Add Deferred::isResolved() (#356)

lib/Deferred.php

@@ -23,6 +23,7 @@ public function __construct()
             use Internal\Placeholder {
                 resolve as public;
                 fail as public;
+                isResolved as public;
             }
         };
 
@@ -64,4 +65,12 @@ public function fail(\Throwable $reason)
         /** @psalm-suppress UndefinedInterfaceMethod */
         $this->resolver->fail($reason);
     }
+
+    /**
+     * @return bool True if the promise has been resolved.
+     */
+    public function isResolved(): bool
+    {
+        return $this->resolver->isResolved();
+    }
 }

lib/Internal/Placeholder.php

@@ -176,4 +176,12 @@ private function fail(\Throwable $reason)
     {
         $this->resolve(new Failure($reason));
     }
+
+    /**
+     * @return bool True if the placeholder has been resolved.
+     */
+    private function isResolved(): bool
+    {
+        return $this->resolved;
+    }
 }

test/DeferredTest.php

@@ -35,8 +35,11 @@ public function testResolve()
             $result = $value;
         });
 
+        $this->assertFalse($this->deferred->isResolved());
+
         $this->deferred->resolve($value);
 
+        $this->assertTrue($this->deferred->isResolved());
         $this->assertTrue($invoked);
         $this->assertSame($value, $result);
     }
@@ -55,8 +58,11 @@ public function testFail()
             $result = $exception;
         });
 
+        $this->assertFalse($this->deferred->isResolved());
+
         $this->deferred->fail($exception);
 
+        $this->assertTrue($this->deferred->isResolved());
         $this->assertTrue($invoked);
         $this->assertSame($exception, $result);
     }

test/MergeTest.php

@@ -87,7 +87,6 @@ public function testMergeWithFailedIterator(): void
 
             try {
                 while (yield $iterator->advance()) {
-                    ;
                 }
                 $this->fail("The exception used to fail the iterator should be thrown from advance()");
             } catch (TestException $reason) {

test/ProducerTest.php

@@ -117,7 +117,6 @@ public function testProducerCoroutineThrows(): void
                 });
 
                 while (yield $producer->advance()) {
-                    ;
                 }
                 $this->fail("The exception thrown from the coroutine should fail the iterator");
             });