More lenient handling of mixed parameterless and nullary methods

When faced with a denotation that combines parameterless and nullary method
definitions (toString is a common example), ignore any redundant () applications.

3 files changed, 48 insertions, 17 deletions

diff --git a/compiler/src/dotty/tools/dotc/typer/Applications.scala b/compiler/src/dotty/tools/dotc/typer/Applications.scala
index 8a18e63c0..42c24ffb7 100644
--- a/compiler/src/dotty/tools/dotc/typer/Applications.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Applications.scala
@@ -657,18 +657,20 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
         case err: ErrorType => untpd.cpy.Apply(tree)(fun1, tree.args).withType(err)
         case TryDynamicCallType => typedDynamicApply(tree, pt)
         case _ =>
-          tryEither {
-            implicit ctx => simpleApply(fun1, proto)
-          } {
-            (failedVal, failedState) =>
-              def fail = { failedState.commit(); failedVal }
-              // Try once with original prototype and once (if different) with tupled one.
-              // The reason we need to try both is that the decision whether to use tupled
-              // or not was already taken but might have to be revised when an implicit
-              // is inserted on the qualifier.
-              tryWithImplicitOnQualifier(fun1, originalProto).getOrElse(
-                if (proto eq originalProto) fail
-                else tryWithImplicitOnQualifier(fun1, proto).getOrElse(fail))
+          if (originalProto.isDropped) fun1
+          else
+            tryEither {
+              implicit ctx => simpleApply(fun1, proto)
+            } {
+              (failedVal, failedState) =>
+                def fail = { failedState.commit(); failedVal }
+                // Try once with original prototype and once (if different) with tupled one.
+                // The reason we need to try both is that the decision whether to use tupled
+                // or not was already taken but might have to be revised when an implicit
+                // is inserted on the qualifier.
+                tryWithImplicitOnQualifier(fun1, originalProto).getOrElse(
+                  if (proto eq originalProto) fail
+                  else tryWithImplicitOnQualifier(fun1, proto).getOrElse(fail))
           }
       }
     }
diff --git a/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala b/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala
index ed6b95c3b..3c318a6af 100644
--- a/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala
+++ b/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala
@@ -250,6 +250,22 @@ object ProtoTypes {
     /** Somebody called the `tupled` method of this prototype */
     def isTupled: Boolean = myTupled.isInstanceOf[FunProto]
 
+    /** If true, the application of this prototype was canceled. */
+    private var toDrop: Boolean = false
+
+    /** Cancel the application of this prototype. This can happen for a nullary
+     *  application `f()` if `f` refers to a symbol that exists both in parameterless
+     *  form `def f` and nullary method form `def f()`. A common example for such
+     *  a method is `toString`. If in that case the type in the denotation is
+     *  parameterless, we compensate by dropping the application.
+     */
+    def markAsDropped() = {
+      assert(args.isEmpty)
+      toDrop = true
+    }
+
+    def isDropped: Boolean = toDrop
+
     override def toString = s"FunProto(${args mkString ","} => $resultType)"
 
     def map(tm: TypeMap)(implicit ctx: Context): FunProto =
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 07a27a498..d054fe803 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -1640,13 +1640,19 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     case _                  => false
   }
 
-  /** Add apply node or implicit conversions. Two strategies are tried, and the first
-   *  that is successful is picked. If neither of the strategies are successful, continues with
-   *  `fallBack`.
+  /** Potentially add apply node or implicit conversions. Before trying either,
+   *  if the function is applied to an empty parameter list (), we try
+   *
+   *  0th strategy: If `tree` overrides a nullary method, mark the prototype
+   *                so that the argument is dropped and return `tree` itself.
+   *
+   *  After that, two strategies are tried, and the firs that is successful is picked.
+   *  If neither of the strategies are successful, continues with`fallBack`.
    *
    *  1st strategy: Try to insert `.apply` so that the result conforms to prototype `pt`.
    *                This strategy is not tried if the prototype represents already
    *                another `.apply` or `.apply()` selection.
+   *
    *  2nd strategy: If tree is a select `qual.name`, try to insert an implicit conversion
    *    around the qualifier part `qual` so that the result conforms to the expected type
    *    with wildcard result type.
@@ -1661,8 +1667,15 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     def tryImplicit =
       tryInsertImplicitOnQualifier(tree, pt).getOrElse(fallBack)
 
-    if (isApplyProto(pt)) tryImplicit
-    else tryEither(tryApply(_))((_, _) => tryImplicit)
+    pt match {
+      case pt @ FunProto(Nil, _, _)
+      if tree.symbol.allOverriddenSymbols.exists(_.info.isNullaryMethod) =>
+        pt.markAsDropped()
+        tree
+      case _ =>
+        if (isApplyProto(pt)) tryImplicit
+        else tryEither(tryApply(_))((_, _) => tryImplicit)
+     }
   }
 
   /** If this tree is a select node `qual.name`, try to insert an implicit conversion