From 212ff60ba7a7a641449f6b6b5d8e5ccfd6efdcb8 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Tue, 16 Feb 2021 19:09:17 +0100
Subject: [PATCH] More robust comparison of type constructors in
 provablyDisjoint

A type comparison with `==` should be used only for exploring opportunities to optimize,
never where it affects the logic. I.e. we need to always have a fallback that checks
via `=:=`.

Fixes #11393
---
 compiler/src/dotty/tools/dotc/config/Printers.scala   | 1 +
 compiler/src/dotty/tools/dotc/core/TypeComparer.scala | 9 +++++----
 compiler/src/dotty/tools/dotc/core/Types.scala        | 4 ++--
 tests/pos/i11393/Format_1.scala                       | 7 +++++++
 tests/pos/i11393/Test_2.scala                         | 5 +++++
 5 files changed, 20 insertions(+), 6 deletions(-)
 create mode 100644 tests/pos/i11393/Format_1.scala
 create mode 100644 tests/pos/i11393/Test_2.scala

diff --git a/compiler/src/dotty/tools/dotc/config/Printers.scala b/compiler/src/dotty/tools/dotc/config/Printers.scala
index 8f9e480e0330..27391154591f 100644
--- a/compiler/src/dotty/tools/dotc/config/Printers.scala
+++ b/compiler/src/dotty/tools/dotc/config/Printers.scala
@@ -31,6 +31,7 @@ object Printers {
   val init = noPrinter
   val inlining = noPrinter
   val interactiv = noPrinter
+  val matchTypes = noPrinter
   val nullables = noPrinter
   val overload = noPrinter
   val patmatch = noPrinter
diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index 91f5c2258ea7..8bf13b9c6f2b 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -11,7 +11,7 @@ import collection.mutable
 import util.Stats
 import config.Config
 import config.Feature.migrateTo3
-import config.Printers.{constr, subtyping, gadts, noPrinter}
+import config.Printers.{constr, subtyping, gadts, matchTypes, noPrinter}
 import TypeErasure.{erasedLub, erasedGlb}
 import TypeApplications._
 import Variances.{Variance, variancesConform}
@@ -2408,7 +2408,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
    *  property that in all possible contexts, the same match type expression
    *  is either stuck or reduces to the same case.
    */
-  def provablyDisjoint(tp1: Type, tp2: Type)(using Context): Boolean = {
+  def provablyDisjoint(tp1: Type, tp2: Type)(using Context): Boolean = trace(i"provable disjoint $tp1, $tp2", matchTypes) {
     // println(s"provablyDisjoint(${tp1.show}, ${tp2.show})")
 
     def isEnumValueOrModule(ref: TermRef): Boolean =
@@ -2452,7 +2452,8 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             decompose(cls2, tp2).forall(x => provablyDisjoint(x, tp1))
           else
             false
-      case (AppliedType(tycon1, args1), AppliedType(tycon2, args2)) if tycon1 == tycon2 =>
+      case (AppliedType(tycon1, args1), AppliedType(tycon2, args2))
+      if tycon1.typeSymbol == tycon2.typeSymbol && tycon1 =:= tycon2 =>
         // It is possible to conclude that two types applies are disjoint by
         // looking at covariant type parameters if the said type parameters
         // are disjoin and correspond to fields.
@@ -2768,7 +2769,7 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
      *           None         if the match fails and we should consider the following cases
      *                        because scrutinee and pattern do not overlap
      */
-    def matchCase(cas: Type): Option[Type] = {
+    def matchCase(cas: Type): Option[Type] = trace(i"match case $cas vs $scrut", matchTypes) {
       val cas1 = cas match {
         case cas: HKTypeLambda =>
           caseLambda = constrained(cas)
diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 971267112723..068330b52a31 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -33,7 +33,7 @@ import config.Config
 import annotation.{tailrec, constructorOnly}
 import language.implicitConversions
 import scala.util.hashing.{ MurmurHash3 => hashing }
-import config.Printers.{core, typr}
+import config.Printers.{core, typr, matchTypes}
 import reporting.{trace, Message}
 import java.lang.ref.WeakReference
 
@@ -4489,7 +4489,7 @@ object Types {
         record("MatchType.reduce computed")
         if (myReduced != null) record("MatchType.reduce cache miss")
         myReduced =
-          trace(i"reduce match type $this $hashCode", typr, show = true) {
+          trace(i"reduce match type $this $hashCode", matchTypes, show = true) {
             def matchCases(cmp: TrackingTypeComparer): Type =
               try cmp.matchCases(scrutinee.normalized, cases)
               catch case ex: Throwable =>
diff --git a/tests/pos/i11393/Format_1.scala b/tests/pos/i11393/Format_1.scala
new file mode 100644
index 000000000000..22abde4ccdd1
--- /dev/null
+++ b/tests/pos/i11393/Format_1.scala
@@ -0,0 +1,7 @@
+object Formatt:
+  type ToFormat[X <: Tuple] = X match
+      case EmptyTuple => String
+      case '%' *: 's' *: ts => (String => ToFormat[ts])
+      case Char *: ts => ToFormat[ts]
+
+
diff --git a/tests/pos/i11393/Test_2.scala b/tests/pos/i11393/Test_2.scala
new file mode 100644
index 000000000000..9b4fed33a1af
--- /dev/null
+++ b/tests/pos/i11393/Test_2.scala
@@ -0,0 +1,5 @@
+@main def hello: Unit = {
+  val x: Formatt.ToFormat['a' *: EmptyTuple] =  ""
+
+
+}