Fix for SI-6263, futile adaptation.

Don't try to implicitly convert an unstable prefix
to a stable one by applying a view. As the matrix spoon kid
says, "that's impossible."

Author: paulp

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

@@ -233,10 +233,11 @@ trait Typers extends Modes with Adaptations with Tags {
      *  @param tree ...
      *  @return     ...
      */
-    def checkStable(tree: Tree): Tree =
+    def checkStable(tree: Tree): Tree = (
       if (treeInfo.isExprSafeToInline(tree)) tree
       else if (tree.isErrorTyped) tree
       else UnstableTreeError(tree)
+    )
 
     /** Would tree be a stable (i.e. a pure expression) if the type
      *  of its symbol was not volatile?
@@ -5201,7 +5202,10 @@ trait Typers extends Modes with Adaptations with Tags {
 
       def typedSingletonTypeTree(tree: SingletonTypeTree) = {
         val ref1 = checkStable(
-          typed(tree.ref, EXPRmode | QUALmode | (mode & TYPEPATmode), AnyRefClass.tpe))
+          context.withImplicitsDisabled(
+            typed(tree.ref, EXPRmode | QUALmode | (mode & TYPEPATmode), AnyRefClass.tpe)
+          )
+        )
         tree setType ref1.tpe.resultType
       }
 

test/files/neg/t6263.check

@@ -0,0 +1,6 @@
+t6263.scala:5: error: type mismatch;
+ found   : A.this.c.type (with underlying type C)
+ required: AnyRef
+  type t = c.type
+           ^
+one error found

test/files/neg/t6263.scala

@@ -0,0 +1,6 @@
+class C(val a: Any) extends AnyVal
+class A {
+  implicit def c2AnyRef(c: C): AnyRef = new {}
+  val c = new C(0)
+  type t = c.type
+}