[Diagnostics] In ExtraneousReturnFailure, insert the return type fix-it

after any effects specifiers like async, throws, rethrows, etc.

Author: hborla

lib/Sema/CSDiagnostics.cpp

@@ -5337,8 +5337,17 @@ bool ExtraneousReturnFailure::diagnoseAsError() {
     if (FD->getResultTypeRepr() == nullptr &&
         FD->getParameters()->getStartLoc().isValid() &&
         !FD->getBaseIdentifier().empty()) {
-      auto fixItLoc = Lexer::getLocForEndOfToken(
-          getASTContext().SourceMgr, FD->getParameters()->getEndLoc());
+      // Insert the fix-it after the parameter list, and after any
+      // effects specifiers.
+      SourceLoc loc = FD->getParameters()->getEndLoc();
+      if (auto asyncLoc = FD->getAsyncLoc())
+        loc = asyncLoc;
+
+      if (auto throwsLoc = FD->getThrowsLoc())
+        if (throwsLoc.getOpaquePointerValue() > loc.getOpaquePointerValue())
+          loc = throwsLoc;
+
+      auto fixItLoc = Lexer::getLocForEndOfToken(getASTContext().SourceMgr, loc);
       emitDiagnostic(diag::add_return_type_note)
           .fixItInsert(fixItLoc, " -> <#Return Type#>");
     }

test/Constraints/diagnostics.swift

@@ -1213,6 +1213,43 @@ func voidFuncWithNestedVoidFunc() {
   }
 }
 
+func voidFuncWithEffects1() throws {
+  return 1
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}{{35-35= -> <#Return Type#>}}
+}
+
+func voidFuncWithEffects2() async throws {
+  return 1
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}{{41-41= -> <#Return Type#>}}
+}
+
+// expected-error@+1 {{'async' must precede 'throws'}}
+func voidFuncWithEffects3() throws async {
+  return 1
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}{{41-41= -> <#Return Type#>}}
+}
+
+func voidFuncWithEffects4() async {
+  return 1
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}{{34-34= -> <#Return Type#>}}
+}
+
+func voidFuncWithEffects5(_ closure: () throws -> Void) rethrows {
+  return 1
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}{{65-65= -> <#Return Type#>}}
+}
+
+func voidGenericFuncWithEffects<T>(arg: T) async where T: CustomStringConvertible {
+  return 1
+  // expected-error@-1 {{unexpected non-void return value in void function}}
+  // expected-note@-2 {{did you mean to add a return type?}}{{49-49= -> <#Return Type#>}}
+}
+
 // Special cases: These should not offer a note + fix-it
 
 func voidFuncExplicitType() -> Void {