Verify that flow inference for blocks only works inside methods

There's no principled reason for this. It's just that our approach
with the new type of Completer seems brittle, so I'd like to guarantee
that is only used for vals that appear inside method bodies, since those
must be forced when the corresponding block is typed (so it seems safe
to use the completion context, as opposed to the creation one).

Author: abeln

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

@@ -2119,7 +2119,8 @@ class Typer extends Namer
             import untpd.modsDeco
             val ctx1 = if (ctx.settings.YexplicitNulls.value) {
               mdef match {
-                case mdef: untpd.ValDef if !mdef.mods.is(Lazy) && !mdef.mods.is(Implicit) => ctx.fresh.addNonNullFacts(facts)
+                case mdef: untpd.ValDef if ctx.owner.is(Method) && !mdef.mods.is(Lazy) && !mdef.mods.is(Implicit) =>
+                  ctx.fresh.addNonNullFacts(facts)
                 case _ => ctx
               }
             } else {

tests/neg/explicit-null-flow5.scala

@@ -5,7 +5,7 @@ class Foo(x: String|Null) {
 
   // Test within constructor
   if (x == null) throw new NullPointerException()
-  val x2: String = x // ok: x == null case handled by throwing
+  val x2: String = x // error: flow inference for blocks doesn't work inside constructors
 
   def foo(): Unit = {
     val y: String|Null = ???