bugfix: Fix issues with Scala JS and coverage (#24769)

Tested it with the reproduction in
#20255

Fixes #20255

Author: tgodzik

compiler/src/dotty/tools/dotc/transform/InstrumentCoverage.scala

@@ -10,6 +10,7 @@ import core.Contexts.{Context, ctx, inContext}
 import core.DenotTransformers.IdentityDenotTransformer
 import core.Symbols.{defn, Symbol}
 import core.Constants.Constant
+import core.NameKinds.DefaultGetterName
 import core.NameOps.isContextFunction
 import core.StdNames.nme
 import core.Types.*
@@ -554,6 +555,7 @@ class InstrumentCoverage extends MacroTransform with IdentityDenotTransformer:
       !sym.isOneOf(ExcludeMethodFlags)
       && !isCompilerIntrinsicMethod(sym)
       && !(sym.isClassConstructor && isSecondaryCtorDelegateCall)
+      && !sym.name.is(DefaultGetterName) // https://github.com/scala/scala3/issues/20255
       && (tree.typeOpt match
         case AppliedType(tycon: NamedType, _) =>
           /* If the last expression in a block is a context function, we'll try to
@@ -590,6 +592,7 @@ class InstrumentCoverage extends MacroTransform with IdentityDenotTransformer:
       && sym.info.isParameterless
       && !isCompilerIntrinsicMethod(sym)
       && !sym.info.typeSymbol.name.isContextFunction // exclude context functions like in canInstrumentApply
+      && !sym.name.is(DefaultGetterName) // https://github.com/scala/scala3/issues/20255
 
     /** Does sym refer to a "compiler intrinsic" method, which only exist during compilation,
       * like Any.isInstanceOf?

tests/coverage/pos/DefaultArgs.scala

@@ -0,0 +1,29 @@
+package covtest
+
+// Test case for default arguments with coverage.
+// Default getter methods ($default$N) should not be instrumented/lifted
+// as this can cause issues with ScalaJS IR (https://github.com/scala/scala3/issues/20255).
+
+class DefaultArgs:
+  def methodWithDefaults(
+    a: String,
+    b: Int = 0,
+    c: Boolean = true
+  ): String =
+    s"$a, $b, $c"
+
+  def caller(): String =
+    // This call uses default arguments for b and c
+    methodWithDefaults("test")
+
+  def callerPartial(): String =
+    // This call uses default argument only for c
+    methodWithDefaults("test", 42)
+
+object DefaultArgs:
+  def staticMethod(x: Int = 10, y: Int = 20): Int =
+    x + y
+
+  def staticCaller(): Int =
+    staticMethod() + staticMethod(5) + staticMethod(5, 15)
+