fix issue with inline fragments over interface in compatibility plugin

Author: dotansimha

packages/plugins/typescript/compatibility/src/selection-set-to-types.ts

@@ -72,7 +72,7 @@ export function selectionSetToTypes(
           const fragmentName = baseVisitor.convertName(typeCondition, { suffix: 'InlineFragment' });
           let inlineFragmentValue;
 
-          if (isUnionType(parentType)) {
+          if (isUnionType(parentType) || isInterfaceType(parentType)) {
             inlineFragmentValue = `DiscriminateUnion<RequireField<${stack}, '__typename'>, { __typename: '${typeCondition}' }>`;
           } else {
             inlineFragmentValue = `{ __typename: '${typeCondition}' } & Pick<${stack}, ${selection.selectionSet.selections

packages/plugins/typescript/compatibility/tests/compatibility.spec.ts

@@ -149,6 +149,49 @@ describe('Compatibility Plugin', () => {
   `);
 
   describe('Issues', () => {
+    it('Issue #1943 - Missing DiscriminateUnion on interface types with strict mode', async () => {
+      const testSchema = buildSchema(/* GraphQL */ `
+        interface Book {
+          title: String
+          author: String
+        }
+
+        type TextBook implements Book {
+          title: String
+          author: String
+          classes: [String]
+        }
+
+        type ColoringBook implements Book {
+          title: String
+          author: String
+          colors: [String]
+        }
+
+        type Query {
+          schoolBooks: [Book]
+        }
+      `);
+      const testQuery = parse(/* GraphQL */ `
+        query GetBooks {
+          schoolBooks {
+            title
+            ... on TextBook {
+              classes
+            }
+            ... on ColoringBook {
+              colors
+            }
+          }
+        }
+      `);
+
+      const operations = [{ filePath: '', content: testQuery }];
+      const config = { strict: true, noNamespaces: true };
+      const result = await plugin(testSchema, operations, config);
+
+      await validateAndCompile(result, testSchema, operations, config, false);
+    });
     it('Issue #1686 - Inline fragments on a union', async () => {
       const testSchema = buildSchema(/* GraphQL */ `
         schema {
@@ -468,7 +511,7 @@ describe('Compatibility Plugin', () => {
 
     expect(result).toContain(`export type Query = Me4Query;`);
     expect(result).toContain(`export type Me = Me4Query['me'];`);
-    validateAndCompile(result, testSchema, ast);
+    await validateAndCompile(result, testSchema, ast);
   });
 
   it('Should work with interfaces and inline fragments', async () => {
@@ -511,7 +554,7 @@ describe('Compatibility Plugin', () => {
       },
     ];
     const result = await plugin(testSchema, ast, {});
-    validateAndCompile(result, testSchema, ast);
+    await validateAndCompile(result, testSchema, ast);
   });
 
   it('Should generate namepsace and the internal types correctly', async () => {
@@ -557,7 +600,7 @@ describe('Compatibility Plugin', () => {
     expect(result).toContain(`export type Friends = MeQuery['me']['friends'][0];`);
     expect(result).toContain(`export type _Friends = MeQuery['me']['friends'][0]['friends'][0];`);
     expect(result).toContain(`export type __Friends = MeQuery['me']['friends'][0]['friends'][0]['friends'][0];`);
-    validateAndCompile(result, schema, ast);
+    await validateAndCompile(result, schema, ast);
   });
 
   it('Should work with fragment spread', async () => {