Check that a Scala 2 macros has a Scala 3 implemetation

We must make sure we have one as the Scala 2 macro implemetations
are not entered into the class.

Author: nicolasstucki

community-build/community-projects/stdLib213

@@ -146,6 +146,10 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
       tree match
         case tree: ValOrDefDef if !tree.symbol.is(Synthetic) =>
           checkInferredWellFormed(tree.tpt)
+          val sym = tree.symbol
+          if sym.isScala2Macro then
+            if !sym.owner.unforcedDecls.exists(p => !p.isScala2Macro && p.name == sym.name && p.signature == sym.signature) then
+              ctx.error("No Scala 3 implementation found for this Scala 2 macro.", tree.sourcePos)
         case _ =>
       processMemberDef(tree)
 

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

@@ -141,6 +141,7 @@ class CompilationTests extends ParallelTesting {
       compileFile("tests/neg-custom-args/i3882.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i4372.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i1754.scala", allowDeepSubtypes),
+      compileFile("tests/neg-custom-args/scala2-macro-compat-no-scala3-implementation.scala", defaultOptions.and("-Yscala2-experimental-compat")),
       compileFile("tests/neg-custom-args/interop-polytypes.scala", allowDeepSubtypes.and("-Yexplicit-nulls")),
       compileFile("tests/neg-custom-args/conditionalWarnings.scala", allowDeepSubtypes.and("-deprecation").and("-Xfatal-warnings")),
       compileFilesInDir("tests/neg-custom-args/isInstanceOf", allowDeepSubtypes and "-Xfatal-warnings"),

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -0,0 +1,17 @@
+
+
+object Macro1 {
+  import scala.language.experimental.macros
+  def lineNumber: Int = macro LineNumberMacro2.thisLineNumberImpl // error: No Scala 3 implementation found for this Scala 2 macro.
+}
+
+object LineNumberMacro2 {
+  class Context: // Dummy scala.reflect.macros.Context
+    type Expr[+T]
+
+  def thisLineNumberImpl(context: Context): context.Expr[Int] = {
+    // val lineNumber = context.enclosingPosition.line
+    // context.literal(lineNumber)
+    ???
+  }
+}

tests/neg-custom-args/scala2-macro-compat-no-scala3-implementation.scala

@@ -6,7 +6,10 @@ class Context: // Dummy scala.reflect.macros.Context
 object Macros {
 
   def foo1(x: Int): Int = macro foo1Impl
+  def foo1(x: Int): Int = ???
+
   def foo2(x: Int, y: String): Int = macro foo2Impl
+  def foo2(x: Int, y: String): Int = ???
 
   def foo1Impl(context: Context)(x: context.Expr[Int]): context.Expr[Int] = ???
   def foo2Impl(context: Context)(x: context.Expr[Int], y: context.Expr[String]): context.Expr[Int] = ???