Uninhabited != NoReturn

Previously, isNever would return true for an equivalence class of
uninhabited enums.  The rest of the compiler, however, is using this in
places that actually expect just the Never type.  This means that code
like this would compile properly

enum Uninhabited {}
func do() -> Uninhabited { /* No body here */ }

and we wouldn’t diagnose the missing return.

Author: CodaFi

include/swift/AST/Types.h

@@ -393,7 +393,7 @@ class alignas(1 << TypeAlignInBits) TypeBase {
   /// semantics?
   bool hasReferenceSemantics();
 
-  /// Is this an uninhabited type, such as 'Never'?
+  /// Is this 'Never'?
   bool isNever();
 
   /// Is this the 'Any' type?

lib/AST/Type.cpp

@@ -92,9 +92,7 @@ bool TypeBase::hasReferenceSemantics() {
 bool TypeBase::isNever() {
   if (auto nominalDecl = getAnyNominal())
     if (auto enumDecl = dyn_cast<EnumDecl>(nominalDecl))
-      if (enumDecl->getAllElements().empty())
-        return true;
-
+      return enumDecl == getASTContext().getNeverDecl();
   return false;
 }
 

lib/IRGen/GenClangType.cpp

@@ -493,7 +493,7 @@ GenClangType::visitBoundGenericType(CanBoundGenericType type) {
 clang::CanQualType GenClangType::visitEnumType(CanEnumType type) {
   // Special case: Uninhabited enums are not @objc, so we don't
   // know what to do below, but we can just convert to 'void'.
-  if (type->isNever())
+  if (type->getDecl()->getAllElements().empty())
     return Converter.convert(IGM, IGM.Context.TheEmptyTupleType);
 
   assert(type->getDecl()->isObjC() && "not an @objc enum?!");

test/IRGen/dllexport.swift

@@ -20,7 +20,7 @@ public var ci : c = c()
 
 open class d {
   private func m() -> Never {
-    fatalError()
+    return fatalError()
   }
 }
 

test/SILOptimizer/return.swift

@@ -9,6 +9,11 @@ func singleBlock2() -> Int {
   y += 1
 } // expected-error {{missing return in a function expected to return 'Int'}}
 
+enum NoCasesButNotNever {}
+
+func diagnoseNoCaseEnumMissingReturn() -> NoCasesButNotNever {
+} // expected-error {{missing return in a function expected to return 'NoCasesButNotNever'}}
+
 class MyClassWithClosure {
   var f : (_ s: String) -> String = { (_ s: String) -> String in } // expected-error {{missing return in a closure expected to return 'String'}}
 }