Fix Promise<void> in non-async function

Fixes #814

source/parser.hera

@@ -1603,8 +1603,8 @@ FunctionSignature
       async,
       generator,
       modifier: {
-        async: !async.length,
-        generator: !generator.length,
+        async: !!async.length,
+        generator: !!generator.length,
       },
       block: null,
       children: !parameters.implicit
@@ -1629,10 +1629,12 @@ FunctionExpression
 
     if (hasAwait(block) && !signature.async.length) {
       signature.async.push("async ")
+      signature.modifier.async = true
     }
 
     if (hasYield(block) && !signature.generator.length) {
       signature.generator.push("*")
+      signature.modifier.generator = true
     }
 
     // Attach the block

test/function.civet

@@ -1228,6 +1228,28 @@ describe "function", ->
       })
     """
 
+    testCase """
+      no async Promise<void>
+      ---
+      (x): Promise<void> ->
+        fetch x
+      ---
+      (function(x): Promise<void> {
+        return fetch(x)
+      })
+    """
+
+    testCase """
+      no async Promise<void> longhand
+      ---
+      function(x): Promise<void>
+        fetch x
+      ---
+      (function(x): Promise<void> {
+        return fetch(x)
+      })
+    """
+
     testCase """
       nested anonymous function
       ---