Disable variance checks in default getters

This matches what Scala 2 does and is needed to compile geny in the
community build which does:

    def count(f: A => Boolean = ((_: Any) => true))

This happened to work before the previous commit because the inferred
type of the getter was `Any => Boolean`.

See the added testcase for a justification of the relaxed variance
checking.

Author: smarter

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

@@ -194,7 +194,8 @@ class VarianceChecker(using Context) {
       def sym = tree.symbol
       // No variance check for private/protected[this] methods/values.
       def skip = !sym.exists
-        || sym.name.is(InlineAccessorName) // TODO: should we exclude all synthetic members?
+        || sym.name.is(InlineAccessorName)
+        || sym.name.is(DefaultGetterName) // see default-getter-variance.scala
         || sym.isAllOf(LocalParamAccessor) // local class parameters are construction only
         || sym.is(TypeParam) && sym.owner.isClass // already taken care of in primary constructor of class
       try tree match {

tests/pos/default-getter-variance.scala

@@ -0,0 +1,26 @@
+class Foo[+A] {
+  def count(f: A => Boolean = _ => true): Unit = {}
+  // The preceding line is valid, even though the generated default getter
+  // has type `A => Boolean` which wouldn't normally pass variance checks
+  // because it's equivalent to the following overloads which are valid:
+  def count2(f: A => Boolean): Unit = {}
+  def count2(): Unit = count(_ => true)
+}
+
+class Bar1[+A] extends Foo[A] {
+  override def count(f: A => Boolean): Unit = {}
+  // This reasoning extends to overrides:
+  override def count2(f: A => Boolean): Unit = {}
+}
+
+class Bar2[+A] extends Foo[A] {
+  override def count(f: A => Boolean = _ => true): Unit = {}
+  // ... including overrides which also override the default getter:
+  override def count2(f: A => Boolean): Unit = {}
+  override def count2(): Unit = count(_ => true)
+}
+
+// This can be contrasted with the need for variance checks in
+// `protected[this] methods (cf tests/neg/t7093.scala),
+// default getters do not have the same problem since they cannot
+// appear in arbitrary contexts.