[Issue 15987] Nicer error message in case a derived method has an explicit term param

compiler/src/dotty/tools/dotc/typer/Deriving.scala

@@ -292,10 +292,21 @@ trait Deriving {
           val companion = companionRef(resultType)
           val module = untpd.ref(companion).withSpan(sym.span)
           val rhs = untpd.Select(module, nme.derived)
-          if companion.termSymbol.exists then typed(rhs, resultType)
-          else errorTree(rhs, em"$resultType cannot be derived since ${resultType.typeSymbol} has no companion object")
+          val derivedMember = companion.member(nme.derived)
+
+          if !companion.termSymbol.exists then 
+            errorTree(rhs, em"$resultType cannot be derived since ${resultType.typeSymbol} has no companion object")
+          else if hasExplicitParams(derivedMember.symbol) then 
+            errorTree(rhs, em"""derived instance $resultType failed to generate:
+            |method `derived` from object ${module} takes explicit term parameters""")
+          else 
+            typed(rhs, resultType)
       end typeclassInstance
 
+      // checks whether any of the params of 'sym' is explicit
+      def hasExplicitParams(sym: Symbol) = 
+        !sym.paramSymss.flatten.forall(sym => sym.isType || sym.is(Flags.Given) || sym.is(Flags.Implicit))
+
       def syntheticDef(sym: Symbol): Tree = inContext(ctx.fresh.setOwner(sym).setNewScope) {
         if sym.is(Method) then tpd.DefDef(sym.asTerm, typeclassInstance(sym))
         else tpd.ValDef(sym.asTerm, typeclassInstance(sym)(Nil))

tests/neg/i15987.check

@@ -0,0 +1,5 @@
+-- Error: tests/neg/i15987.scala:26:40 ---------------------------------------------------------------------------------
+26 |case class Person(name: String) derives ShowWithExplicit, // error
+   |                                        ^
+   |                                    derived instance ShowWithExplicit[Person] failed to generate:
+   |                                    method `derived` from object ShowWithExplicit takes explicit term parameters

tests/neg/i15987.scala

@@ -0,0 +1,30 @@
+trait ShowWithExplicit[A]
+
+object ShowWithExplicit:
+  def derived[A, B](explicit: String)(using DummyImplicit)(implicit dummy: DummyImplicit): ShowWithExplicit[A] = ???
+
+trait ShowUsingAndImplicit[A]
+
+object ShowUsingAndImplicit:
+  def derived[A, B](using DummyImplicit)(implicit dummy: DummyImplicit): ShowUsingAndImplicit[A] = ???
+
+trait ShowUsing[A]
+
+object ShowUsing:
+  def derived[A](using DummyImplicit): ShowUsing[A] = ???
+
+trait ShowImplicit[A]
+
+object ShowImplicit:
+  def derived[A](implicit ev: DummyImplicit): ShowImplicit[A] = ???
+
+trait ShowContra[-A]
+
+object ShowContra:
+  val derived: ShowContra[Any] = ???
+
+case class Person(name: String) derives ShowWithExplicit, // error
+  ShowUsingAndImplicit,
+  ShowUsing,
+  ShowImplicit,
+  ShowContra