Disable derivation for case object

build.sbt

@@ -33,7 +33,7 @@ lazy val weaverVersion = "0.8.4"
 ThisBuild / tlBaseVersion := "1.0"
 ThisBuild / tlCiReleaseBranches := Seq("main") // publish snapshots on `main`
 ThisBuild / tlCiScalafmtCheck := true
-//ThisBuild / scalaVersion := scala212Version
+//ThisBuild / scalaVersion := scala213Version
 ThisBuild / scalaVersion := scala3Version
 ThisBuild / crossScalaVersions := Seq(scala212Version, scala213Version, scala3Version)
 ThisBuild / developers += tlGitHubDev("tpolecat", "Rob Norris")
@@ -98,9 +98,12 @@ lazy val compilerFlags = Seq(
   Compile / doc / scalacOptions --= Seq(
     "-Xfatal-warnings"
   ),
-//  Test / scalacOptions --= Seq(
-//    "-Xfatal-warnings"
-//  ),
+  // Disable warning when @nowarn annotation isn't suppressing a warning
+  // to simplify cross-building
+  // because 2.12 @nowarn doesn't actually do anything.. https://github.com/scala/bug/issues/12313
+  scalacOptions ++= Seq(
+    "-Wconf:cat=unused-nowarn:s"
+  ),
   scalacOptions ++= (if (tlIsScala3.value)
                        // Handle irrefutable patterns in for comprehensions
                        Seq("-source:future", "-language:adhocExtensions")

modules/core/src/main/scala-3/doobie/util/MkReadPlatform.scala

@@ -6,6 +6,7 @@ package doobie.util
 
 import scala.deriving.Mirror
 import doobie.util.shapeless.OrElse
+import scala.util.NotGiven
 
 trait MkReadPlatform:
 
@@ -14,8 +15,10 @@ trait MkReadPlatform:
       implicit
       m: Mirror.ProductOf[P],
       i: A =:= m.MirroredElemTypes,
-      r: Read[A] `OrElse` Derived[MkRead[A]]
+      r: Read[A] `OrElse` Derived[MkRead[A]],
+      isNotCaseObj: NotGiven[m.MirroredElemTypes =:= EmptyTuple]
   ): Derived[MkRead[P]] = {
+    val _ = isNotCaseObj
     val read = r.fold(identity, _.instance).map(a => m.fromProduct(i(a)))
     new Derived(new MkRead(read))
   }

modules/core/src/main/scala-3/doobie/util/MkWritePlatform.scala

@@ -6,6 +6,7 @@ package doobie.util
 
 import scala.deriving.Mirror
 import doobie.util.shapeless.OrElse
+import scala.util.NotGiven
 
 trait MkWritePlatform:
 
@@ -14,8 +15,10 @@ trait MkWritePlatform:
       implicit
       m: Mirror.ProductOf[P],
       i: m.MirroredElemTypes =:= A,
-      w: Write[A] `OrElse` Derived[MkWrite[A]]
+      w: Write[A] `OrElse` Derived[MkWrite[A]],
+      isNotCaseObj: NotGiven[m.MirroredElemTypes =:= EmptyTuple]
   ): Derived[MkWrite[P]] =
+    val _ = isNotCaseObj
     val write: Write[P] = w.fold(identity, _.instance).contramap(p => i(Tuple.fromProductTyped(p)))
     new Derived(
       new MkWrite(write)

modules/core/src/main/scala/doobie/util/read.scala

@@ -63,7 +63,11 @@ object Read extends LowerPriorityRead {
 
   def apply[A](implicit ev: Read[A]): Read[A] = ev
 
-  def derived[A](implicit ev: Derived[MkRead[A]]): Read[A] = ev.instance.underlying
+  def derived[A](implicit
+      @implicitNotFound(
+        "Cannot derive Read instance. Please check that each field in the case class has a Read instance or can derive one")
+      ev: Derived[MkRead[A]]
+  ): Read[A] = ev.instance.underlying
 
   trait Auto extends MkReadInstances
 

modules/core/src/main/scala/doobie/util/write.scala

@@ -76,7 +76,11 @@ sealed trait Write[A] {
 object Write extends LowerPriorityWrite {
   def apply[A](implicit A: Write[A]): Write[A] = A
 
-  def derived[A](implicit ev: Derived[MkWrite[A]]): Write[A] = ev.instance
+  def derived[A](implicit
+      @implicitNotFound(
+        "Cannot derive Write instance. Please check that each field in the case class has a Write instance or can derive one")
+      ev: Derived[MkWrite[A]]
+  ): Write[A] = ev.instance
 
   trait Auto extends MkWriteInstances
 

modules/core/src/test/scala/doobie/util/ReadSuite.scala

@@ -11,6 +11,7 @@ import doobie.testutils.VoidExtensions
 import doobie.syntax.all.*
 import doobie.Query
 import munit.Location
+import scala.annotation.nowarn
 
 class ReadSuite extends munit.FunSuite with ReadSuitePlatform {
 
@@ -107,11 +108,18 @@ class ReadSuite extends munit.FunSuite with ReadSuitePlatform {
   }
 
   test("Read should not be derivable for case objects") {
+    val expectedDeriveError =
+      if (util.Properties.versionString.startsWith("version 2.12"))
+        "could not find implicit"
+      else
+        "Cannot derive"
+    assert(compileErrors("Read.derived[CaseObj.type]").contains(expectedDeriveError))
+    assert(compileErrors("Read.derived[Option[CaseObj.type]]").contains(expectedDeriveError))
+
+    import doobie.implicits.*
     assert(compileErrors("Read[CaseObj.type]").contains("not find or construct"))
-    assert(compileErrors("Read.derived[CaseObj.type]").contains("not find or construct"))
     assert(compileErrors("Read[Option[CaseObj.type]]").contains("not find or construct"))
-    assert(compileErrors("Read.derived[Option[CaseObj.type]]").contains("not find or construct"))
-  }
+  }: @nowarn("msg=.*(u|U)nused import.*")
 
   test("Read should exist for Unit/Option[Unit]") {
     assertEquals(Read[Unit].length, 0)

modules/core/src/test/scala/doobie/util/WriteSuite.scala

@@ -11,6 +11,7 @@ import cats.effect.unsafe.implicits.global
 import doobie.testutils.VoidExtensions
 import doobie.syntax.all.*
 import munit.Location
+import scala.annotation.nowarn
 
 class WriteSuite extends munit.FunSuite with WriteSuitePlatform {
 
@@ -105,11 +106,18 @@ class WriteSuite extends munit.FunSuite with WriteSuitePlatform {
   }
 
   test("Write should not be derivable for case objects") {
-    assert(compileErrors("Write[CaseObj.type]").contains("not find or construct"))
-    assert(compileErrors("Write.derived[CaseObj.type]").contains("not find or construct"))
+    val expectedDeriveError =
+      if (util.Properties.versionString.startsWith("version 2.12"))
+        "could not find implicit"
+      else
+        "Cannot derive"
+    assert(compileErrors("Write.derived[CaseObj.type]").contains(expectedDeriveError))
+    assert(compileErrors("Write.derived[Option[CaseObj.type]]").contains(expectedDeriveError))
+
+    import doobie.implicits.*
     assert(compileErrors("Write[Option[CaseObj.type]]").contains("not find or construct"))
-    assert(compileErrors("Write.derived[Option[CaseObj.type]]").contains("not find or construct"))
-  }
+    assert(compileErrors("Write[CaseObj.type]").contains("not find or construct"))
+  }: @nowarn("msg=.*(u|U)nused import.*")
 
   test("Write should exist for Unit/Option[Unit]") {
     assertEquals(Write[Unit].length, 0)