added getStream() to MultiMap in collection.ts, adjusted evaluati…

…on of precomputed scopes in `DefaultScopeProvider`

packages/langium/src/references/scope-provider.ts

@@ -56,8 +56,8 @@ export class DefaultScopeProvider implements ScopeProvider {
         if (precomputed) {
             let currentNode: AstNode | undefined = context.container;
             do {
-                const allDescriptions = precomputed.get(currentNode);
-                if (allDescriptions.length > 0) {
+                const allDescriptions = precomputed.getStream(currentNode);
+                if (!allDescriptions.isEmpty()) {
                     scopes.push(stream(allDescriptions).filter(
                         desc => this.reflection.isSubtype(desc.type, referenceType)));
                 }

packages/langium/src/utils/collections.ts

@@ -77,6 +77,14 @@ export class MultiMap<K, V> {
         return this.map.get(key) ?? [];
     }
 
+    /**
+     * Returns a stream of all values associated with the given key. If no value exists,
+     * an empty stream is returned.
+     */
+    getStream(key: K): Stream<V> {
+        return stream(this.get(key));
+    }
+
     /**
      * Operates differently depending on whether a `value` is given:
      *  * With a value, this method returns `true` if the specific key / value pair is present in the multimap.