constify constifiable methods

Summary:
Make these const:
- {Future,SemiFuture}::hasValue()
- {Future,SemiFuture}::hasException()
- {Future,SemiFuture}::isActive()
- Promise::throwIfFulfilled()

Reviewed By: yfeldblum

Differential Revision: D7824879

fbshipit-source-id: 035b4bbcfd5b88825580e22b37b13e07158898f3

folly/futures/Future-inl.h

@@ -217,12 +217,12 @@ bool FutureBase<T>::isReady() const {
 }
 
 template <class T>
-bool FutureBase<T>::hasValue() {
+bool FutureBase<T>::hasValue() const {
   return result().hasValue();
 }
 
 template <class T>
-bool FutureBase<T>::hasException() {
+bool FutureBase<T>::hasException() const {
   return result().hasException();
 }
 

folly/futures/Future.h

@@ -114,10 +114,10 @@ class FutureBase {
   bool isReady() const;
 
   /// sugar for getTry().hasValue()
-  bool hasValue();
+  bool hasValue() const;
 
   /// sugar for getTry().hasException()
-  bool hasException();
+  bool hasException() const;
 
   /// If the promise has been fulfilled, return an Optional with the Try<T>.
   /// Otherwise return an empty Optional.
@@ -132,7 +132,7 @@ class FutureBase {
   template <class F>
   void setCallback_(F&& func);
 
-  bool isActive() {
+  bool isActive() const {
     return getCore().isActive();
   }
 

folly/futures/Promise-inl.h

@@ -49,7 +49,7 @@ Promise<T>& Promise<T>::operator=(Promise<T>&& other) noexcept {
 }
 
 template <class T>
-void Promise<T>::throwIfFulfilled() {
+void Promise<T>::throwIfFulfilled() const {
   if (getCore().hasResult()) {
     throwPromiseAlreadySatisfied();
   }

folly/futures/Promise.h

@@ -158,7 +158,7 @@ class Promise {
 
   explicit Promise(futures::detail::EmptyConstruct) noexcept;
 
-  void throwIfFulfilled();
+  void throwIfFulfilled() const;
   void detach();
 };