ModuleInterface: Improve TypedThrows feature guards.

Functions that use typed throws _anywhere_ in their signature (including in
closure types) need to be surrounded with `if $TypedThrows` guards in
swiftinterfaces.

Author: tshortli

lib/AST/ASTPrinter.cpp

@@ -3758,8 +3758,26 @@ static bool usesFeatureFullTypedThrows(Decl *decl) {
 }
 
 static bool usesFeatureTypedThrows(Decl *decl) {
-  if (auto func = dyn_cast<AbstractFunctionDecl>(decl))
-    return func->getThrownTypeRepr() != nullptr;
+  if (auto func = dyn_cast<AbstractFunctionDecl>(decl)) {
+    struct Walker : public TypeWalker {
+      bool hasTypedThrows = false;
+
+      Action walkToTypePre(Type ty) override {
+        if (auto funcType = ty->getAs<AnyFunctionType>()) {
+          if (funcType->hasThrownError()) {
+            hasTypedThrows = true;
+            return Action::Stop;
+          }
+        }
+
+        return Action::Continue;
+      }
+    };
+
+    Walker walker;
+    func->getInterfaceType().walk(walker);
+    return walker.hasTypedThrows;
+  }
 
   return false;
 }

test/ModuleInterface/typed_throws.swift

@@ -11,3 +11,15 @@ public enum MyError: Error {
 // CHECK-NEXT: public func throwsMyError() throws(Test.MyError)
 // CHECK-NEXT: #endif
 public func throwsMyError() throws(MyError) { }
+
+// CHECK:      #if compiler(>=5.3) && $TypedThrows
+// CHECK-NEXT: public func takesClosureThrowingMyError(_ closure: () throws(Test.MyError) -> Swift.Void)
+// CHECK-NEXT: #endif
+public func takesClosureThrowingMyError(_ closure: () throws(MyError) -> Void) {}
+
+public struct HasThrowingInit {
+  // CHECK:      #if compiler(>=5.3) && $TypedThrows
+  // CHECK-NEXT: public init() throws(Test.MyError)
+  // CHECK-NEXT: #endif
+  public init() throws(MyError) { }
+}