Fixes for comparing and avoiding singletons

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

@@ -525,14 +525,10 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       /** if `tp2 == p.type` and `p: q.type` then try `tp1 <:< q.type` as a last effort.*/
       def comparePaths = tp2 match {
         case tp2: TermRef =>
-          tp2.info match {
-            case tp2i: TermRef =>
-              isSubType(tp1, tp2i)
-            case ExprType(tp2i: TermRef) if (ctx.phase.id > ctx.gettersPhase.id) =>
-              // After getters, val x: T becomes def x: T
-              isSubType(tp1, tp2i)
-            case _ =>
-              false
+          tp2.info.widenExpr match {
+            case tp2i: SingletonType =>
+              isSubType(tp1, tp2i) // see z1720.scala for a case where this can arise even in typer.
+            case _ => false
           }
         case _ =>
           false

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

@@ -59,7 +59,10 @@ trait TypeAssigner {
           case _ => false
         }
       def apply(tp: Type): Type = tp match {
-        case tp: TermRef if toAvoid(tp) && variance > 0 =>
+        case tp: TermRef
+        if toAvoid(tp) && (variance > 0 || tp.info.widenExpr <:< tp) =>
+          // Can happen if `x: y.type`, then `x.type =:= y.type`, hence we can widen `x.type`
+          // to y.type in all contexts, not just covariant ones.
           apply(tp.info.widenExpr)
         case tp: TypeRef if toAvoid(tp) =>
           tp.info match {

test/dotc/tests.scala

@@ -111,6 +111,13 @@ class tests extends CompilerTest {
 
   @Test def pos_859 = compileFile(posSpecialDir, "i859", scala2mode)(allowDeepSubtypes)
 
+  @Test def pos_t5545 = {
+    // compile by hand in two batches, since junit lacks the infrastructure to
+    // compile files in multiple batches according to _1, _2, ... suffixes.
+    compileFile(posSpecialDir, "spec-t5545/S_1")
+    compileFile(posSpecialDir, "spec-t5545/S_2")
+  }
+
   @Test def new_all = compileFiles(newDir, twice)
   @Test def repl_all = replFiles(replDir)