From 92aabde4c2da5a2717bd7d7d443415a5c1bbccaf Mon Sep 17 00:00:00 2001
From: Yichen Xu <yichen.x@outlook.com>
Date: Sat, 30 Apr 2022 12:27:06 +0800
Subject: [PATCH] try LHS and type before tryBaseType when comparing applied
 types in GADT mode

---
 .../dotty/tools/dotc/core/TypeComparer.scala  |  9 ++++++++
 tests/neg/i11545.scala                        | 21 +++++++++++++++++++
 2 files changed, 30 insertions(+)
 create mode 100644 tests/neg/i11545.scala

diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index e1605c326a05..ed610a7cedb6 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -1198,6 +1198,14 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             case _ => false
         } && { GADTused = true; true }
 
+      def tryGadtAnd1: Boolean =
+        ctx.mode.is(Mode.GadtConstraintInference) && {
+          tp1 match
+            case AndType(tp11, tp12) =>
+              either(recur(tp11, tp2), recur(tp12, tp2))
+            case _ => false
+        }
+
       tycon2 match {
         case param2: TypeParamRef =>
           isMatchingApply(tp1) ||
@@ -1213,6 +1221,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
               case info2: ClassInfo =>
                 tycon2.name.startsWith("Tuple") &&
                   defn.isTupleNType(tp2) && recur(tp1, tp2.toNestedPairs) ||
+                tryGadtAnd1 ||
                 tryBaseType(info2.cls)
               case _ =>
                 fourthTry
diff --git a/tests/neg/i11545.scala b/tests/neg/i11545.scala
new file mode 100644
index 000000000000..27f4b4f4b559
--- /dev/null
+++ b/tests/neg/i11545.scala
@@ -0,0 +1,21 @@
+// Taken from
+// https://github.com/lampepfl/dotty/issues/11545#issuecomment-787609144
+class test0 {
+  type AA
+  trait S[A]
+  trait Inv[A]
+
+  class P[X] extends S[Inv[X] & AA]
+
+}
+
+@main def test: Unit =
+  new test0:
+    type AA = Inv[String]
+
+    def patmat[A, Y](s: S[Inv[A] & Y]): A = s match {
+      case p: P[x] =>
+        "Hello"  // error
+    }
+
+    val got: Int = patmat[Int, AA](new P)