Fix forward progress analysis losing its place in the node list

Sources/_StringProcessing/ByteCodeGen+DSLList.swift

@@ -351,9 +351,20 @@ fileprivate extension Compiler.ByteCodeGen {
     }
   }
 
+  /// Returns whether the node at `position` is guaranteed to consume input,
+  /// leaving `position` just past that node's entire subtree, so that a caller
+  /// walking a list of nodes reads the next sibling.
+  ///
+  /// Some of the cases below stop looking at children as soon as the answer is
+  /// known, and others don't look at them at all, so the end of the subtree is
+  /// found up front rather than left to the traversal to reach.
   func _guaranteesForwardProgressImpl(_ list: ArraySlice<DSLTree.Node>, position: inout Int) -> Bool {
     guard position < list.endIndex else { return false }
     let node = list[position]
+    var endOfSubtree = position
+    list.skipNode(&endOfSubtree)
+    endOfSubtree += 1
+    defer { position = endOfSubtree }
     position += 1
     switch node {
     case .orderedChoice(let count):
@@ -369,8 +380,6 @@ fileprivate extension Compiler.ByteCodeGen {
     case .nonCapturingGroup(let kind):
       switch kind.ast {
       case .lookahead, .negativeLookahead, .lookbehind, .negativeLookbehind:
-        list.skipNode(&position)
-        position += 1
         return false
       default:
         return _guaranteesForwardProgressImpl(list, position: &position)
@@ -396,8 +405,6 @@ fileprivate extension Compiler.ByteCodeGen {
       if let atLeast, atLeast > 0 {
         return _guaranteesForwardProgressImpl(list, position: &position)
       } else {
-        list.skipNode(&position)
-        position += 1
         return false
       }
     case .limitCaptureNesting, .ignoreCapturesInTypedOutput:

Tests/RegexTests/CompileTests.swift

@@ -556,6 +556,24 @@ extension RegexTests {
     // Inner node is a quantification that guarantees forward progress
     expectProgram(for: #"(a+)*"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
     expectProgram(for: #"(a{1,})*"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+
+    // An empty alternative next to one that is a concatenation of more than
+    // one node. Deciding the concatenation takes only its first child, so the
+    // walk has to skip the rest of it to find the empty alternative.
+    // https://github.com/swiftlang/swift-experimental-string-processing/issues/897
+    expectProgram(for: #"(?:.c|)*"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:\sA|)*?"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:[ab]c|)*"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    // https://github.com/swiftlang/swift-experimental-string-processing/issues/894
+    expectProgram(for: #"(?:.b|)+"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    // Every alternative progresses, so no check is needed even though one of
+    // them is a concatenation.
+    expectProgram(for: #"(?:.c|b)*"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:.c|b|[de]f)*"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+    // A concatenation progresses as soon as one of its children does, wherever
+    // in the concatenation that child is.
+    expectProgram(for: #"(?:(?=a)\b.)*"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:(?=a)\b(?:))*"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
   }
 
   func testCanOnlyMatchAtStart() throws {

Tests/RegexTests/MatchTests.swift

@@ -2917,6 +2917,32 @@ extension RegexTests {
     expectCompletion(regex: #"(?:(?=\d)\w*)*"#, in: "a")
     expectCompletion(regex: #"(?:(?!b)\d*)*"#, in: "a")
     expectCompletion(regex: #"(?:(?!\d)[0-9]*)*"#, in: "a")
+
+    // An empty alternative next to one that is a concatenation of more than
+    // one node.
+    // https://github.com/swiftlang/swift-experimental-string-processing/issues/897
+    expectCompletion(regex: #"(?:.c|)*x"#, in: "b")
+    expectCompletion(regex: #"(?:\sA|)*?x"#, in: "b")
+    expectCompletion(regex: #"(?:[ab]c|)*x"#, in: "b")
+    // https://github.com/swiftlang/swift-experimental-string-processing/issues/894
+    expectCompletion(regex: #"(?:.b|)+."#, in: "a")
+  }
+
+  func testEmptyAlternativeInQuantification() {
+    // https://github.com/swiftlang/swift-experimental-string-processing/issues/897
+    firstMatchTests(
+      #"(?:.c|)*x"#,
+      ("b", nil), ("x", "x"), ("bcx", "bcx"), ("acbcx", "acbcx"))
+    firstMatchTests(
+      #"(?:\sA|)*?x"#,
+      ("b", nil), ("x", "x"), (" Ax", " Ax"))
+    firstMatchTests(
+      #"(?:[ab]c|)*x"#,
+      ("b", nil), ("acbcx", "acbcx"))
+    // https://github.com/swiftlang/swift-experimental-string-processing/issues/894
+    firstMatchTests(
+      #"(?:.b|)+."#,
+      ("a", "a"), ("ab", "a"), ("abb", "abb"))
   }
 
   func testQuantifyOptimization() throws {