From 7cad6cb5547b1325967652dacf2d8536603b4172 Mon Sep 17 00:00:00 2001
From: Alejandro Alonso <alejandro_alonso@apple.com>
Date: Tue, 5 Jul 2022 11:25:44 -0700
Subject: [PATCH] Throw error when Output is wrong type

better diagnostic
---
 Sources/_StringProcessing/Compiler.swift         |  4 ++++
 .../_StringProcessing/Regex/AnyRegexOutput.swift | 16 ++++++++++++++--
 .../Utility/TypeVerification.swift               | 11 ++++++-----
 .../RegexBuilderTests/AnyRegexOutputTests.swift  | 11 +++++++++++
 4 files changed, 35 insertions(+), 7 deletions(-)

diff --git a/Sources/_StringProcessing/Compiler.swift b/Sources/_StringProcessing/Compiler.swift
index f47898e4e..530126a32 100644
--- a/Sources/_StringProcessing/Compiler.swift
+++ b/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)>'"
     }
   }
 }
diff --git a/Sources/_StringProcessing/Regex/AnyRegexOutput.swift b/Sources/_StringProcessing/Regex/AnyRegexOutput.swift
index 20731ad39..0cf27af6b 100644
--- a/Sources/_StringProcessing/Regex/AnyRegexOutput.swift
+++ b/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
     }
   }
diff --git a/Sources/_StringProcessing/Utility/TypeVerification.swift b/Sources/_StringProcessing/Utility/TypeVerification.swift
index c3aa53c7a..c5043bf97 100644
--- a/Sources/_StringProcessing/Utility/TypeVerification.swift
+++ b/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)
   }
 }
diff --git a/Tests/RegexBuilderTests/AnyRegexOutputTests.swift b/Tests/RegexBuilderTests/AnyRegexOutputTests.swift
index d628ce0d9..e6c3214b9 100644
--- a/Tests/RegexBuilderTests/AnyRegexOutputTests.swift
+++ b/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 {