diff --git a/include/swift/AST/Decl.h b/include/swift/AST/Decl.h
index 75776ff3610f8..e8b6870d21025 100644
--- a/include/swift/AST/Decl.h
+++ b/include/swift/AST/Decl.h
@@ -3762,6 +3762,7 @@ struct SelfReferencePosition final {
     case Contravariant:
       return Covariant;
     }
+    llvm_unreachable("unhandled self reference position!");
   }
 
   explicit operator bool() const { return kind > None; }
diff --git a/include/swift/AST/ForeignErrorConvention.h b/include/swift/AST/ForeignErrorConvention.h
index d09a7a80c0d86..3d9d95280c0e2 100644
--- a/include/swift/AST/ForeignErrorConvention.h
+++ b/include/swift/AST/ForeignErrorConvention.h
@@ -198,6 +198,7 @@ class ForeignErrorConvention {
     case NonNilError:
       return false;
     }
+    llvm_unreachable("unhandled foreign error kind!");
   }
 
   bool operator==(ForeignErrorConvention other) const {
diff --git a/include/swift/Demangling/TypeLookupError.h b/include/swift/Demangling/TypeLookupError.h
index c67fc35598be8..cf2c75087e0fa 100644
--- a/include/swift/Demangling/TypeLookupError.h
+++ b/include/swift/Demangling/TypeLookupError.h
@@ -150,6 +150,7 @@ class TypeLookupError {
         delete castContext;
         return nullptr;
       }
+      llvm_unreachable("unhandled command!");
     };
   }
 
diff --git a/include/swift/Sema/ConstraintSystem.h b/include/swift/Sema/ConstraintSystem.h
index 0642dedac73ba..5dd0778a492ee 100644
--- a/include/swift/Sema/ConstraintSystem.h
+++ b/include/swift/Sema/ConstraintSystem.h
@@ -1907,6 +1907,7 @@ class SolutionApplicationTarget {
     case Kind::patternBinding:
       return patternBinding;
     }
+    llvm_unreachable("invalid case label type");
   }
 
   VarDecl *getAsUninitializedWrappedVar() const {
@@ -1921,6 +1922,7 @@ class SolutionApplicationTarget {
     case Kind::uninitializedWrappedVar:
       return uninitializedWrappedVar;
     }
+    llvm_unreachable("invalid case label type");
   }
 
   BraceStmt *getFunctionBody() const {
diff --git a/lib/AST/AutoDiff.cpp b/lib/AST/AutoDiff.cpp
index 492e38a2cb861..566de24dae297 100644
--- a/lib/AST/AutoDiff.cpp
+++ b/lib/AST/AutoDiff.cpp
@@ -462,6 +462,7 @@ bool swift::operator==(const TangentPropertyInfo::Error &lhs,
   case TangentPropertyInfo::Error::Kind::TangentPropertyWrongType:
     return lhs.getType()->isEqual(rhs.getType());
   }
+  llvm_unreachable("unhandled tangent property!");
 }
 
 void swift::simple_display(llvm::raw_ostream &os, TangentPropertyInfo info) {
diff --git a/lib/AST/ClangTypeConverter.cpp b/lib/AST/ClangTypeConverter.cpp
index 9fd13f797798d..365d9b72e3d03 100644
--- a/lib/AST/ClangTypeConverter.cpp
+++ b/lib/AST/ClangTypeConverter.cpp
@@ -207,6 +207,7 @@ const clang::Type *ClangTypeConverter::getFunctionType(
   case SILFunctionType::Representation::Closure:
     llvm_unreachable("Expected a C-compatible representation.");
   }
+  llvm_unreachable("unhandled representation!");
 }
 
 clang::QualType ClangTypeConverter::convertMemberType(NominalTypeDecl *DC,
diff --git a/lib/AST/TypeCheckRequests.cpp b/lib/AST/TypeCheckRequests.cpp
index 707b0cdecb62c..30a5d1c84a02d 100644
--- a/lib/AST/TypeCheckRequests.cpp
+++ b/lib/AST/TypeCheckRequests.cpp
@@ -1497,6 +1497,7 @@ bool ActorIsolation::requiresSubstitution() const {
   case GlobalActor:
     return getGlobalActor()->hasTypeParameter();
   }
+  llvm_unreachable("unhandled actor isolation kind!");
 }
 
 ActorIsolation ActorIsolation::subst(SubstitutionMap subs) const {
@@ -1509,6 +1510,7 @@ ActorIsolation ActorIsolation::subst(SubstitutionMap subs) const {
   case GlobalActor:
     return forGlobalActor(getGlobalActor().subst(subs));
   }
+  llvm_unreachable("unhandled actor isolation kind!");
 }
 
 void swift::simple_display(
diff --git a/lib/Driver/DarwinToolChains.cpp b/lib/Driver/DarwinToolChains.cpp
index 0f4ffda29b3fb..95accad29f2bd 100644
--- a/lib/Driver/DarwinToolChains.cpp
+++ b/lib/Driver/DarwinToolChains.cpp
@@ -364,6 +364,7 @@ bool jobMatchesFilter(LinkKind jobKind, BackDeployLibFilter filter) {
   case BackDeployLibFilter::all:
     return true;
   }
+  llvm_unreachable("unhandled back deploy lib filter!");
 }
 
 }
diff --git a/lib/IDE/APIDigesterData.cpp b/lib/IDE/APIDigesterData.cpp
index 5bda079279477..d4d9a6655aba1 100644
--- a/lib/IDE/APIDigesterData.cpp
+++ b/lib/IDE/APIDigesterData.cpp
@@ -56,6 +56,7 @@ StringRef swift::ide::api::getDeclKindStr(const DeclKind Value, bool lower) {
     }
 #include "swift/AST/DeclNodes.def"
     }
+    llvm_unreachable("Unhandled DeclKind in switch.");
   } else {
     return getDeclKindStrRaw(Value);
   }
diff --git a/lib/IRGen/MetadataLayout.cpp b/lib/IRGen/MetadataLayout.cpp
index e93d370240ea4..5ea2eb6ee2be4 100644
--- a/lib/IRGen/MetadataLayout.cpp
+++ b/lib/IRGen/MetadataLayout.cpp
@@ -414,6 +414,7 @@ ClassMetadataLayout::getMethodInfo(IRGenFunction &IGF, SILDeclRef method) const{
   case MethodInfo::Kind::DirectImpl:
     return MethodInfo(stored.TheImpl);
   }
+  llvm_unreachable("unhandled method info kind!");
 }
 
 Offset ClassMetadataLayout::getFieldOffset(IRGenFunction &IGF,
diff --git a/lib/IRGen/MetadataRequest.cpp b/lib/IRGen/MetadataRequest.cpp
index d3f9fe58b6a16..34b1b05b042dc 100644
--- a/lib/IRGen/MetadataRequest.cpp
+++ b/lib/IRGen/MetadataRequest.cpp
@@ -616,6 +616,7 @@ static MetadataResponse emitNominalPrespecializedGenericMetadataRef(
     return MetadataResponse::handle(IGF, request, call);
   }
   }
+  llvm_unreachable("unhandled metadata canonicality");
 }
 
 static llvm::Value *
diff --git a/lib/SILOptimizer/Differentiation/PullbackCloner.cpp b/lib/SILOptimizer/Differentiation/PullbackCloner.cpp
index 29adb43cfca57..38f94d73f8fec 100644
--- a/lib/SILOptimizer/Differentiation/PullbackCloner.cpp
+++ b/lib/SILOptimizer/Differentiation/PullbackCloner.cpp
@@ -336,6 +336,7 @@ class PullbackCloner::Implementation final
     case AdjointValueKind::Concrete:
       return val.getConcreteValue();
     }
+    llvm_unreachable("unhandled adjoint value kind!");
   }
 
   /// Materializes an adjoint value indirectly to a SIL buffer.
diff --git a/lib/SILOptimizer/SemanticARC/OwnershipPhiOperand.h b/lib/SILOptimizer/SemanticARC/OwnershipPhiOperand.h
index 169eb9859bea8..88ba1007d2ea2 100644
--- a/lib/SILOptimizer/SemanticARC/OwnershipPhiOperand.h
+++ b/lib/SILOptimizer/SemanticARC/OwnershipPhiOperand.h
@@ -88,6 +88,7 @@ class LLVM_LIBRARY_VISIBILITY OwnershipPhiOperand {
     case Kind::Struct:
       return false;
     }
+    llvm_unreachable("unhandled operand kind!");
   }
 
   bool operator<(const OwnershipPhiOperand &other) const {
@@ -113,6 +114,7 @@ class LLVM_LIBRARY_VISIBILITY OwnershipPhiOperand {
           });
     }
     }
+    llvm_unreachable("unhandled operand kind!");
   }
 };
 
diff --git a/lib/SILOptimizer/Transforms/AllocBoxToStack.cpp b/lib/SILOptimizer/Transforms/AllocBoxToStack.cpp
index 6483b47a12a6f..fd7695b164640 100644
--- a/lib/SILOptimizer/Transforms/AllocBoxToStack.cpp
+++ b/lib/SILOptimizer/Transforms/AllocBoxToStack.cpp
@@ -984,6 +984,7 @@ specializeApplySite(SILOptFunctionBuilder &FuncBuilder, ApplySite Apply,
         GenericSpecializationInformation::create(ApplyInst, Builder));
   }
   }
+  llvm_unreachable("unhandled apply inst kind!");
 }
 
 static void rewriteApplySites(AllocBoxToStackState &pass) {
diff --git a/lib/Sema/TypeCheckConcurrency.cpp b/lib/Sema/TypeCheckConcurrency.cpp
index 474512979861c..0215c6687e89d 100644
--- a/lib/Sema/TypeCheckConcurrency.cpp
+++ b/lib/Sema/TypeCheckConcurrency.cpp
@@ -825,6 +825,7 @@ void swift::checkActorIsolation(const Expr *expr, const DeclContext *dc) {
         // Okay.
         return false;
       }
+      llvm_unreachable("unhandled actor isolation kind!");
     }
 
     /// Check a reference to a local or global.
@@ -868,6 +869,7 @@ void swift::checkActorIsolation(const Expr *expr, const DeclContext *dc) {
       case ActorIsolationRestriction::Unsafe:
         return diagnoseReferenceToUnsafe(value, loc);
       }
+      llvm_unreachable("unhandled actor isolation kind!");
     }
 
     /// Check a reference with the given base expression to the given member.
@@ -964,6 +966,7 @@ void swift::checkActorIsolation(const Expr *expr, const DeclContext *dc) {
       case ActorIsolationRestriction::Unsafe:
         return diagnoseReferenceToUnsafe(member, memberLoc);
       }
+      llvm_unreachable("unhandled actor isolation kind!");
     }
   };
 
diff --git a/lib/Sema/TypeCheckRequestFunctions.cpp b/lib/Sema/TypeCheckRequestFunctions.cpp
index c3abd1a067820..78d4c0c231f2d 100644
--- a/lib/Sema/TypeCheckRequestFunctions.cpp
+++ b/lib/Sema/TypeCheckRequestFunctions.cpp
@@ -262,6 +262,7 @@ static Type inferFunctionBuilderType(ValueDecl *decl)  {
       case DynamicReplacement:
         return dynamicReplacement->getName();
       }
+      llvm_unreachable("unhandled decl name kind!");
     }
   };