Don't instantiate variables to Nothing in canDefineFurther

The purpose of canDefineFurther is to discover new members. That is not
helped by instantiating a type variable to `Nothing`. Avoiding the instantiation
helpos error messages. There was one test (i864.scala) that fails now
but compiled previously with a surprising instantiation to Nothing. I believe
it is better to issue an error message in this case.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Inferencing.scala

@@ -33,7 +33,10 @@ object Inferencing {
    */
   def isFullyDefined(tp: Type, force: ForceDegree.Value)(using Context): Boolean = {
     val nestedCtx = ctx.fresh.setNewTyperState()
-    val result = new IsFullyDefinedAccumulator(force)(using nestedCtx).process(tp)
+    val result =
+      try new IsFullyDefinedAccumulator(force)(using nestedCtx).process(tp)
+      catch case ex: StackOverflowError =>
+        false // can happen for programs with illegal recusions, e.g. neg/recursive-lower-constraint.scala
     if (result) nestedCtx.typerState.commit()
     result
   }
@@ -43,7 +46,7 @@ object Inferencing {
    */
   def canDefineFurther(tp: Type)(using Context): Boolean =
     val prevConstraint = ctx.typerState.constraint
-    isFullyDefined(tp, force = ForceDegree.all)
+    isFullyDefined(tp, force = ForceDegree.failBottom)
     && (ctx.typerState.constraint ne prevConstraint)
 
   /** The fully defined type, where all type variables are forced.

tests/neg/i4986a.check

@@ -4,6 +4,6 @@
   |Cannot construct a collection of type List[String] with elements of type Int based on a collection of type List[Int]..
   |I found:
   |
-  |    collection.BuildFrom.buildFromIterableOps[Nothing, Nothing, Nothing]
+  |    collection.BuildFrom.buildFromIterableOps[CC, A0, A]
   |
   |But method buildFromIterableOps in trait BuildFromLowPriority2 does not match type collection.BuildFrom[List[Int], Int, List[String]].

tests/neg/i7056.check

@@ -1,7 +1,18 @@
--- [E008] Not Found Error: tests/neg/i7056.scala:19:10 -----------------------------------------------------------------
+-- [E007] Type Mismatch Error: tests/neg/i7056.scala:19:8 --------------------------------------------------------------
 19 |val z = x.idnt1  // error
-   |        ^^^^^^^
-   |        value idnt1 is not a member of B.
-   |        An extension method was tried, but could not be fully constructed:
+   |        ^
+   |        Found:    (given_PartialId_B : B)
+   |        Required: PartialId[T]
    |
-   |            given_T1_T[T](given_PartialId_B).idnt1()
+   |        where:    T is a type variable with constraint <: A
+   |
+   |
+   |        Note: a match type could not be fully reduced:
+   |
+   |          trying to reduce  PartialId[T]
+   |          failed since selector  T
+   |          matches none of the cases
+   |
+   |            case B => T
+
+longer explanation available when compiling with `-explain`

tests/pos/i864.scala renamed to tests/neg/i864.scala

@@ -6,5 +6,5 @@ object C {
   trait X[T]
   implicit def u[A, B]: X[A | B] = new X[A | B] {}
   def y[T](implicit x: X[T]): T = ???
-  val x: a.type & b.type | b.type & c.type = y
+  val x: a.type & b.type | b.type & c.type = y // error
 }