSig(prefix, name, sig), prefix)
+    override def newLikeThis(prefix: Type)(implicit ctx: Context): TermRef = {
+      // If symbol exists, the new signature is the symbol's signature as seen
+      // from the new prefix, modulo consistency
+      val newSig =
+        if (sig == Signature.NotAMethod || !symbol.exists)
+          sig
+        else
+          sig.updateWith(symbol.info.asSeenFrom(prefix, symbol.owner).signature)
+      if (newSig ne sig)
+        core.println(i"sig change at ${ctx.phase} for $this, pre = $prefix, sig: $sig --> $newSig")
+      fixDenot(TermRef.withSig(prefix, name, newSig), prefix)
+    }
 
     override def shadowed(implicit ctx: Context): NamedType =
       fixDenot(TermRef.withSig(prefix, name.derived(ShadowedName), sig), prefix)

compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala

@@ -96,6 +96,12 @@ object ErrorReporting {
 
     def exprStr(tree: Tree): String = refStr(tree.tpe)
 
+    def takesNoParamsStr(tree: Tree, kind: String) =
+      if (tree.tpe.widen.exists)
+        i"${exprStr(tree)} does not take ${kind}parameters"
+      else
+        i"undefined: $tree # ${tree.uniqueId}: ${tree.tpe.toString}"
+
     def patternConstrStr(tree: Tree): String = ???
 
     def typeMismatch(tree: Tree, pt: Type, implicitFailure: SearchFailure = NoImplicitMatches): Tree =

compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala

@@ -340,7 +340,7 @@ trait TypeAssigner {
         else
           errorType(i"wrong number of arguments for $fntpe: ${fn.tpe}, expected: ${fntpe.paramInfos.length}, found: ${args.length}", tree.pos)
       case t =>
-        errorType(i"${err.exprStr(fn)} does not take parameters", tree.pos)
+        errorType(err.takesNoParamsStr(fn, ""), tree.pos)
     }
     tree.withType(ownType)
   }
@@ -396,7 +396,7 @@ trait TypeAssigner {
           else wrongNumberOfTypeArgs(fn.tpe, pt.typeParams, args, tree.pos)
         }
       case _ =>
-        errorType(i"${err.exprStr(fn)} does not take type parameters", tree.pos)
+        errorType(err.takesNoParamsStr(fn, "type "), tree.pos)
     }
 
     tree.withType(ownType)