enhance(stitch) Test merge conflicts (#2006)

* test merge conflicts.

* fix linting error.

* update test descriptions.

packages/stitch/tests/mergeConflicts.test.ts

@@ -0,0 +1,81 @@
+import { makeExecutableSchema } from '@graphql-tools/schema';
+import { stitchSchemas } from '@graphql-tools/stitch';
+
+describe('merge conflict handlers', () => {
+  const listings1Schema = makeExecutableSchema({
+    typeDefs: `
+      "ignore! - do not document here"
+      type Listing implements IListing {
+        "type identifier"
+        id: ID!
+      }
+      interface IListing {
+        "interface identifier"
+        id: ID!
+      }
+      "An input"
+      input ListingInput {
+        "input identifier"
+        id: ID!
+      }
+    `
+  });
+
+  const listings2Schema = makeExecutableSchema({
+    typeDefs: `
+      "A type"
+      type Listing implements IListing {
+        "ignore! - do not document here"
+        id: ID!
+      }
+      "An interface"
+      interface IListing {
+        id: ID!
+      }
+      input ListingInput {
+        "ignore! - do not document here"
+        id: ID!
+      }
+    `
+  });
+
+  it('handles description merges', () => {
+    const gatewaySchema = stitchSchemas({
+      subschemas: [
+        { schema: listings1Schema },
+        { schema: listings2Schema },
+      ],
+      typeMergingOptions: {
+        typeDescriptionsMerger(candidates) {
+          const candidate = candidates.find(({ type }) => {
+            const description = (type.description || '').trim();
+            return description.length && !description.startsWith('ignore!');
+          }) || candidates[candidates.length-1];
+          return candidate.type.description;
+        },
+        fieldConfigMerger(candidates) {
+          const fieldConfigs = candidates.map(c => c.fieldConfig);
+          return fieldConfigs.find(({ description }) => {
+            description = (description || '').trim();
+            return description && !description.startsWith('ignore!');
+          }) || fieldConfigs[fieldConfigs.length-1];
+        },
+        inputFieldConfigMerger(candidates) {
+          const inputFieldConfig = candidates.map(c => c.inputFieldConfig);
+          return inputFieldConfig.find(({ description }) => {
+            description = (description || '').trim();
+            return description && !description.startsWith('ignore!');
+          }) || inputFieldConfig[inputFieldConfig.length-1];
+        }
+      },
+      mergeTypes: true
+    });
+
+    expect(gatewaySchema.getType('Listing').description).toEqual('A type');
+    expect(gatewaySchema.getType('IListing').description).toEqual('An interface');
+    expect(gatewaySchema.getType('ListingInput').description).toEqual('An input');
+    expect(gatewaySchema.getType('Listing').getFields().id.description).toEqual('type identifier');
+    expect(gatewaySchema.getType('IListing').getFields().id.description).toEqual('interface identifier');
+    expect(gatewaySchema.getType('ListingInput').getFields().id.description).toEqual('input identifier');
+  });
+});