[CSDiagnostics] Offer a fix-it to insert a return type placeholder when returning from a void function

include/swift/AST/DiagnosticsSema.def

@@ -703,6 +703,8 @@ NOTE(selector_construction_suppress_warning,none,
 
 ERROR(cannot_return_value_from_void_func,none,
       "unexpected non-void return value in void function", ())
+NOTE(add_return_type_note,none,
+     "did you mean to add a return type?", ())
 
 //------------------------------------------------------------------------------
 // MARK: Name Binding

lib/Sema/CSDiagnostics.cpp

@@ -4792,6 +4792,23 @@ bool InvalidUseOfAddressOf::diagnoseAsError() {
 bool ExtraneousReturnFailure::diagnoseAsError() {
   auto *anchor = getAnchor();
   emitDiagnostic(anchor->getLoc(), diag::cannot_return_value_from_void_func);
+  if (auto FD = dyn_cast<FuncDecl>(getDC())) {
+    // We only want to emit the note + fix-it if the function does not
+    // have an explicit return type. The reason we also need to check
+    // whether the parameter list has a valid loc is to guard against
+    // cases like like 'var foo: () { return 1 }' as here that loc will
+    // be invalid. We also need to check that the name is not empty,
+    // because certain decls will have empty name (like setters).
+    if (FD->getBodyResultTypeLoc().getLoc().isInvalid() &&
+        FD->getParameters()->getStartLoc().isValid() &&
+        !FD->getName().empty()) {
+      auto fixItLoc = Lexer::getLocForEndOfToken(
+          getASTContext().SourceMgr, FD->getParameters()->getEndLoc());
+      emitDiagnostic(anchor->getLoc(), diag::add_return_type_note)
+          .fixItInsert(fixItLoc, " -> <#Return Type#>");
+    }
+  }
+
   return true;
 }
 

test/Constraints/diagnostics.swift

@@ -127,7 +127,9 @@ protocol Shoes {
 
 // Here the opaque value has type (metatype_type (archetype_type ... ))
 func f(_ x: Shoes, asType t: Shoes.Type) {
-  return t.select(x) // expected-error{{unexpected non-void return value in void function}}
+  return t.select(x) 
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}
 }
 
 precedencegroup Starry {
@@ -184,7 +186,9 @@ func perform<T>() {}  // expected-error {{generic parameter 'T' is not used in f
 
 // <rdar://problem/17080659> Error Message QOI - wrong return type in an overload
 func recArea(_ h: Int, w : Int) {
-  return h * w  // expected-error {{unexpected non-void return value in void function}}
+  return h * w  
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}
 }
 
 // <rdar://problem/17224804> QoI: Error In Ternary Condition is Wrong
@@ -752,7 +756,9 @@ func segfault23433271(_ a : UnsafeMutableRawPointer) {
 // <rdar://problem/23272739> Poor diagnostic due to contextual constraint
 func r23272739(_ contentType: String) {
   let actualAcceptableContentTypes: Set<String> = []
-  return actualAcceptableContentTypes.contains(contentType)  // expected-error {{unexpected non-void return value in void function}}
+  return actualAcceptableContentTypes.contains(contentType)  
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}
 }
 
 // <rdar://problem/23641896> QoI: Strings in Swift cannot be indexed directly with integer offsets
@@ -863,8 +869,8 @@ class Foo23752537 {
 extension Foo23752537 {
   func isEquivalent(other: Foo23752537) {
     // TODO: <rdar://problem/27391581> QoI: Nonsensical "binary operator '&&' cannot be applied to two 'Bool' operands"
-    // expected-error @+1 {{unexpected non-void return value in void function}}
-    return (self.title != other.title && self.message != other.message)
+    // expected-error@+1 {{unexpected non-void return value in void function}} 
+    return (self.title != other.title && self.message != other.message) // expected-note {{did you mean to add a return type?}}
   }
 }
 
@@ -890,7 +896,9 @@ func f23213302() {
 
 // <rdar://problem/24202058> QoI: Return of call to overloaded function in void-return context
 func rdar24202058(a : Int) {
-  return a <= 480 // expected-error {{unexpected non-void return value in void function}}
+  return a <= 480 
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}
 }
 
 // SR-1752: Warning about unused result with ternary operator
@@ -953,7 +961,9 @@ r27212391(a: 1, 3, x: 5)    // expected-error {{argument 'x' must precede unname
 
 // SR-1255
 func foo1255_1() {
-  return true || false // expected-error {{unexpected non-void return value in void function}}
+  return true || false 
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}
 }
 func foo1255_2() -> Int {
   return true || false // expected-error {{cannot convert return expression of type 'Bool' to return type 'Int'}}
@@ -1150,6 +1160,7 @@ func sr5045() {
   let doubles: [Double] = [1, 2, 3]
   return doubles.reduce(0, +)
   // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}
 }
 
 // rdar://problem/32934129 - QoI: misleading diagnostic
@@ -1165,7 +1176,9 @@ class L_32934129<T : Comparable> {
 
   func length() -> Int {
     func inner(_ list: L_32934129<T>?, _ count: Int) {
-    guard let list = list else { return count } // expected-error {{unexpected non-void return value in void function}}
+    guard let list = list else { return count } 
+      // expected-error@-1 {{unexpected non-void return value in void function}}
+      // expected-note@-2 {{did you mean to add a return type?}}
       return inner(list.next, count + 1)
     }
 
@@ -1315,3 +1328,60 @@ takesGenericFunction(true) // expected-error {{cannot convert value of type 'Boo
 func takesTuple<T>(_ x: ([T], [T])) {} // expected-note {{in call to function 'takesTuple'}}
 takesTuple(true) // expected-error {{cannot convert value of type 'Bool' to expected argument type '([T], [T])'}}
 // expected-error@-1 {{generic parameter 'T' could not be inferred}}
+
+// Void function returns non-void result fix-it
+
+func voidFunc() {
+  return 1 
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}{{16-16= -> <#Return Type#>}}
+}
+
+func voidFuncWithArgs(arg1: Int) {
+  return 1 
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}{{33-33= -> <#Return Type#>}}
+}
+
+func voidFuncWithCondFlow() {
+  if Bool.random() {
+    return 1
+    // expected-error@-1 {{unexpected non-void return value in void function}}
+    // expected-note@-2 {{did you mean to add a return type?}}{{28-28= -> <#Return Type#>}}
+  } else {
+    return 2
+    // expected-error@-1 {{unexpected non-void return value in void function}}
+    // expected-note@-2 {{did you mean to add a return type?}}{{28-28= -> <#Return Type#>}}
+  }
+}
+
+func voidFuncWithNestedVoidFunc() {
+  func nestedVoidFunc() {
+    return 1
+    // expected-error@-1 {{unexpected non-void return value in void function}}
+    // expected-note@-2 {{did you mean to add a return type?}}{{24-24= -> <#Return Type#>}}
+  }
+}
+
+// Special cases: These should not offer a note + fix-it
+
+func voidFuncExplicitType() -> Void {
+  return 1 // expected-error {{unexpected non-void return value in void function}}
+}
+
+class ClassWithDeinit {
+  deinit {
+    return 0 // expected-error {{unexpected non-void return value in void function}}
+  }
+}
+
+class ClassWithVoidProp {
+  var propertyWithVoidType: () { return 5 } // expected-error {{unexpected non-void return value in void function}}
+}
+
+class ClassWithPropContainingSetter {
+  var propWithSetter: Int {
+    get { return 0 }
+    set { return 1 } // expected-error {{unexpected non-void return value in void function}}
+  }
+}

test/Constraints/overload.swift

@@ -135,7 +135,9 @@ func test_contextual_result_1() {
 }
 
 func test_contextual_result_2() {
-  return overloaded_identity(1)  // expected-error {{unexpected non-void return value in void function}}
+  return overloaded_identity(1)
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}
 }
 
 // rdar://problem/24128153

test/Misc/misc_diagnostics.swift

@@ -56,7 +56,9 @@ class A {
     }
 }
 
-func retV() { return true } // expected-error {{unexpected non-void return value in void function}}
+func retV() { return true } 
+// expected-error@-1 {{unexpected non-void return value in void function}}
+// expected-note@-2 {{did you mean to add a return type?}}
 
 func retAI() -> Int {
     let a = [""]
@@ -65,7 +67,9 @@ func retAI() -> Int {
 }
 
 func bad_return1() {
-  return 42  // expected-error {{unexpected non-void return value in void function}}
+  return 42  
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}
 }
 
 func bad_return2() -> (Int, Int) {
@@ -74,15 +78,19 @@ func bad_return2() -> (Int, Int) {
 
 // <rdar://problem/14096697> QoI: Diagnostics for trying to return values from void functions
 func bad_return3(lhs: Int, rhs: Int) {
-  return lhs != 0  // expected-error {{unexpected non-void return value in void function}}
+  return lhs != 0  
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}
 }
 
 class MyBadReturnClass {
   static var intProperty = 42
 }
 
 func ==(lhs:MyBadReturnClass, rhs:MyBadReturnClass) {
-  return MyBadReturnClass.intProperty == MyBadReturnClass.intProperty  // expected-error{{unexpected non-void return value in void function}}
+  return MyBadReturnClass.intProperty == MyBadReturnClass.intProperty
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}
 }
 
 
@@ -156,4 +164,3 @@ func tuple_splat2(_ q : (a : Int, b : Int)) {
 func is_foreign(a: AnyObject) -> Bool {
   return a is CGColor // expected-warning {{'is' test is always true because 'CGColor' is a Core Foundation type}}
 }
-