Clarify the .multilineCompilerLiteral syntax option

This should always be set in a multi-line literal,
with extended syntax potentially being set and
unset as we parse.

Author: hamishknight

Sources/_RegexParser/Regex/Parse/LexicalAnalysis.swift

@@ -597,7 +597,7 @@ extension Source {
         }.value
 
         // In multi-line literals, the quote may not span multiple lines.
-        if context.syntax.contains(.multilineExtendedSyntax),
+        if context.syntax.contains(.multilineCompilerLiteral),
             contents.spansMultipleLinesInRegexLiteral {
           throw ParseError.quoteMayNotSpanMultipleLines
         }
@@ -841,7 +841,7 @@ extension Source {
           throw ParseError.cannotRemoveSemanticsOptions
         }
         // Extended syntax may not be removed if in multi-line mode.
-        if context.syntax.contains(.multilineExtendedSyntax) &&
+        if context.syntax.contains(.multilineCompilerLiteral) &&
             opt.isAnyExtended {
           throw ParseError.cannotRemoveExtendedSyntaxInMultilineMode
         }

Sources/_RegexParser/Regex/Parse/Parse.swift

@@ -317,7 +317,7 @@ extension Parser {
     // engines such as Oniguruma, Java, and ICU do this under (?x). Therefore,
     // treat (?x) and (?xx) as the same option here. If we ever get a strict
     // PCRE mode, we will need to change this to handle that.
-    if !context.syntax.contains(.multilineExtendedSyntax) {
+    if !context.syntax.contains(.multilineCompilerLiteral) {
       mapOption(.extendedSyntax, \.isAnyExtended)
     }
   }
@@ -609,7 +609,7 @@ fileprivate func defaultSyntaxOptions(
     // For an extended syntax forward slash e.g #/.../#, extended syntax is
     // permitted if it spans multiple lines.
     if delim.poundCount > 0 && contents.spansMultipleLinesInRegexLiteral {
-      return .multilineExtendedSyntax
+      return [.multilineCompilerLiteral, .extendedSyntax]
     }
     return .traditional
   case .reSingleQuote:

Sources/_RegexParser/Regex/Parse/SyntaxOptions.swift

@@ -58,10 +58,10 @@ public struct SyntaxOptions: OptionSet {
   /// `(_: .*)` == `(?:.*)`
   public static var experimentalCaptures: Self { Self(1 << 5) }
 
-  /// The default syntax for a multi-line regex literal.
-  public static var multilineExtendedSyntax: Self {
-    return [Self(1 << 6), .extendedSyntax]
-  }
+  /// The syntax kind of a multi-line literal. This will always be set when
+  /// parsing a multi-line `#/.../#` literal. Note this does not imply extended
+  /// syntax, as that may be temporarily disabled while parsing.
+  public static var multilineCompilerLiteral: Self { Self(1 << 6) }
 
   /// `(?n)`
   public static var namedCapturesOnly: Self { Self(1 << 7) }