[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 +4792,21 @@ 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.
+    if (FD->getBodyResultTypeLoc().getLoc().isInvalid() &&
+        FD->getParameters()->getStartLoc().isValid()) {
+      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

@@ -1315,3 +1315,53 @@ 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?}}{{15-15= -> <# 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}}
+}