Better diagnose unsupported group conditions

Author: hamishknight

Sources/_MatchingEngine/Regex/Parse/Diagnostics.swift

@@ -29,6 +29,7 @@ enum ParseError: Error, Hashable {
   case misc(String)
 
   case tooManyBranchesInConditional(Int)
+  case unsupportedCondition(String)
 
   case expectedASCII(Character)
 
@@ -81,6 +82,8 @@ extension ParseError: CustomStringConvertible {
       return "cannot refer to whole pattern here"
     case let .tooManyBranchesInConditional(i):
       return "expected 2 branches in conditional, have \(i)"
+    case let .unsupportedCondition(str):
+      return "\(str) cannot be used as condition"
     case let .unknownGroupKind(str):
       return "unknown group kind '(\(str)'"
     case let .invalidMatchingOption(c):

Sources/_MatchingEngine/Regex/Parse/LexicalAnalysis.swift

@@ -872,10 +872,16 @@ extension Source {
   ///
   mutating func lexGroupConditionalStart() throws -> Located<AST.Group.Kind>? {
     try tryEating { src in
-      guard src.tryEat(sequence: "(?"),
-            let group = try src.lexGroupStart(),
-            !group.value.hasImplicitScope
+      guard src.tryEat(sequence: "(?"), let group = try src.lexGroupStart()
       else { return nil }
+
+      // Implicitly scoped groups are not supported here.
+      guard !group.value.hasImplicitScope else {
+        throw LocatedError(
+          ParseError.unsupportedCondition("implicitly scoped group"),
+          group.location
+        )
+      }
       return group
     }
   }

Tests/RegexTests/ParseTests.swift

@@ -1276,5 +1276,6 @@ extension RegexTests {
 
     diagnosticTest(#"(?(1)a|b|c)"#, .tooManyBranchesInConditional(3))
     diagnosticTest(#"(?(1)||)"#, .tooManyBranchesInConditional(3))
+    diagnosticTest(#"(?(?i))"#, .unsupportedCondition("implicitly scoped group"))
   }
 }