Fix path error disgnastics

Fix wording so that it works for nested errors as well.
Incorparte Tiark's latest example.

Author: odersky

src/dotty/tools/dotc/core/TypeOps.scala

@@ -633,7 +633,6 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
     if (scala2Mode) migrationWarning(msg, pos)
     scala2Mode
   }
-
 }
 
 object TypeOps {
@@ -646,16 +645,16 @@ object TypeOps {
 
   object Realizable extends Realizability("")
 
-  object NotConcrete extends Realizability("it is not a concrete type")
+  object NotConcrete extends Realizability(" is not a concrete type")
 
-  object NotStable extends Realizability("it is not a stable reference")
+  object NotStable extends Realizability(" is not a stable reference")
 
   class NotFinal(sym: Symbol)(implicit ctx: Context)
-  extends Realizability(i"it refers to nonfinal $sym")
+  extends Realizability(i" refers to nonfinal $sym")
 
   class HasProblemBounds(mbr: SingleDenotation)(implicit ctx: Context)
-  extends Realizability(i"it has a member $mbr with possibly conflicting bounds ${mbr.info.bounds.lo} <: ... <: ${mbr.info.bounds.hi}")
+  extends Realizability(i" has a member $mbr with possibly conflicting bounds ${mbr.info.bounds.lo} <: ... <: ${mbr.info.bounds.hi}")
 
   class ProblemInUnderlying(tp: Type, problem: Realizability)(implicit ctx: Context)
-  extends Realizability(i"its underlying type ${tp} ${problem.msg}")
+  extends Realizability(i"s underlying type ${tp}${problem.msg}")
 }

src/dotty/tools/dotc/typer/Checking.scala

@@ -325,7 +325,7 @@ trait Checking {
   def checkRealizable(tp: Type, pos: Position)(implicit ctx: Context): Unit = {
     val rstatus = ctx.realizability(tp)
     if (rstatus ne TypeOps.Realizable) {
-      def msg = d"$tp is not a legal path since ${rstatus.msg}"
+      def msg = d"$tp is not a legal path since it${rstatus.msg}"
       if (ctx.scala2Mode) ctx.migrationWarning(msg, pos) else ctx.error(msg, pos)
     }
   }

test/dotc/tests.scala

@@ -156,7 +156,7 @@ class tests extends CompilerTest {
   @Test def neg_i705 = compileFile(negDir, "i705-inner-value-class", xerrors = 7)
   @Test def neg_i866 = compileFile(negDir, "i866", xerrors = 2)
   @Test def neg_i974 = compileFile(negDir, "i974", xerrors = 2)
-  @Test def neg_i1050 = compileFile(negDir, "i1050", xerrors = 5)
+  @Test def neg_i1050 = compileFile(negDir, "i1050", xerrors = 6)
   @Test def neg_i1050a = compileFile(negDir, "i1050a", xerrors = 2)
   @Test def neg_moduleSubtyping = compileFile(negDir, "moduleSubtyping", xerrors = 4)
   @Test def neg_escapingRefs = compileFile(negDir, "escapingRefs", xerrors = 2)

tests/neg/i1050.scala

@@ -63,6 +63,22 @@ object Tiark2 {
     val v = new V {}
     v.brand("boom!"): Nothing
 }
+object Tiark3 {
+    trait A { type L <: Nothing }
+    trait B { type L >: Any}
+    trait U {
+      type X <: B
+      def p2: X
+      final lazy val p: X = p2
+      def brand(x: Any): p.L = x
+    }
+    trait V extends U {
+      type X = B with A
+      def p2: X = ???
+    }
+    val v = new V {}
+    v.brand("boom!"): Nothing
+}
 /*
 object Import {
     trait A { type L <: Nothing }