diff --git a/include/swift/ABI/MetadataValues.h b/include/swift/ABI/MetadataValues.h
index 20e7dd3af63..8f597c20ee9 100644
--- a/include/swift/ABI/MetadataValues.h
+++ b/include/swift/ABI/MetadataValues.h
@@ -20,7 +20,7 @@
 #define SWIFT_ABI_METADATAVALUES_H
 
 #include "swift/AST/Ownership.h"
-#include "swift/Basic/Unreachable.h"
+#include "swift/Runtime/Unreachable.h"
 
 #include <stdlib.h>
 #include <stdint.h>
@@ -381,7 +381,7 @@ public:
       return true;
     }
 
-    swift_unreachable("Unhandled ProtocolDispatchStrategy in switch.");
+    swift_runtime_unreachable("Unhandled ProtocolDispatchStrategy in switch.");
   }
   
   /// Return the identifier if this is a special runtime-known protocol.
diff --git a/include/swift/Reflection/ReflectionContext.h b/include/swift/Reflection/ReflectionContext.h
index 114a08f0b30..8f30096532c 100644
--- a/include/swift/Reflection/ReflectionContext.h
+++ b/include/swift/Reflection/ReflectionContext.h
@@ -18,13 +18,13 @@
 #ifndef SWIFT_REFLECTION_REFLECTIONCONTEXT_H
 #define SWIFT_REFLECTION_REFLECTIONCONTEXT_H
 
-#include "swift/Basic/Unreachable.h"
 #include "swift/Remote/MemoryReader.h"
 #include "swift/Remote/MetadataReader.h"
 #include "swift/Reflection/Records.h"
 #include "swift/Reflection/TypeLowering.h"
 #include "swift/Reflection/TypeRef.h"
 #include "swift/Reflection/TypeRefBuilder.h"
+#include "swift/Runtime/Unreachable.h"
 
 #include <iostream>
 #include <set>
@@ -454,7 +454,7 @@ private:
       return true;
     }
 
-    swift_unreachable("Unhandled MetadataSourceKind in switch.");
+    swift_runtime_unreachable("Unhandled MetadataSourceKind in switch.");
   }
 
   /// Read metadata for a captured generic type from a closure context.
diff --git a/include/swift/Reflection/TypeRef.h b/include/swift/Reflection/TypeRef.h
index fd24dcf68ad..9f4c7157da4 100644
--- a/include/swift/Reflection/TypeRef.h
+++ b/include/swift/Reflection/TypeRef.h
@@ -21,7 +21,7 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/Support/Casting.h"
 #include "swift/ABI/MetadataValues.h"
-#include "swift/Basic/Unreachable.h"
+#include "swift/Runtime/Unreachable.h"
 
 #include <iostream>
 
@@ -783,7 +783,7 @@ public:
 #include "swift/Reflection/TypeRefs.def"
     }
 
-    swift_unreachable("Unhandled TypeRefKind in switch.");
+    swift_runtime_unreachable("Unhandled TypeRefKind in switch.");
   }
 };
 
diff --git a/include/swift/Remote/MetadataReader.h b/include/swift/Remote/MetadataReader.h
index e0a9210ff49..6f51e38f659 100644
--- a/include/swift/Remote/MetadataReader.h
+++ b/include/swift/Remote/MetadataReader.h
@@ -21,7 +21,7 @@
 #include "swift/Remote/MemoryReader.h"
 #include "swift/Basic/Demangle.h"
 #include "swift/Basic/LLVM.h"
-#include "swift/Basic/Unreachable.h"
+#include "swift/Runtime/Unreachable.h"
 
 #include <vector>
 #include <unordered_map>
@@ -763,7 +763,7 @@ public:
     }
     }
 
-    swift_unreachable("Unhandled MetadataKind in switch");
+    swift_runtime_unreachable("Unhandled MetadataKind in switch");
   }
 
   BuiltType readTypeFromMangledName(const char *MangledTypeName,
diff --git a/include/swift/Runtime/Debug.h b/include/swift/Runtime/Debug.h
index d8ce205491f..a6904529fad 100644
--- a/include/swift/Runtime/Debug.h
+++ b/include/swift/Runtime/Debug.h
@@ -19,9 +19,9 @@
 
 #include <llvm/Support/Compiler.h>
 #include <stdint.h>
-#include "swift/Basic/Unreachable.h"
 #include "swift/Runtime/Config.h"
 #include "swift/Runtime/Metadata.h"
+#include "swift/Runtime/Unreachable.h"
 
 #ifdef SWIFT_HAVE_CRASHREPORTERCLIENT
 
@@ -72,7 +72,7 @@ static inline void crash(const char *message) {
   CRSetCrashLogMessage(message);
 
   LLVM_BUILTIN_TRAP;
-  swift_unreachable("Expected compiler to crash.");
+  swift_runtime_unreachable("Expected compiler to crash.");
 }
 
 /// Report a corrupted type object.
diff --git a/include/swift/Runtime/Metadata.h b/include/swift/Runtime/Metadata.h
index 5a1d4f9a4ab..b9dbfe0a285 100644
--- a/include/swift/Runtime/Metadata.h
+++ b/include/swift/Runtime/Metadata.h
@@ -33,7 +33,7 @@
 #include "swift/Basic/FlaggedPointer.h"
 #include "swift/Basic/RelativePointer.h"
 #include "swift/Basic/ManglingMacros.h"
-#include "swift/Basic/Unreachable.h"
+#include "swift/Runtime/Unreachable.h"
 #include "../../../stdlib/public/SwiftShims/HeapObject.h"
 
 namespace swift {
@@ -1160,7 +1160,7 @@ public:
       return false;
     }
     
-    swift_unreachable("Unhandled MetadataKind in switch.");
+    swift_runtime_unreachable("Unhandled MetadataKind in switch.");
   }
   
   /// Is this metadata for an existential type?
@@ -1186,7 +1186,7 @@ public:
       return false;
     }
 
-    swift_unreachable("Unhandled MetadataKind in switch.");
+    swift_runtime_unreachable("Unhandled MetadataKind in switch.");
   }
   
   /// Is this either type metadata or a class object for any kind of class?
@@ -1267,7 +1267,7 @@ public:
       return RelativeDirectPointerNullPtrRef;
     }
 
-    swift_unreachable("Unhandled MetadataKind in switch.");
+    swift_runtime_unreachable("Unhandled MetadataKind in switch.");
   }
   
   /// Get the generic metadata pattern from which this generic type instance was
diff --git a/include/swift/Basic/Unreachable.h b/include/swift/Runtime/Unreachable.h
similarity index 66%
rename from include/swift/Basic/Unreachable.h
rename to include/swift/Runtime/Unreachable.h
index 6ed40eda1c3..671e2616207 100644
--- a/include/swift/Basic/Unreachable.h
+++ b/include/swift/Runtime/Unreachable.h
@@ -1,4 +1,4 @@
-//===--- Unreachable.h - Implements swift_unreachable -----------*- C++ -*-===//
+//===--- Unreachable.h - Implements swift_runtime_unreachable ---*- C++ -*-===//
 //
 // This source file is part of the Swift.org open source project
 //
@@ -10,23 +10,23 @@
 //
 //===----------------------------------------------------------------------===//
 //
-//  This file defines swift_unreachable, an LLVM-independent implementation of
-//  llvm_unreachable.
+//  This file defines swift_runtime_unreachable, an LLVM-independent
+//  implementation of llvm_unreachable.
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef SWIFT_BASIC_UNREACHABLE_H
-#define SWIFT_BASIC_UNREACHABLE_H
+#ifndef SWIFT_RUNTIME_UNREACHABLE_H
+#define SWIFT_RUNTIME_UNREACHABLE_H
 
 #include "llvm/Support/Compiler.h"
 #include <assert.h>
 #include <stdlib.h>
 
 LLVM_ATTRIBUTE_NORETURN
-inline static void swift_unreachable(const char* msg) {
+inline static void swift_runtime_unreachable(const char *msg) {
   assert(false && msg);
   (void)msg;
   abort();
 }
 
-#endif // SWIFT_BASIC_UNREACHABLE_H
+#endif // SWIFT_RUNTIME_UNREACHABLE_H
diff --git a/stdlib/public/Reflection/TypeLowering.cpp b/stdlib/public/Reflection/TypeLowering.cpp
index 4a83c0ceb9d..81bcaa45e8c 100644
--- a/stdlib/public/Reflection/TypeLowering.cpp
+++ b/stdlib/public/Reflection/TypeLowering.cpp
@@ -19,9 +19,9 @@
 //===----------------------------------------------------------------------===//
 
 #include "swift/Reflection/TypeLowering.h"
-#include "swift/Basic/Unreachable.h"
 #include "swift/Reflection/TypeRef.h"
 #include "swift/Reflection/TypeRefBuilder.h"
+#include "swift/Runtime/Unreachable.h"
 
 #include <iostream>
 
@@ -186,7 +186,7 @@ public:
     }
     }
 
-    swift_unreachable("Bad TypeInfo kind");
+    swift_runtime_unreachable("Bad TypeInfo kind");
   }
 };
 
@@ -1043,7 +1043,7 @@ public:
       return nullptr;
     }
 
-    swift_unreachable("Unhandled FieldDescriptorKind in switch.");
+    swift_runtime_unreachable("Unhandled FieldDescriptorKind in switch.");
   }
 
   const TypeInfo *visitNominalTypeRef(const NominalTypeRef *N) {
@@ -1074,7 +1074,7 @@ public:
       return TC.getTypeInfo(TC.getThinFunctionTypeRef());
     }
 
-    swift_unreachable("Unhandled FunctionMetadataConvention in switch.");
+    swift_runtime_unreachable("Unhandled FunctionMetadataConvention in switch.");
   }
 
   const TypeInfo *visitProtocolTypeRef(const ProtocolTypeRef *P) {
@@ -1102,7 +1102,7 @@ public:
       return TC.getTypeInfo(TC.getAnyMetatypeTypeRef());
     }
 
-    swift_unreachable("Unhandled MetatypeRepresentation in switch.");
+    swift_runtime_unreachable("Unhandled MetatypeRepresentation in switch.");
   }
 
   const TypeInfo *
@@ -1285,7 +1285,7 @@ const TypeInfo *TypeConverter::getClassInstanceTypeInfo(const TypeRef *TR,
     return nullptr;
   }
 
-  swift_unreachable("Unhandled FieldDescriptorKind in switch.");
+  swift_runtime_unreachable("Unhandled FieldDescriptorKind in switch.");
 }
 
 } // namespace reflection
diff --git a/stdlib/public/SwiftRemoteMirror/SwiftRemoteMirror.cpp b/stdlib/public/SwiftRemoteMirror/SwiftRemoteMirror.cpp
index e994dce9a24..e202f43dc72 100644
--- a/stdlib/public/SwiftRemoteMirror/SwiftRemoteMirror.cpp
+++ b/stdlib/public/SwiftRemoteMirror/SwiftRemoteMirror.cpp
@@ -10,10 +10,10 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "swift/Basic/Unreachable.h"
 #include "swift/Reflection/ReflectionContext.h"
 #include "swift/Reflection/TypeLowering.h"
 #include "swift/Remote/CMemoryReader.h"
+#include "swift/Runtime/Unreachable.h"
 #include "swift/SwiftRemoteMirror/SwiftRemoteMirror.h"
 
 using namespace swift;
@@ -177,7 +177,7 @@ swift_layout_kind_t getTypeInfoKind(const TypeInfo &TI) {
   }
   }
 
-  swift_unreachable("Unhandled TypeInfoKind in switch");
+  swift_runtime_unreachable("Unhandled TypeInfoKind in switch");
 }
 
 static swift_typeinfo_t convertTypeInfo(const TypeInfo *TI) {
diff --git a/stdlib/public/runtime/Casting.cpp b/stdlib/public/runtime/Casting.cpp
index 7a23e353ff7..80783f53c14 100644
--- a/stdlib/public/runtime/Casting.cpp
+++ b/stdlib/public/runtime/Casting.cpp
@@ -18,12 +18,12 @@
 #include "swift/Basic/Demangle.h"
 #include "swift/Basic/Fallthrough.h"
 #include "swift/Basic/Lazy.h"
-#include "swift/Basic/Unreachable.h"
 #include "swift/Runtime/Config.h"
 #include "swift/Runtime/Enum.h"
 #include "swift/Runtime/HeapObject.h"
 #include "swift/Runtime/Metadata.h"
 #include "swift/Runtime/Mutex.h"
+#include "swift/Runtime/Unreachable.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/PointerIntPair.h"
 #include "swift/Runtime/Debug.h"
@@ -1013,7 +1013,7 @@ static bool _dynamicCastToExistential(OpaqueValue *dest,
   }
   }
 
-  swift_unreachable("Unhandled ExistentialTypeRepresentation in switch.");
+  swift_runtime_unreachable("Unhandled ExistentialTypeRepresentation in switch.");
 }
 
 /******************************************************************************/
@@ -1276,7 +1276,7 @@ swift::swift_dynamicCastMetatype(const Metadata *sourceType,
     return origSourceType;
   }
 
-  swift_unreachable("Unhandled MetadataKind in switch.");
+  swift_runtime_unreachable("Unhandled MetadataKind in switch.");
 }
 
 const Metadata *
@@ -1388,7 +1388,7 @@ swift::swift_dynamicCastMetatypeUnconditional(const Metadata *sourceType,
     return origSourceType;
   }
 
-  swift_unreachable("Unhandled MetadataKind in switch.");
+  swift_runtime_unreachable("Unhandled MetadataKind in switch.");
 }
 
 /******************************************************************************/
@@ -1523,7 +1523,8 @@ static bool _dynamicCastToUnknownClassFromExistential(OpaqueValue *dest,
   }
   }
 
-  swift_unreachable("Unhandled ExistentialTypeRepresentation in switch.");
+  swift_runtime_unreachable(
+      "Unhandled ExistentialTypeRepresentation in switch.");
 }
 
 static void unwrapExistential(OpaqueValue *src,
@@ -1994,7 +1995,7 @@ static bool _dynamicCastToFunction(OpaqueValue *dest,
     return _fail(src, srcType, targetType, flags);
   }
 
-  swift_unreachable("Unhandled MetadataKind in switch.");
+  swift_runtime_unreachable("Unhandled MetadataKind in switch.");
 }
 
 /******************************************************************************/
diff --git a/stdlib/public/runtime/Metadata.cpp b/stdlib/public/runtime/Metadata.cpp
index 817d838b5e6..59c317ef183 100644
--- a/stdlib/public/runtime/Metadata.cpp
+++ b/stdlib/public/runtime/Metadata.cpp
@@ -2200,7 +2200,7 @@ getExistentialValueWitnesses(ProtocolClassConstraint classConstraint,
     return getOpaqueExistentialValueWitnesses(numWitnessTables);
   }
 
-  swift_unreachable("Unhandled ProtocolClassConstraint in switch.");
+  swift_runtime_unreachable("Unhandled ProtocolClassConstraint in switch.");
 }
 
 template<> ExistentialTypeRepresentation
@@ -2243,7 +2243,8 @@ ExistentialTypeMetadata::mayTakeValue(const OpaqueValue *container) const {
   }
   }
 
-  swift_unreachable("Unhandled ExistentialTypeRepresentation in switch.");
+  swift_runtime_unreachable(
+      "Unhandled ExistentialTypeRepresentation in switch.");
 }
 
 template<> void
@@ -2294,7 +2295,8 @@ ExistentialTypeMetadata::projectValue(const OpaqueValue *container) const {
   }
   }
 
-  swift_unreachable("Unhandled ExistentialTypeRepresentation in switch.");
+  swift_runtime_unreachable(
+      "Unhandled ExistentialTypeRepresentation in switch.");
 }
 
 template<> const Metadata *
@@ -2318,7 +2320,8 @@ ExistentialTypeMetadata::getDynamicType(const OpaqueValue *container) const {
   }
   }
 
-  swift_unreachable("Unhandled ExistentialTypeRepresentation in switch.");
+  swift_runtime_unreachable(
+      "Unhandled ExistentialTypeRepresentation in switch.");
 }
 
 template<> const WitnessTable *
@@ -2609,7 +2612,7 @@ Metadata::getClassObject() const {
     return nullptr;
   }
 
-  swift_unreachable("Unhandled MetadataKind in switch.");
+  swift_runtime_unreachable("Unhandled MetadataKind in switch.");
 }
 
 #ifndef NDEBUG
diff --git a/stdlib/public/runtime/ProtocolConformance.cpp b/stdlib/public/runtime/ProtocolConformance.cpp
index f0a9798c72b..70660a53154 100644
--- a/stdlib/public/runtime/ProtocolConformance.cpp
+++ b/stdlib/public/runtime/ProtocolConformance.cpp
@@ -16,10 +16,10 @@
 
 #include "swift/Basic/LLVM.h"
 #include "swift/Basic/Lazy.h"
-#include "swift/Basic/Unreachable.h"
 #include "swift/Runtime/Concurrent.h"
 #include "swift/Runtime/Metadata.h"
 #include "swift/Runtime/Mutex.h"
+#include "swift/Runtime/Unreachable.h"
 #include "ImageInspection.h"
 #include "Private.h"
 
@@ -118,7 +118,7 @@ const {
     return nullptr;
   }
 
-  swift_unreachable("Unhandled TypeMetadataRecordKind in switch.");
+  swift_runtime_unreachable("Unhandled TypeMetadataRecordKind in switch.");
 }
 
 template<>
@@ -133,7 +133,8 @@ const {
     return getWitnessTableAccessor()(type);
   }
 
-  swift_unreachable("Unhandled ProtocolConformanceReferenceKind in switch.");
+  swift_runtime_unreachable(
+      "Unhandled ProtocolConformanceReferenceKind in switch.");
 }
 
 namespace {
diff --git a/stdlib/public/runtime/Reflection.mm b/stdlib/public/runtime/Reflection.mm
index ead123a1bde..9b6ce1a416f 100644
--- a/stdlib/public/runtime/Reflection.mm
+++ b/stdlib/public/runtime/Reflection.mm
@@ -11,12 +11,12 @@
 //===----------------------------------------------------------------------===//
 
 #include "swift/Basic/Fallthrough.h"
-#include "swift/Basic/Unreachable.h"
 #include "swift/Runtime/Reflection.h"
 #include "swift/Runtime/Config.h"
 #include "swift/Runtime/HeapObject.h"
 #include "swift/Runtime/Metadata.h"
 #include "swift/Runtime/Enum.h"
+#include "swift/Runtime/Unreachable.h"
 #include "swift/Basic/Demangle.h"
 #include "swift/Runtime/Debug.h"
 #include "swift/Runtime/Portability.h"
@@ -1100,7 +1100,7 @@ getImplementationForType(const Metadata *T, const OpaqueValue *Value) {
     swift::crash("Swift mirror lookup failure");
   }
 
-  swift_unreachable("Unhandled MetadataKind in switch.");
+  swift_runtime_unreachable("Unhandled MetadataKind in switch.");
 }
 
 /// MagicMirror ownership-taking whole-value constructor.