SILGen: Don't emit unreachable code warnings for explicit return statement

Author: slavapestov

lib/SILGen/SILGenStmt.cpp

@@ -404,6 +404,9 @@ void StmtEmitter::visitReturnStmt(ReturnStmt *S) {
   if (!S->hasResult())
     // Void return.
     SGF.Cleanups.emitBranchAndCleanups(SGF.ReturnDest, Loc);
+  else if (S->getResult()->getType()->isUninhabited())
+    // Never return.
+    SGF.emitIgnoredExpr(S->getResult());
   else
     SGF.emitReturnExpr(Loc, S->getResult());
 }

lib/SILOptimizer/Mandatory/DiagnoseUnreachable.cpp

@@ -423,6 +423,20 @@ static SILInstruction *getAsCallToNoReturn(SILInstruction *I) {
     if (BI->getModule().isNoReturnBuiltinOrIntrinsic(BI->getName()))
       return BI;
   }
+
+  // These appear in accessors for stored properties with uninhabited
+  // type. Since a type containing an uninhabited stored property is
+  // itself uninhabited, we treat these identically to fatalError(), etc.
+  if (auto *SEI = dyn_cast<StructExtractInst>(I)) {
+    if (SEI->getType().getASTType()->isUninhabited())
+      return SEI;
+  }
+
+  if (auto *SEAI = dyn_cast<StructElementAddrInst>(I)) {
+    if (SEAI->getType().getASTType()->isUninhabited())
+      return SEAI;
+  }
+
   return nullptr;
 }
 

test/SILOptimizer/unreachable_code.swift

@@ -374,3 +374,37 @@ struct SillyStruct {
     takesInOut(value: &self)
   }
 }
+
+// This triggers various problems
+public func genericMightBeNever<R>(
+  _ body: () -> R) -> R {
+  while true {}
+
+}
+
+func sillyGenericExample() -> Never {
+  return genericMightBeNever {
+    return genericMightBeNever {
+      return fatalError()
+    }
+  }
+}
+
+// https://bugs.swift.org/browse/SR-7472
+
+protocol P {
+    static var theThing: Self { get }
+}
+
+extension Never : P {
+    static var theThing: Never { return fatalError() }
+}
+
+func test<T: P>(_ type: T.Type) -> T {
+    return type.theThing
+}
+
+func f(i: Int?) {
+    guard i != nil else { Never.theThing }
+    guard i != nil else { test(Never.self) }
+}