diff --git a/Sources/_StringProcessing/ByteCodeGen.swift b/Sources/_StringProcessing/ByteCodeGen.swift
index 5e85c7b3d..931e62bbd 100644
--- a/Sources/_StringProcessing/ByteCodeGen.swift
+++ b/Sources/_StringProcessing/ByteCodeGen.swift
@@ -156,6 +156,10 @@ fileprivate extension Compiler.ByteCodeGen {
         name, isScalarMode: options.semanticLevel == .unicodeScalar)
     case .relative:
       throw Unsupported("Backreference kind: \(ref)")
+    #if RESILIENT_LIBRARIES
+    @unknown default:
+      fatalError()
+    #endif
     }
   }
 
@@ -657,6 +661,10 @@ fileprivate extension Compiler.ByteCodeGen {
       // quantification break if trying to restore to a prior
       // iteration because the register got overwritten?
       //
+    #if RESILIENT_LIBRARIES
+    @unknown default:
+      fatalError()
+    #endif
     }
 
     builder.label(exit)
diff --git a/Sources/_StringProcessing/ConsumerInterface.swift b/Sources/_StringProcessing/ConsumerInterface.swift
index 808a1e498..99d1b3cae 100644
--- a/Sources/_StringProcessing/ConsumerInterface.swift
+++ b/Sources/_StringProcessing/ConsumerInterface.swift
@@ -324,6 +324,11 @@ extension AST.Atom {
         .backtrackingDirective, .changeMatchingOptions, .invalid:
       // FIXME: implement
       return nil
+
+    #if RESILIENT_LIBRARIES
+    @unknown default:
+      fatalError()
+    #endif
     }
   }
 }
@@ -686,7 +691,13 @@ extension AST.Atom.CharacterProperty {
 
       case .invalid:
         throw Unreachable("Expected valid property")
+
+      #if RESILIENT_LIBRARIES
+      @unknown default:
+        throw Unreachable("Unknown kind \(kind)")
+      #endif
       }
+
     }()
 
     if !isInverted { return preInversion }
@@ -856,6 +867,10 @@ extension Unicode.BinaryProperty {
     case .expandsOnNFC, .expandsOnNFD, .expandsOnNFKD,
         .expandsOnNFKC:
       throw Unsupported("Unicode-deprecated: \(self)")
+    #if RESILIENT_LIBRARIES
+    @unknown default:
+      break
+    #endif
     }
 
     throw Unsupported("TODO: map prop \(self)")
@@ -906,6 +921,10 @@ extension Unicode.POSIXProperty {
     case .xdigit:
       return consume(propertyScalarPredicate(\.isHexDigit)) // or number
 
+    #if RESILIENT_LIBRARIES
+    @unknown default:
+      fatalError()
+    #endif
     }
   }
 }
@@ -1024,6 +1043,11 @@ extension Unicode.ExtendedGeneralCategory {
       return consume(categoryScalarPredicate(.paragraphSeparator))
     case .spaceSeparator:
       return consume(categoryScalarPredicate(.spaceSeparator))
+
+    #if RESILIENT_LIBRARIES
+    @unknown default:
+      fatalError()
+    #endif
     }
   }
 }
diff --git a/Sources/_StringProcessing/Engine/InstPayload.swift b/Sources/_StringProcessing/Engine/InstPayload.swift
index 78baf9ce1..5f038449c 100644
--- a/Sources/_StringProcessing/Engine/InstPayload.swift
+++ b/Sources/_StringProcessing/Engine/InstPayload.swift
@@ -423,6 +423,10 @@ struct QuantifyPayload: RawRepresentable {
       kindVal = 1
     case .possessive:
       kindVal = 2
+    #if RESILIENT_LIBRARIES
+    @unknown default:
+      fatalError()
+    #endif
     }
     // TODO: refactor / reimplement
     let maxExtraTripsVal: UInt64 = maxExtraTrips == nil ? 1 : UInt64(maxExtraTrips!) << 1
diff --git a/Sources/_StringProcessing/LiteralPrinter.swift b/Sources/_StringProcessing/LiteralPrinter.swift
index 87b747585..4c85c4b49 100644
--- a/Sources/_StringProcessing/LiteralPrinter.swift
+++ b/Sources/_StringProcessing/LiteralPrinter.swift
@@ -216,6 +216,10 @@ extension LiteralPrinter {
       output("{,\(n.value!)}")
     case let .range(low, high):
       output("{\(low.value!),\(high.value!)}")
+    #if RESILIENT_LIBRARIES
+    @unknown default:
+      fatalError()
+    #endif
     }
     
     outputQuantificationKind(kind)
@@ -235,6 +239,10 @@ extension LiteralPrinter {
         output(options.isReluctantByDefault ? "" : "?")
       case .possessive:
         output("+")
+      #if RESILIENT_LIBRARIES
+      @unknown default:
+        fatalError()
+      #endif
       }
     case let .syntax(kind):
       // Syntactically-specified quantification modifiers can stay as-is.
@@ -245,6 +253,10 @@ extension LiteralPrinter {
         output("?")
       case .possessive:
         output("+")
+      #if RESILIENT_LIBRARIES
+      @unknown default:
+        fatalError()
+      #endif
       }
     }
   }
@@ -351,6 +363,10 @@ extension LiteralPrinter {
       output("\\g{\(prefix)\(abs(value))}")
     case .named(let name):
       output("\\g{\(name)}")
+    #if RESILIENT_LIBRARIES
+    @unknown default:
+      fatalError()
+    #endif
     }
   }
   
@@ -558,6 +574,11 @@ extension AST.MatchingOption.Kind {
       
     // NSRE Compatibility option; no literal representation
     case .nsreCompatibleDot: return nil
+
+    #if RESILIENT_LIBRARIES
+    @unknown default:
+      fatalError()
+    #endif
     }
   }
 }
@@ -597,6 +618,11 @@ extension DSLTree._AST.GroupKind {
       
     case let .changeMatchingOptions(sequence):
       return sequence._patternString + ":"
+
+    #if RESILIENT_LIBRARIES
+    @unknown default:
+      fatalError()
+    #endif
     }
   }
 }
diff --git a/Sources/_StringProcessing/MatchingOptions.swift b/Sources/_StringProcessing/MatchingOptions.swift
index 893131657..a5f470ec0 100644
--- a/Sources/_StringProcessing/MatchingOptions.swift
+++ b/Sources/_StringProcessing/MatchingOptions.swift
@@ -217,6 +217,10 @@ extension MatchingOptions {
         self = .extended
       case .extraExtended:
         self = .extraExtended
+      #if RESILIENT_LIBRARIES
+      @unknown default:
+        fatalError()
+      #endif
       }
     }
     
diff --git a/Sources/_StringProcessing/PrintAsPattern.swift b/Sources/_StringProcessing/PrintAsPattern.swift
index c729b647a..31321ef16 100644
--- a/Sources/_StringProcessing/PrintAsPattern.swift
+++ b/Sources/_StringProcessing/PrintAsPattern.swift
@@ -1105,6 +1105,11 @@ extension AST.Atom {
       
     case .changeMatchingOptions:
       return "/* TODO: change matching options */"
+
+    #if RESILIENT_LIBRARIES
+    @unknown default:
+      fatalError()
+    #endif
     }
   }
 }
@@ -1125,6 +1130,9 @@ extension AST.Quantification.Amount {
     case let .nOrMore(n):  return "Repeat(\(n._patternBase)...)"
     case let .upToN(n):    return "Repeat(...\(n._patternBase))"
     case let .range(n, m): return "Repeat(\(n._patternBase)...\(m._patternBase))"
+    #if RESILIENT_LIBRARIES
+    @unknown default: fatalError()
+    #endif
     }
   }
   
@@ -1144,6 +1152,9 @@ extension AST.Quantification.Kind {
     case .eager: return ".eager"
     case .reluctant: return ".reluctant"
     case .possessive: return ".possessive"
+    #if RESILIENT_LIBRARIES
+    @unknown default: fatalError()
+    #endif
     }
   }
 }
diff --git a/Sources/_StringProcessing/Regex/ASTConversion.swift b/Sources/_StringProcessing/Regex/ASTConversion.swift
index 6af0924cb..d9ba470e6 100644
--- a/Sources/_StringProcessing/Regex/ASTConversion.swift
+++ b/Sources/_StringProcessing/Regex/ASTConversion.swift
@@ -97,6 +97,11 @@ extension AST.Node {
       case let .absentFunction(abs):
         // TODO: What should this map to?
         return .absentFunction(.init(ast: abs))
+
+      #if RESILIENT_LIBRARIES
+      @unknown default:
+        fatalError()
+      #endif
       }
     }
 
@@ -141,9 +146,18 @@ extension AST.CustomCharacterClass {
           return .intersection(lhs, rhs)
         case .symmetricDifference:
           return .symmetricDifference(lhs, rhs)
+        #if RESILIENT_LIBRARIES
+        @unknown default:
+          fatalError()
+        #endif
         }
       case let .trivia(t):
         return .trivia(t.contents)
+
+      #if RESILIENT_LIBRARIES
+      @unknown default:
+        fatalError()
+      #endif
       }
     }
 
diff --git a/Sources/_StringProcessing/Regex/DSLTree.swift b/Sources/_StringProcessing/Regex/DSLTree.swift
index f24b87d09..9f0e2df23 100644
--- a/Sources/_StringProcessing/Regex/DSLTree.swift
+++ b/Sources/_StringProcessing/Regex/DSLTree.swift
@@ -651,6 +651,10 @@ extension CaptureList.Builder {
         addCaptures(of: child, optionalNesting: nesting, visibleInTypedOutput: visibleInTypedOutput)
       case .clearer, .repeater, .stopper:
         break
+      #if RESILIENT_LIBRARIES
+      @unknown default:
+        fatalError()
+      #endif
       }
 
     case let .convertedRegexLiteral(n, _):
@@ -926,6 +930,10 @@ extension DSLTree {
           return true
         case .exactly(let num), .nOrMore(let num), .range(let num, _):
           return num.value.map { $0 > 0 } ?? false
+        #if RESILIENT_LIBRARIES
+        @unknown default:
+          fatalError()
+        #endif
         }
       }
     }