SI-6912 Avoid a typer cycle in overload resolution.

c800d1f, and followup commits 1ddc935 and b10b582
modified error handling in `Infer#inferExprAlternative`.
After these changes, this method could fail to resolve
the overloaded alternative if:

     best != NoSymbol
  && !competing.isEmpty
  && !noAlternatives
  && pt.isErroneous

This commit calls `setError` in that case, which
prevents the cycle in `adapt`.

While I didn't extract a reproduction from the
original code base, I've included a test case that
exhibits the same symptom.

It was actually pretty tough to find an program that
got close to this code path, but luckilly we've been
pretty close to this bug in SI-5553 / 4f99c2e, and those
test cases formed the basis for this one.

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

@@ -1510,6 +1510,13 @@ trait Infer extends Checkable {
         } else if (!competing.isEmpty) {
           if (noAlternatives) NoBestExprAlternativeError(tree, pt, isSecondTry)
           else if (!pt.isErroneous) AmbiguousExprAlternativeError(tree, pre, best, competing.head, pt, isSecondTry)
+          else {
+            // SI-6912 Don't give up and leave an OverloadedType on the tree.
+            //         Originally I wrote this as `if (secondTry) ... `, but `tryTwice` won't attempt the second try
+            //         unless an error is issued. We're not issuing an error, in the assumption that it would be
+            //         spurious in light of the erroneous expected type
+            setError(tree)
+          }
         } else {
 //          val applicable = alts1 filter (alt =>
 //            global.typer.infer.isWeaklyCompatible(pre.memberType(alt), pt))

test/files/neg/t6912.check

@@ -0,0 +1,4 @@
+t6912.scala:8: error: not found: type Xxxx
+  def test[T]: Xxxx = Foo1[T]
+               ^
+one error found

test/files/neg/t6912.scala

@@ -0,0 +1,9 @@
+object Foo1 {
+  def apply[T](a: Int = 0): Nothing = sys.error("")
+  def apply[T](z: String = ""): Nothing = sys.error("")
+}
+
+object Test {
+  // Triggered a cycle in Typers#adapt
+  def test[T]: Xxxx = Foo1[T]
+}