Fix #4709 and #3253: report CyclicError in implicit search better

- Record in CyclicReference if it's emitted during implicit search.
- Before checking if the error is on an implicit, check if we have an error
  during implicit search that happens while inferring a return type; that can
  happen whether the member we're typechecking is implicit or not, and needs the
  same cure.
- Add ErrorMessagesTests as requested

This improves the error message, and also puts more effort into diagnosing the
cycle: we now use similar logic as for recursive/overloaded members to check
we're indeed dealing with a (potentially) recursive implicit.

Remaining `CyclicReferenceInvolvingImplicit` are unexpected and might involve
bugs, so make the error message and explanation more tentative, following
`CyclicReferenceInvolving`.

*However*, this does not yet handle mutually recursive methods in a reasonable
way.

Author: Blaisorblade

compiler/src/dotty/tools/dotc/core/TypeErrors.scala

@@ -98,13 +98,28 @@ object handleRecursive {
   }
 }
 
+/**
+ * This TypeError signals that completing denot encountered a cycle: it asked for denot.info (or similar),
+ * so it requires knowing denot already.
+ * @param denot
+ */
 class CyclicReference private (val denot: SymDenotation) extends TypeError {
+  var inImplicitSearch: Boolean = false
 
-  override def toMessage(implicit ctx: Context) = {
+  override def toMessage(implicit ctx: Context): Message = {
+    val cycleSym = denot.symbol
 
-    def errorMsg(cx: Context): Message =
+    /* This CyclicReference might have arisen from asking for `m`'s type while trying to infer it.
+     * To try to diagnose this, walk the context chain searching for context in
+     * Mode.InferringReturnType for the innermost member without type
+     * annotations (!tree.tpt.typeOpt.exists).
+     */
+    def errorMsg(cx: Context): Message = {
       if (cx.mode is Mode.InferringReturnType) {
         cx.tree match {
+          case tree: untpd.ValOrDefDef if inImplicitSearch && !tree.tpt.typeOpt.exists =>
+            // Can happen in implicit defs (#4709) or outside (#3253).
+            TermMemberNeedsResultTypeForImplicitSearch(cycleSym)
           case tree: untpd.DefDef if !tree.tpt.typeOpt.exists =>
             OverloadedOrRecursiveMethodNeedsResultType(tree.name)
           case tree: untpd.ValDef if !tree.tpt.typeOpt.exists =>
@@ -113,13 +128,14 @@ class CyclicReference private (val denot: SymDenotation) extends TypeError {
             errorMsg(cx.outer)
         }
       }
-      else CyclicReferenceInvolving(denot)
+      // Give up and give generic errors.
+      else if (cycleSym.is(Implicit, butNot = Method) && cycleSym.owner.isTerm)
+        CyclicReferenceInvolvingImplicit(cycleSym)
+      else
+        CyclicReferenceInvolving(denot)
+    }
 
-    val cycleSym = denot.symbol
-    if (cycleSym.is(Implicit, butNot = Method) && cycleSym.owner.isTerm)
-      CyclicReferenceInvolvingImplicit(cycleSym)
-    else
-      errorMsg(ctx)
+    errorMsg(ctx)
   }
 }
 

compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java

@@ -131,6 +131,7 @@ public enum ErrorMessageID {
     MatchCaseOnlyNullWarningID,
     ImportRenamedTwiceID,
     TypeTestAlwaysSucceedsID,
+    TermMemberNeedsNeedsResultTypeForImplicitSearchID
     ;
 
     public int errorNumber() {

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -1260,7 +1260,7 @@ object messages {
            |""".stripMargin
   }
 
-  case class OverloadedOrRecursiveMethodNeedsResultType(method: Names.TermName)(implicit ctx: Context)
+  case class OverloadedOrRecursiveMethodNeedsResultType(method: Names.Name)(implicit ctx: Context)
   extends Message(OverloadedOrRecursiveMethodNeedsResultTypeID) {
     val kind = "Syntax"
     val msg = hl"""overloaded or recursive method ${method} needs return type"""
@@ -1300,8 +1300,10 @@ object messages {
     val kind = "Syntax"
     val msg = hl"""cyclic reference involving implicit $cycleSym"""
     val explanation =
-      hl"""|This happens when the right hand-side of $cycleSym's definition involves an implicit search.
-           |To avoid this error, give `${cycleSym.name}` an explicit type.
+      hl"""|$cycleSym is declared as part of a cycle which makes it impossible for the
+           |compiler to decide upon ${cycleSym.name}'s type.
+           |This might happen when the right hand-side of $cycleSym's definition involves an implicit search.
+           |To avoid this error, try giving `${cycleSym.name}` an explicit type.
            |""".stripMargin
   }
 
@@ -2107,4 +2109,15 @@ object messages {
     }
     val explanation = ""
   }
+
+  // Relative of CyclicReferenceInvolvingImplicit and RecursiveValueNeedsResultType
+  case class TermMemberNeedsResultTypeForImplicitSearch(cycleSym: Symbol)(implicit ctx: Context)
+    extends Message(TermMemberNeedsNeedsResultTypeForImplicitSearchID) {
+    val kind = "Syntax"
+    val msg = hl"""$cycleSym needs result type because its right-hand side attempts implicit search"""
+    val explanation =
+      hl"""|The right hand-side of $cycleSym's definition requires an implicit search at the highlighted position.
+           |To avoid this error, give `${cycleSym.name}` an explicit type.
+           |""".stripMargin
+  }
 }

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

@@ -845,7 +845,15 @@ trait Implicits { self: Typer =>
       else i"type error: ${argument.tpe} does not conform to $pt${err.whyNoMatchStr(argument.tpe, pt)}")
     trace(s"search implicit ${pt.show}, arg = ${argument.show}: ${argument.tpe.show}", implicits, show = true) {
       assert(!pt.isInstanceOf[ExprType])
-      val result = new ImplicitSearch(pt, argument, pos).bestImplicit(contextual = true)
+      val result =
+        try {
+          new ImplicitSearch(pt, argument, pos).bestImplicit(contextual = true)
+        } catch {
+          case ce: CyclicReference =>
+            ce.inImplicitSearch = true
+            throw ce
+        }
+
       result match {
         case result: SearchSuccess =>
           result.tstate.commit()

compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala

@@ -260,7 +260,65 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       assertEquals("value x", denot.show)
     }
 
-  @Test def cyclicReferenceInvolvingImplicit =
+  @Test def cyclicReferenceInvolving2 =
+    checkMessagesAfter(FrontEnd.name) {
+      """
+        |class A {
+        |  implicit val x: T = ???
+        |  type T <: x.type // error: cyclic reference involving value x
+        |}
+      """.stripMargin
+    }
+      .expect { (ictx, messages) =>
+        implicit val ctx: Context = ictx
+
+        assertMessageCount(1, messages)
+        val CyclicReferenceInvolving(denot) :: Nil = messages
+        assertEquals("value x", denot.show)
+      }
+
+  @Test def mutualRecursionre_i2001 =
+    checkMessagesAfter(FrontEnd.name) {
+      """
+        |class A {
+        |  def odd(x: Int) = if (x == 0) false else !even(x-1)
+        |  def even(x: Int) = if (x == 0) true else !odd(x-1) // error: overloaded or recursive method needs result type
+        |}
+      """.stripMargin
+    }
+      .expect { (ictx, messages) =>
+        implicit val ctx: Context = ictx
+
+        assertMessageCount(1, messages)
+        val OverloadedOrRecursiveMethodNeedsResultType(name) :: Nil = messages
+        assertEquals("even", name.show)
+      }
+
+  @Test def mutualRecursion_i2001a =
+    checkMessagesAfter(FrontEnd.name) {
+      """
+        |class A {
+        |  def odd(x: Int) = if (x == 0) false else !even(x-1)
+        |  def even(x: Int) = {
+        |    def foo = {
+        |      if (x == 0) true else !odd(x-1) // error: overloaded or recursive method needs result type
+        |    }
+        |    false
+        |  }
+        |}
+      """.stripMargin
+    }
+      .expect { (ictx, messages) =>
+        implicit val ctx: Context = ictx
+
+        assertMessageCount(1, messages)
+        val OverloadedOrRecursiveMethodNeedsResultType(denot) :: Nil = messages
+        // Not ideal behavior
+        assertEquals("foo", denot.show)
+      }
+
+
+  @Test def termMemberNeedsNeedsResultTypeForImplicitSearch =
     checkMessagesAfter(FrontEnd.name) {
       """
         |object implicitDefs {
@@ -276,10 +334,53 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       implicit val ctx: Context = ictx
 
       assertMessageCount(1, messages)
-      val CyclicReferenceInvolvingImplicit(tree) :: Nil = messages
+      val TermMemberNeedsResultTypeForImplicitSearch(tree) :: Nil = messages
       assertEquals("x", tree.name.show)
     }
 
+  @Test def implicitSearchForcesImplicitRetType_i4709 =
+    checkMessagesAfter(FrontEnd.name) {
+      """
+        |import scala.language.implicitConversions
+        |
+        |class Context
+        |class ContextBase { def settings = 1 }
+        |
+        |class Test {
+        |  implicit def toBase(ctx: Context): ContextBase = ???
+        |
+        |  def test(ctx0: Context) = {
+        |    implicit val ctx = { ctx0.settings; ??? }
+        |  }
+        |}
+      """.stripMargin
+    }
+    .expect{ (ictx, messages) =>
+      implicit val ctx: Context = ictx
+
+      assertMessageCount(1, messages)
+      val TermMemberNeedsResultTypeForImplicitSearch(tree) :: Nil = messages
+      assertEquals("ctx", tree.name.show)
+    }
+
+  @Test def implicitSearchForcesNonImplicitRetTypeOnExplicitImport_i3253 =
+    checkMessagesAfter(FrontEnd.name) {
+      """
+        |import Test.test
+        |
+        |object Test {
+        |  def test = "  " * 10
+        |}
+      """.stripMargin
+    }
+      .expect{ (ictx, messages) =>
+        implicit val ctx: Context = ictx
+
+        assertMessageCount(1, messages)
+        val TermMemberNeedsResultTypeForImplicitSearch(tree) :: Nil = messages
+        assertEquals("test", tree.name.show)
+      }
+
   @Test def superQualMustBeParent =
     checkMessagesAfter(FrontEnd.name) {
       """

tests/neg/i2001a.scala

@@ -0,0 +1,13 @@
+class A {
+  def odd(x: Int) = if (x == 0) false else !even(x-1)
+  def even(x: Int) = {
+    def foo = {
+      if (x == 0) true else !odd(x-1) // error: overloaded or recursive method needs result type
+    }
+    foo
+  }
+  lazy val x = {
+    def foo = x // error
+    foo
+  }
+}

tests/neg/i3253.scala

@@ -0,0 +1,5 @@
+import Test.test
+
+object Test {
+  def test = "  " * 10 // error
+}

tests/neg/i4709.scala

@@ -0,0 +1,11 @@
+class Context
+class ContextBase { def settings = 1 }
+
+class Test {
+  implicit def toBase(ctx: Context): ContextBase = ???
+
+  def test(ctx0: Context) = {
+    implicit val ctx = { ctx0.settings; ??? } // error
+  }
+  def f: Unit = { implicitly[Int]; implicit val i = implicitly[Int] } // error
+}