Decouple Product and pattern-matching

Product pattern use to:
- have a `<: Product` requirement
- compute the arity of a pattern by looking at `N` in a `ProductN` superclass.

This commit changes `<: Product`, instead we look for a `_1` member. The arity is determined by inspecting `_1` to `_N` members instead.

---

Here another attempt to formalize Dotty's pattern-matching (base on #1805). This covers the *Extractor Patterns* [section of the spec](https://www.scala-lang.org/files/archive/spec/2.12/08-pattern-matching.html#extractor-patterns). Dotty support 4 different extractor patterns: Boolean Pattern, Product Pattern, Seq Pattern and Name Based Pattern.

Boolean Pattern

- Extractor defines `def unapply(x: T): Boolean`
- Pattern-matching on exactly `0` patterns

Product Pattern

- Extractor defines `def unapply(x: T): U`
- `N > 0` is the maximum number of consecutive (parameterless `def` or `val`) `_1: P1` ... `_N: PN` members in `U`
- Pattern-matching on exactly `N` patterns with types `P1, P2, ..., PN`

Seq Pattern

- Extractor defines `def unapplySeq(x: T): U`
- `U` has (parameterless `def` or `val`) members `isEmpty: Boolean` and `get: S`
- `S <: Seq[V]`
- Pattern-matching on any number of pattern with types `V, V, ..., V`

Name Based Pattern

- Extractor defines `def unapply(x: T): U`
- `U` has (parameterless `def` or `val`) members `isEmpty: Boolean` and `get: S`
- If there is exactly `1` pattern, pattern-matching on `1` pattern with type `S`
- Otherwise fallback to Product Pattern on type `U`

In case of ambiguities, *Product Pattern* is preferred over *Name Based Pattern*.

1 files changed, 2 insertions, 3 deletions

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 2760ceba9..c86b15871 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -761,10 +761,9 @@ 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) = {
-        val pclass = defn.ProductNType(params.length).symbol
         isFullyDefined(formal, ForceDegree.noBottom) &&
-        formal.derivesFrom(pclass) &&
-        formal.baseArgTypes(pclass).corresponds(params) {
+        defn.isProductSubType(formal) &&
+        Applications.productSelectorTypes(formal).corresponds(params) {
           (argType, param) =>
             param.tpt.isEmpty || argType <:< typedAheadType(param.tpt).tpe
         }