fix: include default deprecation reason in the composed supergraph

.changeset/green-chairs-nail.md

@@ -0,0 +1,10 @@
+---
+"@apollo/composition": patch
+"@apollo/federation-internals": patch
+---
+
+Include default deprecation reason in the composed supergraph
+
+`@deprecated` reason used to be optional so if a subgraph does not specify the reason, it is omitted from the supergraph
+schema. [Reason is no longer optional](https://github.com/graphql/graphql-spec/pull/1040) so we should always include it
+(even if it has a default value).

composition-js/src/__tests__/compose.test.ts

@@ -1939,7 +1939,7 @@ describe('composition', () => {
       const [_, api] = schemas(result);
       expect(printSchema(api)).toMatchString(`
         type Query {
-          a: String @deprecated
+          a: String @deprecated(reason: "No longer supported")
           b: String
         }
       `);

internals-js/src/definitions.ts

@@ -3260,6 +3260,12 @@ export class Directive<
     const args = entries.length == 0 ? '' : '(' + entries.map(([n, v]) => `${n}: ${valueToString(v, this.argumentType(n))}`).join(', ') + ')';
     return `@${this.name}${args}`;
   }
+
+  toStringWithDefaultValues(): string {
+    const entries = Object.entries(this.arguments(true)).filter(([_, v]) => v !== undefined);
+    const args = entries.length == 0 ? '' : '(' + entries.map(([n, v]) => `${n}: ${valueToString(v, this.argumentType(n))}`).join(', ') + ')';
+    return `@${this.name}${args}`;
+  }
 }
 
 /**

internals-js/src/print.ts

@@ -24,6 +24,7 @@ import {
 } from "./definitions";
 import { assert } from "./utils";
 import { valueToString } from "./values";
+import { GraphQLDeprecatedDirective } from 'graphql/type';
 
 export type PrintOptions = {
   indentString: string;
@@ -235,7 +236,13 @@ function printAppliedDirectives(
     return "";
   }
   const joinStr = onNewLines ? '\n' + options.indentString : ' ';
-  const directives = appliedDirectives.map(d => d.toString()).join(joinStr);
+  const directives = appliedDirectives.map(d => {
+    if (GraphQLDeprecatedDirective.name === d.name) {
+      return d.toStringWithDefaultValues();
+    } else {
+      return d.toString();
+    }
+  }).join(joinStr);
   return onNewLines ? '\n' + options.indentString + directives + (endWithNewLine ? '\n' : '') : ' ' + directives;
 }