Fix #2745: Dealias before either

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

@@ -473,6 +473,12 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       }
       compareTypeLambda
     case OrType(tp21, tp22) =>
+      val tp1a = tp1.widenDealias
+      if (tp1a ne tp1)
+        // Follow the alias; this might avoid truncating the search space in the either below
+        // Note that it's safe to widen here because singleton types cannot be part of `|`.
+        return isSubType(tp1a, tp2)
+
       // Rewrite T1 <: (T211 & T212) | T22 to T1 <: (T211 | T22) and T1 <: (T212 | T22)
       // and analogously for T1 <: T21 | (T221 & T222)
       // `|' types to the right of <: are problematic, because
@@ -591,6 +597,10 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       }
       compareHKLambda
     case AndType(tp11, tp12) =>
+      val tp2a = tp2.dealias
+      if (tp2a ne tp2) // Follow the alias; this might avoid truncating the search space in the either below
+        return isSubType(tp1, tp2a)
+
       // Rewrite (T111 | T112) & T12 <: T2 to (T111 & T12) <: T2 and (T112 | T12) <: T2
       // and analogously for T11 & (T121 | T122) & T12 <: T2
       // `&' types to the left of <: are problematic, because

tests/pos/i2745.scala

@@ -0,0 +1,17 @@
+object Test {
+  type Result[A] = Errors | A
+  final case class Errors(msgs: Seq[String])
+
+  implicit class RichRes[A](val res: Result[A]) extends AnyVal {
+    def map[B](f: A => B): Result[B] = res match {
+      case xs: Errors => xs
+      case a: A => f(a)
+    }
+  }
+
+  var foo: Result[String] = ???
+  def f(str: String): Int = ???
+
+  foo.map(f(_)) // error
+
+}