Merge pull request scala#5845 from adriaanm/t10261

Actually retract clashing synthetic apply/unapply

src/compiler/scala/tools/nsc/typechecker/Namers.scala

@@ -664,7 +664,17 @@ trait Namers extends MethodSynthesis {
 
           if (suppress) {
             sym setInfo ErrorType
+
+            // There are two ways in which we exclude the symbol from being added in typedStats::addSynthetics,
+            // because we don't know when the completer runs with respect to this loop in addSynthetics
+            //  for (sym <- scope)
+            //    for (tree <- context.unit.synthetics.get(sym) if shouldAdd(sym)) {
+            //      if (!sym.initialize.hasFlag(IS_ERROR))
+            //        newStats += typedStat(tree)
+            // If we're already in the loop, set the IS_ERROR flag and trigger the condition `sym.initialize.hasFlag(IS_ERROR)`
             sym setFlag IS_ERROR
+            // Or, if we are not yet in the addSynthetics loop, we can just retract our symbol from the synthetics for this unit.
+            companionContext.unit.synthetics -= sym
 
             // Don't unlink in an error situation to generate less confusing error messages.
             // Ideally, our error reporting would distinguish overloaded from recursive user-defined apply methods without signature,

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -3168,7 +3168,9 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
           for (sym <- scope)
             // OPT: shouldAdd is usually true. Call it here, rather than in the outer loop
             for (tree <- context.unit.synthetics.get(sym) if shouldAdd(sym)) {
-              newStats += typedStat(tree) // might add even more synthetics to the scope
+              // if the completer set the IS_ERROR flag, retract the stat (currently only used by applyUnapplyMethodCompleter)
+              if (!sym.initialize.hasFlag(IS_ERROR))
+                newStats += typedStat(tree) // might add even more synthetics to the scope
               context.unit.synthetics -= sym
             }
           // the type completer of a synthetic might add more synthetics. example: if the

test/files/run/t10261/Companion_1.scala

@@ -0,0 +1,4 @@
+trait Companion[T] {
+  def parse(value: String): Option[T]
+  def apply(value: String): T = parse(value).get
+}

test/files/run/t10261/Test_2.scala

@@ -0,0 +1,14 @@
+import scala.util.Try
+
+object C extends Companion[C] {
+  def parse(v: String) = if (v.nonEmpty) Some(new C(v)) else None
+}
+
+case class C(value: String)
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    assert(Try{C("")}.isFailure, "Empty value should fail to parse") // check that parse is used to validate input
+    assert(C("a").value == "a", "Unexpected value")
+  }
+}