fix: ignore lazy definition generated from by-name implicits

Preamble:

by-name implicit parameters used for shapeless magic and were
introduced in SIP-31:
https://docs.scala-lang.org/sips/byname-implicits.html

Using scoverage with such code may result in the following warning from
the plugin:

[warn] Could not instrument [Select/value rec$1].

Originally discovered by using scoverage with the code that derives
typeclass instances using kittens library:

case class Bar()
case class Foo(bars: List[Bar])
object Foo {
  implicit eq: cats.Eq[Foo] = cats.derived.semiauto.eq
}

Solution:

Ignore LazyDefns$1.rec$1 from instrumentation like other synthetic code

Other changes:

It was necessary to disable position validation phase for the test case
that reproduces the issue because the code generated by scala compiler
fails such validation. It could be also checked outside testing
environment by manually setting -Yvalidate-pos:typer option to the
compiler. Not sure whether it's expected compiler behavior or not

Author: danielleontiev

plugin/src/main/scala/scoverage/ScoveragePlugin.scala

@@ -790,6 +790,14 @@ class ScoverageInstrumentationComponent(
           */
         case s: Select if s.symbol.isLazy => tree
 
+        // Generated by compiler for lazy definitions involving
+        // by-name implicit parameters. More on that here:
+        // https://docs.scala-lang.org/sips/byname-implicits.html
+        //
+        // final <synthetic> val lazyDefns$1: LazyDefns$1 = new LazyDefns$1();
+        // lazyDefns$1.rec$1()
+        case s: Select if s.symbol.isSynthetic => tree
+
         case s: Select =>
           instrument(
             treeCopy.Select(s, traverseApplication(s.qualifier), s.name),

plugin/src/test/scala/scoverage/PluginCoverageTest.scala

@@ -373,4 +373,31 @@ class PluginCoverageTest extends FunSuite with MacroSupport {
     assert(!compiler.reporter.hasWarnings)
     compiler.assertNMeasuredStatements(11)
   }
+
+  test(
+    "scoverage should ignore synthetic lazy definitions generated by compiler from by-name implicits"
+  ) {
+    val compiler = ScoverageCompiler.noPositionValidation
+    compiler.compileCodeSnippet(
+      """
+        |object test {
+        |
+        |  trait Foo {
+        |    def next: Foo
+        |  }
+        |
+        |  object Foo {
+        |    implicit def foo(implicit rec: => Foo): Foo =
+        |      new Foo { def next = rec }
+        |  }
+        |
+        |  val foo = implicitly[Foo]
+        |
+        |}
+        |
+      """.stripMargin
+    )
+    assert(!compiler.reporter.hasErrors)
+    assert(!compiler.reporter.hasWarnings)
+  }
 }

plugin/src/test/scala/scoverage/ScoverageCompiler.scala

@@ -57,12 +57,17 @@ private[scoverage] object ScoverageCompiler {
 
   def default: ScoverageCompiler = {
     val reporter = new scala.tools.nsc.reporters.ConsoleReporter(settings)
-    new ScoverageCompiler(settings, reporter)
+    new ScoverageCompiler(settings, reporter, validatePositions = true)
+  }
+
+  def noPositionValidation: ScoverageCompiler = {
+    val reporter = new scala.tools.nsc.reporters.ConsoleReporter(settings)
+    new ScoverageCompiler(settings, reporter, validatePositions = false)
   }
 
   def defaultJS: ScoverageCompiler = {
     val reporter = new scala.tools.nsc.reporters.ConsoleReporter(jsSettings)
-    new ScoverageCompiler(jsSettings, reporter)
+    new ScoverageCompiler(jsSettings, reporter, validatePositions = true)
   }
 
   def locationCompiler: LocationCompiler = {
@@ -152,7 +157,8 @@ private[scoverage] object ScoverageCompiler {
 
 class ScoverageCompiler(
     settings: scala.tools.nsc.Settings,
-    rep: scala.tools.nsc.reporters.Reporter
+    rep: scala.tools.nsc.reporters.Reporter,
+    validatePositions: Boolean
 ) extends scala.tools.nsc.Global(settings, rep) {
 
   def addToClassPath(file: File): Unit = {
@@ -268,7 +274,8 @@ class ScoverageCompiler(
 
   override def computeInternalPhases(): Unit = {
     super.computeInternalPhases()
-    addToPhasesSet(validator, "scoverage validator")
+    if (validatePositions)
+      addToPhasesSet(validator, "scoverage validator")
     addToPhasesSet(
       instrumentationComponent,
       "scoverage instrumentationComponent"