bugfix: scoverage does not instrument pat-mat assignment properly

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

@@ -842,6 +842,23 @@ class ScoverageInstrumentationComponent(
           */
         case v: ValDef if v.symbol.isLazy => tree
 
+        /** Pattern matching assignment instrumentation (see https://github.com/scoverage/scalac-scoverage-plugin/issues/123)
+          *
+          * User code: val (a, b) = { if (c) 1 -> 1 else 2 -> 2 }
+          *
+          * After typer, this desugars to:
+          *   <synthetic> val x$1 = (if (c) 1 -> 1 else 2 -> 2) match { case (a, b) => Tuple2(a, b) }
+          *   val a = x$1._1
+          *   val b = x$1._2
+          *
+          * This will instrument the user expression (if-else with arrow calls).
+          */
+        case v: ValDef
+            if v.symbol.isSynthetic && v.rhs.pos.isDefined && containsNonSynthetic(
+              v.rhs
+            ) =>
+          treeCopy.ValDef(tree, v.mods, v.name, v.tpt, process(v.rhs))
+
         /** <synthetic> val default: A1 => B1 =
           * <synthetic> val x1: Any = _
           */

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

@@ -380,4 +380,34 @@ class PluginCoverageTest extends FunSuite with MacroSupport {
     assert(!compiler.reporter.hasWarnings)
     compiler.assertNMeasuredStatements(11)
   }
+
+  test(
+    "plugin should handle return pattern matching assignment https://github.com/scoverage/scalac-scoverage-plugin/issues/123"
+  ) {
+    val compiler = ScoverageCompiler.default
+    compiler.compileCodeSnippet(
+      """
+        |object TestObject {
+        |    def test(c: Boolean): Unit = {
+        |        val (a, b) = {
+        |            if (c) 1 -> 1 else 2 -> 2
+        |        }
+        |    }
+        |}
+      """.stripMargin
+    )
+    assert(!compiler.reporter.hasErrors)
+    assert(!compiler.reporter.hasWarnings)
+
+    /** WITHOUT the bugfix:
+      * 2 when assigning value to "a" and "b"
+      * 1 at the end of the function
+      * WITH the bugfix, it will additionally include
+      * 2 from then branch
+      * 2 from else branch
+      * 2 from synthetic code generated for pattern matching assignment
+      */
+    compiler.assertNMeasuredStatements(9)
+  }
+
 }