Throw error when Output is wrong type

Sources/_StringProcessing/Compiler.swift

@@ -47,10 +47,14 @@ enum RegexCompilationError: Error, CustomStringConvertible {
   // TODO: Source location?
   case uncapturedReference
 
+  case incorrectOutputType(incorrect: Any.Type, correct: Any.Type)
+
   var description: String {
     switch self {
     case .uncapturedReference:
       return "Found a reference used before it captured any match."
+    case .incorrectOutputType(let incorrect, let correct):
+      return "Cast to incorrect type 'Regex<\(incorrect)>', expected 'Regex<\(correct)>'"
     }
   }
 }

Sources/_StringProcessing/Regex/AnyRegexOutput.swift

@@ -161,7 +161,18 @@ extension Regex {
     _ pattern: String,
     as: Output.Type = Output.self
   ) throws {
-    self.init(ast: try parse(pattern, .traditional))
+    let regex = Regex(ast: try parse(pattern, .traditional))
+
+    let (isSuccess, correctType) = regex._verifyType()
+
+    guard isSuccess else {
+      throw RegexCompilationError.incorrectOutputType(
+        incorrect: Output.self,
+        correct: correctType
+      )
+    }
+
+    self = regex
   }
 
   /// Produces a regex that matches `verbatim` exactly, as though every
@@ -217,7 +228,8 @@ extension Regex {
     as: Output.Type = Output.self
   ) {
     self.init(node: erased.root)
-    guard self._verifyType() else {
+
+    guard _verifyType().0 else {
       return nil
     }
   }

Sources/_StringProcessing/Utility/TypeVerification.swift

@@ -13,16 +13,16 @@
 
 @available(SwiftStdlib 5.7, *)
 extension Regex {
-  internal func _verifyType() -> Bool {
+  internal func _verifyType() -> (Bool, Any.Type) {
     guard Output.self != AnyRegexOutput.self else {
-      return true
+      return (true, Output.self)
     }
 
     var tupleElements: [Any.Type] = []
     var labels = ""
 
     for capture in program.tree.captureList.captures {
-      var captureType: Any.Type = capture.type ?? Substring.self
+      var captureType = capture.type
       var i = capture.optionalDepth
 
       while i != 0 {
@@ -41,7 +41,8 @@ extension Regex {
 
     // If we have no captures, then our Regex must be Regex<Substring>.
     if tupleElements.count == 1 {
-      return Output.self == program.tree.root.wholeMatchType
+      let wholeMatchType = program.tree.root.wholeMatchType
+      return (Output.self == wholeMatchType, wholeMatchType)
     }
 
     let createdType = TypeConstruction.tupleType(
@@ -52,6 +53,6 @@ extension Regex {
       labels: labels.all { $0 == " " } ? nil : labels
     )
 
-    return Output.self == createdType
+    return (Output.self == createdType, createdType)
   }
 }

Tests/RegexBuilderTests/AnyRegexOutputTests.swift

@@ -218,6 +218,17 @@ extension RegexDSLTests {
       noteOutput
     )
 
+    // Run-time strings (errors)
+    XCTAssertThrowsError(try Regex("abc", as: (Substring, Substring).self))
+    XCTAssertThrowsError(try Regex("(abc)", as: Substring.self))
+    XCTAssertThrowsError(try Regex("(?<test>abc)", as: (Substring, Substring).self))
+    XCTAssertThrowsError(try Regex("(?<test>abc)?", as: (Substring, test: Substring).self))
+
+    XCTAssertNoThrow(try Regex("abc", as: Substring.self))
+    XCTAssertNoThrow(try Regex("(abc)", as: (Substring, Substring).self))
+    XCTAssertNoThrow(try Regex("(?<test>abc)", as: (Substring, test: Substring).self))
+    XCTAssertNoThrow(try Regex("(?<test>abc)?", as: (Substring, test: Substring?).self))
+
     // Builders
     check(
       Regex {