[Variadic Generics] Add parsing support for a contextual 'each' keyword applied

to type parameter pack references.

Author: hborla

Sources/SwiftParser/Patterns.swift

@@ -350,7 +350,8 @@ extension Parser.Lookahead {
       // If the first name wasn't "isolated", we're done.
       if !self.atContextualKeyword("isolated") &&
           !self.atContextualKeyword("some") &&
-          !self.atContextualKeyword("any") {
+          !self.atContextualKeyword("any") &&
+          !self.atContextualKeyword("each") {
         return true
       }
 

Sources/SwiftParser/Types.swift

@@ -120,6 +120,13 @@ extension Parser {
   ///     protocol-composition-continuation → type-identifier | protocol-composition-type
   @_spi(RawSyntax)
   public mutating func parseSimpleOrCompositionType() -> RawTypeSyntax {
+    // 'each' is a contextual keyword for a pack reference.
+    if let each = consume(ifAny: [], contextualKeywords: ["each"]) {
+      let packType = parseSimpleType()
+      return RawTypeSyntax(RawPackReferenceTypeSyntax(
+        eachKeyword: each, packType: packType, arena: self.arena))
+    }
+
     let someOrAny = self.consume(ifAny: [], contextualKeywords: ["some", "any"])
 
     var base = self.parseSimpleType()