Show GADT bounds in type mismatch message

The inferred GADT bounds can be non-trivial so it makes sense to display
them.

This commit also showcases the now-improved GADT support in Dotty (both
tests/pos/gadt-eval.scala and tests/neg/gadt-eval.scala used to compile,
and both compile in Scala 2).

compiler/src/dotty/tools/dotc/printing/Formatting.scala

@@ -168,7 +168,12 @@ object Formatting {
       case param: TypeParamRef =>
         s"is a type variable${addendum("constraint", ctx.typeComparer.bounds(param))}"
       case sym: Symbol =>
-        s"is a ${ctx.printer.kindString(sym)}${sym.showExtendedLocation}${addendum("bounds", sym.info)}"
+        val info =
+          if (ctx.gadt.bounds.contains(sym))
+            sym.info & ctx.gadt.bounds(sym)
+          else
+            sym.info
+        s"is a ${ctx.printer.kindString(sym)}${sym.showExtendedLocation}${addendum("bounds", info)}"
       case tp: SkolemType =>
         s"is an unknown value of type ${tp.widen.show}"
     }
@@ -183,7 +188,10 @@ object Formatting {
     def needsExplanation(entry: Recorded) = entry match {
       case param: TypeParamRef => ctx.typerState.constraint.contains(param)
       case skolem: SkolemType => true
-      case _ => false
+      case sym: Symbol =>
+        ctx.gadt.bounds.contains(sym) && ctx.gadt.bounds(sym) != TypeBounds.empty
+      case _ =>
+        assert(false, "unreachable")
     }
 
     val toExplain: List[(String, Recorded)] = seen.toList.flatMap {

tests/neg/gadt-eval.scala

@@ -0,0 +1,33 @@
+sealed trait Exp[T]
+case class Lit(value: Int) extends Exp[Int]
+case class Pair[A, B](fst: Exp[A], snd: Exp[B]) extends Exp[(A, B)]
+
+object Test {
+  def eval[T](e: Exp[T]): T = e match {
+    case Lit(x) =>
+      x
+    case Pair(a, b) =>
+      (eval(a), eval(a)) // error:
+        // -- [E007] Type Mismatch Error: tests/neg/gadt-eval.scala:10:6 ------------------
+        // 10 |      (eval(a), eval(a))
+        // |      ^^^^^^^^^^^^^^^^^^
+        // |    found:    (_$1, _$1)
+        // |    required: T
+        // |
+        // |    where:    T is a type in method eval which is an alias of (_$1, _$2)
+  }
+
+  def eval2[T](e: Exp[T]): T = e match {
+    case e: Lit =>
+      e.value
+    case e: Pair[t1, t2] =>
+      (eval(e.fst), eval(e.fst)) // error:
+        //-- [E007] Type Mismatch Error: tests/neg/gadt-eval.scala:24:6 ------------------
+        //24 |      (eval(e.fst), eval(e.fst))
+        //   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^
+        //   |     found:    (t1, t1)
+        //   |     required: T
+        //   |
+        //   |     where:    T is a type in method eval2 which is an alias of (t1, t2)
+  }
+}

tests/pos/gadt-eval.scala

@@ -0,0 +1,19 @@
+sealed trait Exp[T]
+case class Lit(value: Int) extends Exp[Int]
+case class Pair[A, B](fst: Exp[A], snd: Exp[B]) extends Exp[(A, B)]
+
+object Test {
+  def eval[T](e: Exp[T]): T = e match {
+    case Lit(x) =>
+      x
+    case Pair(a, b) =>
+      (eval(a), eval(b))
+  }
+
+  def eval2[T](e: Exp[T]): T = e match {
+    case e: Lit =>
+      e.value
+    case e: Pair[t1, t2] =>
+      (eval(e.fst), eval(e.snd))
+  }
+}