fixed #347. Added option -Xlog-implicits.

3 files changed, 16 insertions, 7 deletions

diff --git a/src/compiler/scala/tools/nsc/Settings.scala b/src/compiler/scala/tools/nsc/Settings.scala
index c1373c59d4..90246cead5 100644
--- a/src/compiler/scala/tools/nsc/Settings.scala
+++ b/src/compiler/scala/tools/nsc/Settings.scala
@@ -108,6 +108,7 @@ class Settings(error: String => Unit) {
   val Xchecknull    = BooleanSetting    ("-Xcheck-null", "Emit warning on selection of nullable reference")
   val noassertions  = BooleanSetting    ("-Xdisable-assertions", "Generate no assertions and assumptions")
   val Xexperimental = BooleanSetting    ("-Xexperimental", "Enable experimental extensions")
+  val XlogImplicits = BooleanSetting    ("-Xlog-implicits", "Show more info on why some implicits are not applicable")
   val Xnojline      = new BooleanSetting("-Xnojline", "Do not use JLine for editing").hideToIDE
   val nouescape     = BooleanSetting    ("-Xno-uescape", "Disables handling of \\u unicode escapes")
   val plugin        = MultiStringSetting("-Xplugin", "file", "Load a plugin from a file")
diff --git a/src/compiler/scala/tools/nsc/typechecker/Infer.scala b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
index c4eba1f7b5..bf5c599ae8 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Infer.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
@@ -503,8 +503,13 @@ trait Infer {
       // check first whether type variables can be fully defined from
       // expected result type.
       if (!isWeaklyCompatible(restpe.instantiateTypeParams(tparams, tvars), pt)) {
-        throw new DeferredNoInstance(() =>
-          "result type " + normalize(restpe) + " is incompatible with expected type " + pt)
+//      just wait and instantiate form the arguments.
+//      that way, we can try to apply an implicit conversion afterwards.
+//      This case could happen if restpe is not fully defined, so that
+//      search for an implicit from it to pt fails because of an ambiguity.
+//      See #0347. Therefore, the following two lines are commented out.
+//        throw new DeferredNoInstance(() =>
+//          "result type " + normalize(restpe) + " is incompatible with expected type " + pt)
       }
       for (tvar <- tvars)
         if (!isFullyDefined(tvar)) tvar.constr.inst = NoType
diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index cace90f489..6d57902b4b 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -3313,9 +3313,9 @@ trait Typers { self: Analyzer =>
           else Select(gen.mkAttributedQualifier(info.pre), info.name)
         }
         // println("typed impl?? "+info.name+":"+info.tpe+" ==> "+tree+" with "+pt)
-        def fail(reason: String, sym1: Symbol, sym2: Symbol): Tree = {
-          if (settings.debug.value)
-            println(tree+" is not a valid implicit value because:\n"+reason + sym1+" "+sym2)
+        def fail(reason: String): Tree = {
+          if (settings.XlogImplicits.value)
+            inform(tree+" is not a valid implicit value for "+pt+" because:\n"+reason)
           EmptyTree
         }
         try {
@@ -3340,9 +3340,12 @@ trait Typers { self: Analyzer =>
           }
           if (tree2.tpe.isError) EmptyTree
           else if (hasMatchingSymbol(tree1)) tree2
-          else fail("syms differ: ", tree1.symbol, info.sym)
+          else if (settings.XlogImplicits.value)
+            fail("candidate implicit "+info.sym+info.sym.locationString+
+                 " is shadowed by other implicit: "+tree1.symbol+tree1.symbol.locationString)
+          else EmptyTree
         } catch {
-          case ex: TypeError => fail(ex.getMessage(), NoSymbol, NoSymbol)
+          case ex: TypeError => fail(ex.getMessage())
         }
       } else EmptyTree
     }