Use nil instead of .some(nil) for failed captures

Previously we would wrap a `nil` in
`optionalCount - 1` outer layers of `.some(...)`.
Change this to return an `optionalCount` nested
optional with a top-level value of `nil`.

Author: hamishknight

Sources/_StringProcessing/Capture.swift

@@ -17,23 +17,24 @@ func constructExistentialOutputComponent(
   component: (range: Range<String.Index>, value: Any?)?,
   optionalCount: Int
 ) -> Any {
-  let someCount: Int
-  var underlying: Any
   if let component = component {
-    underlying = component.value ?? input[component.range]
-    someCount = optionalCount
+    var underlying = component.value ?? input[component.range]
+    for _ in 0 ..< optionalCount {
+      func wrap<T>(_ x: T) {
+        underlying = Optional(x) as Any
+      }
+      _openExistential(underlying, do: wrap)
+    }
+    return underlying
   } else {
-    // Ok since we Any-box every step up the ladder
-    underlying = Optional<Any>(nil) as Any
-    someCount = optionalCount - 1
-  }
-  for _ in 0..<someCount {
-    func wrap<T>(_ x: T) {
-      underlying = Optional(x) as Any
+    precondition(optionalCount > 0, "Must have optional type")
+    func makeNil<T>(_ x: T.Type) -> Any {
+      T?.none as Any
     }
-    _openExistential(underlying, do: wrap)
+    let underlyingTy = TypeConstruction.optionalType(
+      of: Substring.self, depth: optionalCount - 1)
+    return _openExistential(underlyingTy, do: makeNil)
   }
-  return underlying
 }
 
 @available(SwiftStdlib 5.7, *)

Tests/RegexBuilderTests/RegexDSLTests.swift

@@ -438,6 +438,8 @@ class RegexDSLTests: XCTestCase {
 
     try _testDSLCaptures(
       ("abcdef2", ("abcdef2", "f")),
+      ("2", ("2", nil)),
+      ("", ("", nil)),
       matchType: (Substring, Substring??).self, ==)
     {
       Optionally {
@@ -1216,6 +1218,25 @@ class RegexDSLTests: XCTestCase {
       }
     }
   }
+
+  func testOptionalNesting() throws {
+    let r = Regex {
+      Optionally {
+        Optionally {
+          Capture {
+            "a"
+          }
+        }
+      }
+    }
+    if let _ = try r.wholeMatch(in: "")!.output.1 {
+      XCTFail("Unexpected capture match")
+    }
+    if let _ = try r.wholeMatch(in: "a")!.output.1 {}
+    else {
+      XCTFail("Expected to match capture")
+    }
+  }
 }
 
 extension Unicode.Scalar {