Issue an error when trying to use a method as a qualifier.

The check for `implicitsEnabled` here seems older even than
that method; it appears to originally have looked at a flag
`reportGeneralErrors`, which seems a reasonable flag if you
want to check whether to report an error. An erroneous tree
that we do not error on, however, can be use as a qualifier
which will not necessarily cause an error until past typer,
when something finally notices the mistake and blows up.

Fixes scala/bug#10474.

Author: hrhino

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

@@ -894,7 +894,10 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
 
         def cantAdapt =
           if (context.implicitsEnabled) MissingArgsForMethodTpeError(tree, meth)
-          else setError(tree)
+          else if (mode.inQualMode) UnstableTreeError(tree)
+          else setError(tree) // scala/bug#10474 implies that not issuing an
+                              // error here might be harmful... can we get here
+                              // with implicits disabled and not in QUALmode?
 
         // constructors do not eta-expand
         if (meth.isConstructor) cantAdapt

test/files/neg/t10474.check

@@ -0,0 +1,10 @@
+t10474.scala:9: error: stable identifier required, but Test.this.Foo found.
+    case Foo.Bar ⇒ true
+         ^
+t10474.scala:13: error: stable identifier required, but Test.this.Foo found.
+  type Crash = Foo.type
+               ^
+t10474.scala:17: error: stable identifier required, but Test.this.bingo found.
+  val `bingo`.Fuzz = 1
+      ^
+three errors found

test/files/neg/t10474.scala

@@ -0,0 +1,18 @@
+package t10474
+
+object Test {
+  def Foo(a: Int): Char = ???
+
+  object Bar
+
+  def crash[A](): Boolean = Bar match {
+    case Foo.Bar ⇒ true
+    case _ ⇒ false
+  }
+
+  type Crash = Foo.type
+
+  def bingo(l: Int, r: Int) = l + r
+
+  val `bingo`.Fuzz = 1
+}