Report the correct errors in the IDE

In some situations (like when interpolating type variables) the logic
changes depending on whether we have unreported errors or not.
Before this commit, we assumed that any error in a StoreReporter was
unreported, but this is wrong: the IDE, the REPL and other tools use
a StoreReporter as their root reporter.

This commit fixes this by making sure all the code that looks for
unreported errors does so by calling `Reporter#hasPendingErrors` and
changing the implementation of `StoreReporter#hasPendingErrors` to
return false when there is no outer reporter.

Author: smarter

compiler/src/dotty/tools/dotc/reporting/StoreReporter.scala

@@ -28,8 +28,11 @@ class StoreReporter(outer: Reporter) extends Reporter {
     infos += m
   }
 
+  /** Does this reporter contains errors that have yet to be reported by its outer reporter ?
+   *  Note: this is always false when `outer` is null.
+   */
   override def hasPendingErrors: Boolean =
-    infos != null && infos.exists(_.isInstanceOf[Error])
+    outer != null && infos != null && infos.exists(_.isInstanceOf[Error])
 
   override def removeBufferedMessages(implicit ctx: Context): List[MessageContainer] =
     if (infos != null) try infos.toList finally infos = null

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

@@ -432,10 +432,7 @@ trait Inferencing { this: Typer =>
       //     found   : Int(1)
       //     required: String
       //     val y: List[List[String]] = List(List(1))
-      val hasUnreportedErrors = state.reporter match {
-        case r: StoreReporter if r.hasErrors => true
-        case _ => false
-      }
+      val hasUnreportedErrors = state.reporter.hasPendingErrors
       def constraint = state.constraint
       for (tvar <- qualifying)
         if (!tvar.isInstantiated && state.constraint.contains(tvar)) {

language-server/test/dotty/tools/languageserver/DiagnosticsTest.scala

@@ -15,4 +15,14 @@ class DiagnosticsTest {
         (m1 to m2, """found:    String("foo")
                      |required: Int""".stripMargin, Error, Some(TypeMismatchID))
       )
+
+  @Test def diagnosticMissingLambdaBody: Unit =
+    code"""object Test {
+          |  Nil.map(x => x).filter(x$m1 =>$m2)
+          |$m3}""".withSource
+      .diagnostics(m1,
+        (m2 to m3, "illegal start of simple expression", Error, Some(IllegalStartSimpleExprID)),
+        (m1 to m1, """found:    Null
+                     |required: Boolean""".stripMargin, Error, Some(TypeMismatchID))
+      )
 }