From 9dca4a61a6d903a5de0072bca657dbf151d437ec Mon Sep 17 00:00:00 2001
From: Yichen Xu <yichen.x@outlook.com>
Date: Tue, 31 May 2022 08:08:47 +0800
Subject: [PATCH 1/2] Consider GADT upper bounds when upcasting the scrutinee
 type

(cherry picked from commit 172e957422f31ab0b5305e5127a52203cc9ea409)
---
 .../dotc/core/PatternTypeConstrainer.scala    | 27 ++++++++++++++-----
 tests/pos/i15274.orig.scala                   | 21 +++++++++++++++
 tests/pos/i15274.scala                        |  6 +++++
 3 files changed, 48 insertions(+), 6 deletions(-)
 create mode 100644 tests/pos/i15274.orig.scala
 create mode 100644 tests/pos/i15274.scala

diff --git a/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala b/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala
index ac7cbf6c36a6..de2d3d658508 100644
--- a/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala
+++ b/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala
@@ -138,13 +138,28 @@ trait PatternTypeConstrainer { self: TypeComparer =>
           val andType = buildAndType(baseClasses)
           !andType.exists || constrainPatternType(pat, andType)
         case _ =>
-          val upcasted: Type = scrut match {
-            case scrut: TypeProxy => scrut.superType
-            case _ => NoType
+          def tryGadtBounds = scrut match {
+            case scrut: TypeRef =>
+              ctx.gadt.bounds(scrut.symbol) match {
+                case tb: TypeBounds =>
+                  val hi = tb.hi
+                  constrainPatternType(pat, hi)
+                case null => false
+              }
+            case _ => false
           }
-          if (upcasted.exists)
-            tryConstrainSimplePatternType(pat, upcasted) || constrainUpcasted(upcasted)
-          else true
+
+          def trySuperType =
+            val upcasted: Type = scrut match {
+              case scrut: TypeProxy =>
+                scrut.superType
+              case _ => NoType
+            }
+            if (upcasted.exists)
+              tryConstrainSimplePatternType(pat, upcasted) || constrainUpcasted(upcasted)
+            else true
+
+          tryGadtBounds || trySuperType
       }
     }
 
diff --git a/tests/pos/i15274.orig.scala b/tests/pos/i15274.orig.scala
new file mode 100644
index 000000000000..5d6887028f4f
--- /dev/null
+++ b/tests/pos/i15274.orig.scala
@@ -0,0 +1,21 @@
+enum Format[A]:
+  case Str[Next](next: Format[Next]) extends Format[(String, Next)]
+  case Num[Next](next: Format[Next]) extends Format[(Int, Next)]
+  case Constant[Next](value: String, next: Format[Next]) extends Format[Next]
+  case Done extends Format[Unit]
+
+def printf[A](format: Format[A], params: A): Unit = (format, params) match
+  case (Format.Done, ()) =>
+    ()
+
+  case (Format.Constant(value, next), params) =>
+    println(value)
+    printf(next, params)
+
+  case (Format.Str(next), (str, rest)) =>
+    println(str)
+    printf(next, rest)
+
+  case (Format.Num(next), (i, rest)) =>
+    println(i)
+    printf(next, rest)
diff --git a/tests/pos/i15274.scala b/tests/pos/i15274.scala
new file mode 100644
index 000000000000..2862fd8f8cbc
--- /dev/null
+++ b/tests/pos/i15274.scala
@@ -0,0 +1,6 @@
+enum Format[A]:
+  case Str[Next](next: Format[Next]) extends Format[(String, Next)]
+
+def printf[A](format: Format[A], params: A): Unit = (format, params) match
+  case (Format.Str(next), (str, rest)) =>
+    val s: String = str

From 1c3a2866ca450716af07c2827883d448bdf70a78 Mon Sep 17 00:00:00 2001
From: Dale Wijnand <dale.wijnand@gmail.com>
Date: Tue, 7 Jun 2022 10:38:15 +0100
Subject: [PATCH 2/2] Constrain patterns by both GADT upper bound & upcast

---
 .../src/dotty/tools/dotc/core/PatternTypeConstrainer.scala  | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala b/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala
index de2d3d658508..119c3342fa81 100644
--- a/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala
+++ b/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala
@@ -144,9 +144,9 @@ trait PatternTypeConstrainer { self: TypeComparer =>
                 case tb: TypeBounds =>
                   val hi = tb.hi
                   constrainPatternType(pat, hi)
-                case null => false
+                case null => true
               }
-            case _ => false
+            case _ => true
           }
 
           def trySuperType =
@@ -159,7 +159,7 @@ trait PatternTypeConstrainer { self: TypeComparer =>
               tryConstrainSimplePatternType(pat, upcasted) || constrainUpcasted(upcasted)
             else true
 
-          tryGadtBounds || trySuperType
+          tryGadtBounds && trySuperType
       }
     }