Obtain match output elements without materializing the output.

Sources/_RegexParser/Utility/TypeConstruction.swift

@@ -139,3 +139,62 @@ public enum TypeConstruction {
     return _openExistential(childType, do: helper)
   }
 }
+
+extension TypeConstruction {
+  public static func optionalType<Base>(
+    of base: Base.Type, depth: Int = 1
+  ) -> Any.Type {
+    switch depth {
+    case 0: return base
+    case 1: return Base?.self
+    case 2: return Base??.self
+    case 3: return Base???.self
+    case 4: return Base????.self
+    default:
+      return optionalType(of: Base????.self, depth: depth - 4)
+    }
+  }
+}
+
+extension MemoryLayout {
+  /// Returns the element index that corresponnds to the given tuple element key
+  /// path.
+  /// - Parameters:
+  ///   - keyPath: The key path from a tuple to one of its elements.
+  ///   - elementTypes: The element type of the tuple type.
+  // TODO: It possible to get element types from the type metadata, but it's
+  // more efficient to pass them in since we already know them in the matching
+  // engine.
+  public static func tupleElementIndex<ElementTypes: Collection>(
+    of keyPath: PartialKeyPath<T>,
+    elementTypes: ElementTypes
+  ) -> Int? where ElementTypes.Element == Any.Type {
+    guard let byteOffset = offset(of: keyPath) else {
+      return nil
+    }
+    if byteOffset == 0 { return 0 }
+    var currentOffset = 0
+    for (index, type) in elementTypes.enumerated() {
+      func sizeAndAlignMask<T>(_: T.Type) -> (Int, Int) {
+        (MemoryLayout<T>.size, MemoryLayout<T>.alignment - 1)
+      }
+      // The ABI of an offset-based key path only stores the byte offset, so
+      // this doesn't work if there's a 0-sized element, e.g. `Void`,
+      // `(Void, Void)`. (rdar://63819465)
+      if size == 0 {
+        return nil
+      }
+      let (size, alignMask) = _openExistential(type, do: sizeAndAlignMask)
+      // Align up the offset for this type.
+      currentOffset = (currentOffset + alignMask) & ~alignMask
+      // If it matches the offset we are looking for, `index` is the tuple
+      // element index.
+      if currentOffset == byteOffset {
+        return index
+      }
+      // Advance to the past-the-end offset for this element.
+      currentOffset += size
+    }
+    return nil
+  }
+}

Sources/_StringProcessing/Regex/AnyRegexOutput.swift

@@ -62,9 +62,11 @@ extension AnyRegexOutput: RandomAccessCollection {
 
     /// The captured value, `nil` for no-capture
     public var value: Any? {
-      // FIXME: Should this return the substring for default-typed
-      // values?
-      representation.value
+      representation.value ?? substring
+    }
+
+    internal var type: Any.Type {
+      representation.type
     }
 
     /// The name of this capture, if it has one, otherwise `nil`.
@@ -263,4 +265,9 @@ extension AnyRegexOutput.ElementRepresentation {
       optionalCount: optionalDepth
     )
   }
+
+  var type: Any.Type {
+    value.map { Swift.type(of: $0) }
+      ?? TypeConstruction.optionalType(of: Substring.self, depth: optionalDepth)
+  }
 }

Sources/_StringProcessing/Regex/Match.swift

@@ -28,6 +28,10 @@ extension Regex {
 
 @available(SwiftStdlib 5.7, *)
 extension Regex.Match {
+  var input: String {
+    anyRegexOutput.input
+  }
+
   /// The output produced from the match operation.
   public var output: Output {
     if Output.self == AnyRegexOutput.self {
@@ -37,33 +41,48 @@ extension Regex.Match {
       )
 
       let output = AnyRegexOutput(
-        input: anyRegexOutput.input,
+        input: input,
         elements: [wholeMatchCapture] + anyRegexOutput._elements
       )
 
       return output as! Output
     } else if Output.self == Substring.self {
       // FIXME: Plumb whole match (`.0`) through the matching engine.
-      return anyRegexOutput.input[range] as! Output
-    } else if anyRegexOutput.isEmpty, value != nil {
+      return input[range] as! Output
+    } else if anyRegexOutput.isEmpty, let value {
       // FIXME: This is a workaround for whole-match values not
       // being modeled as part of captures. We might want to
       // switch to a model where results are alongside captures
-      return value! as! Output
+      return value as! Output
     } else {
       guard value == nil else {
         fatalError("FIXME: what would this mean?")
       }
       let typeErasedMatch = anyRegexOutput.existentialOutput(
-        from: anyRegexOutput.input[range]
+        from: input[range]
       )
       return typeErasedMatch as! Output
     }
   }
 
+  var wholeMatchType: Any.Type {
+    value.map { type(of: $0) } ?? Substring.self
+  }
+
   /// Accesses a capture by its name or number.
   public subscript<T>(dynamicMember keyPath: KeyPath<Output, T>) -> T {
-    output[keyPath: keyPath]
+    // Note: We should be able to get the element offset from the key path
+    // itself even at compile time. We need a better way of doing this.
+    guard let outputTupleOffset = MemoryLayout.tupleElementIndex(
+      of: keyPath, elementTypes: [wholeMatchType] + anyRegexOutput.map(\.type)
+    ) else {
+      return output[keyPath: keyPath]
+    }
+    if outputTupleOffset == 0 {
+      return value.map { $0 as! T } ?? (input[range] as! T)
+    } else {
+      return anyRegexOutput[outputTupleOffset - 1].value as! T
+    }
   }
 
   /// Accesses a capture using the `.0` syntax, even when the match isn't a tuple.
@@ -83,7 +102,7 @@ extension Regex.Match {
     }
 
     return element.existentialOutputComponent(
-      from: anyRegexOutput.input[...]
+      from: input[...]
     ) as! Capture
   }
 }

Tests/RegexBuilderTests/RegexDSLTests.swift

@@ -427,7 +427,19 @@ class RegexDSLTests: XCTestCase {
         CharacterClass.digit
       }
     }
-
+
+    try _testDSLCaptures(
+      ("abcdef2", ("abcdef2", "f")),
+      matchType: (Substring, Substring??).self, ==)
+    {
+      Optionally {
+        ZeroOrMore {
+          Capture(CharacterClass.word)
+        }
+        CharacterClass.digit
+      }
+    }
+
     try _testDSLCaptures(
       ("aaabbbcccdddeeefff", "aaabbbcccdddeeefff"),
       ("aaaabbbcccdddeeefff", nil),