[Variadic Generics] Add parser tests for variadic generics syntax.

Author: hborla

Tests/SwiftParserTest/VariadicGenericsTests.swift

@@ -0,0 +1,63 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+
+import XCTest
+
+final class VariadicGenericsTests: XCTestCase {
+  func testSimple() {
+    AssertParse(
+      """
+      func tuplify<T...>(_ t: (each T)...) -> ((each T)...) {
+      }
+      """
+    )
+  }
+
+  func testRequirement() {
+    AssertParse(
+      """
+      func requirement<T...>() where each T: P {
+      }
+      """
+    )
+  }
+
+  func testElementOutsideExpansion() {
+    // This is valid to parse, and becomes invalid during type resolution.
+    AssertParse(
+      """
+      func invalid<T...>(_ t: each T) -> each T {}
+      """
+    )
+  }
+
+  func testInvalidPackElement() {
+    AssertParse(
+      """
+      func invalid<T...>() -> (each any 1️⃣T) {}
+      """,
+      diagnostics: [
+        DiagnosticSpec(message: "expected ',' in tuple type")
+      ]
+    )
+
+    AssertParse(
+      """
+      func invalid<T...>(_: each T 1️⃣& P) {}
+      """,
+      diagnostics: [
+        DiagnosticSpec(message: "unexpected code '& P' in parameter clause")
+      ]
+    )
+  }
+}