From 3e04b6f3aa4e4088220f199bd6aa5c6644c22354 Mon Sep 17 00:00:00 2001
From: Olivier Blanvillain <olivier.blanvillain@gmail.com>
Date: Thu, 13 Apr 2017 09:18:46 +0200
Subject: Revert <: Product requierment in pattern matching

The change in question broke the following pattern, commonly used in name based pattern matching:

```scala
object ProdEmpty {
  def _1: Int = ???
  def _2: String = ???
  def isEmpty = true
  def get = this
}
```

This type define both `_1` and `get` + `isEmpty` (but is not <: Product). After #1938, `ProdEmpty` became eligibles for both product and name based pattern. Because "in case of ambiguities, *Product Pattern* is preferred over *Name Based Pattern*", isEmpty wouldn't be used, breaking the scalac semantics.
---
 compiler/src/dotty/tools/dotc/core/Definitions.scala         | 4 +++-
 compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala | 2 +-
 compiler/src/dotty/tools/dotc/typer/Applications.scala       | 7 ++-----
 compiler/src/dotty/tools/dotc/typer/Typer.scala              | 2 +-
 4 files changed, 7 insertions(+), 8 deletions(-)

(limited to 'compiler')

diff --git a/compiler/src/dotty/tools/dotc/core/Definitions.scala b/compiler/src/dotty/tools/dotc/core/Definitions.scala
index a97589d73..eee6ba785 100644
--- a/compiler/src/dotty/tools/dotc/core/Definitions.scala
+++ b/compiler/src/dotty/tools/dotc/core/Definitions.scala
@@ -10,7 +10,6 @@ import scala.collection.{ mutable, immutable }
 import PartialFunction._
 import collection.mutable
 import util.common.alwaysZero
-import typer.Applications
 
 object Definitions {
 
@@ -846,6 +845,9 @@ class Definitions {
     TupleType(elems.size).appliedTo(elems)
   }
 
+  def isProductSubType(tp: Type)(implicit ctx: Context) =
+    tp.derivesFrom(ProductType.symbol)
+
   /** Is `tp` (an alias) of either a scala.FunctionN or a scala.ImplicitFunctionN? */
   def isFunctionType(tp: Type)(implicit ctx: Context) = {
     val arity = functionArity(tp)
diff --git a/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala b/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
index 41a1218eb..447a003e7 100644
--- a/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
+++ b/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
@@ -1408,7 +1408,7 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {
       protected def seqTree(binder: Symbol)                = tupleSel(binder)(firstIndexingBinder + 1)
       protected def tupleSel(binder: Symbol)(i: Int): Tree = {
         val accessors =
-          if (Applications.canProductMatch(binder.info))
+          if (defn.isProductSubType(binder.info))
             productSelectors(binder.info)
           else binder.caseAccessors
         val res =
diff --git a/compiler/src/dotty/tools/dotc/typer/Applications.scala b/compiler/src/dotty/tools/dotc/typer/Applications.scala
index c4d3e2292..7e17abbcd 100644
--- a/compiler/src/dotty/tools/dotc/typer/Applications.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Applications.scala
@@ -48,9 +48,6 @@ object Applications {
     ref.info.widenExpr.dealias
   }
 
-  def canProductMatch(tp: Type)(implicit ctx: Context) =
-    extractorMemberType(tp, nme._1).exists
-
   /** Does `tp` fit the "product match" conditions as an unapply result type
    *  for a pattern with `numArgs` subpatterns?
    *  This is the case of `tp` has members `_1` to `_N` where `N == numArgs`.
@@ -72,7 +69,7 @@ object Applications {
   }
 
   def productArity(tp: Type)(implicit ctx: Context) =
-    if (canProductMatch(tp)) productSelectorTypes(tp).size else -1
+    if (defn.isProductSubType(tp)) productSelectorTypes(tp).size else -1
 
   def productSelectors(tp: Type)(implicit ctx: Context): List[Symbol] = {
     val sels = for (n <- Iterator.from(0)) yield tp.member(nme.selectorName(n)).symbol
@@ -114,7 +111,7 @@ object Applications {
         getUnapplySelectors(getTp, args, pos)
       else if (unapplyResult isRef defn.BooleanClass)
         Nil
-      else if (canProductMatch(unapplyResult))
+      else if (defn.isProductSubType(unapplyResult))
         productSelectorTypes(unapplyResult)
           // this will cause a "wrong number of arguments in pattern" error later on,
           // which is better than the message in `fail`.
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 4bf938fd4..02538671e 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -759,7 +759,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
       /** Is `formal` a product type which is elementwise compatible with `params`? */
       def ptIsCorrectProduct(formal: Type) = {
         isFullyDefined(formal, ForceDegree.noBottom) &&
-        Applications.canProductMatch(formal) &&
+        defn.isProductSubType(formal) &&
         Applications.productSelectorTypes(formal).corresponds(params) {
           (argType, param) =>
             param.tpt.isEmpty || argType <:< typedAheadType(param.tpt).tpe
-- 
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