fix(jsii): fail compilation when two or more enum members have same val

packages/jsii/lib/assembler.ts

@@ -1738,6 +1738,9 @@ export class Assembler implements Emitter {
       return undefined;
     }
 
+    // check the enum to see if there are duplicate enum values
+    this.assertNoDuplicateEnumValues(decl);
+
     this._warnAboutReservedWords(symbol);
 
     const flags = ts.getCombinedModifierFlags(decl);
@@ -1791,6 +1794,57 @@ export class Assembler implements Emitter {
     return jsiiType;
   }
 
+  private assertNoDuplicateEnumValues(decl: ts.EnumDeclaration): void {
+    type EnumValue = {
+      name: string;
+      value: string;
+      decl: ts.DeclarationName | undefined;
+    };
+
+    const enumValues = decl.members
+      .filter((m) => m.initializer)
+      .map((member): EnumValue => {
+        return {
+          value: member.initializer!.getText(),
+          name: member.name.getText(),
+          decl: ts.getNameOfDeclaration(member),
+        };
+      });
+
+    const hasDuplicateEnumValues = enumValues.some(
+      (val, _, arr) => arr.filter((e) => val.value === e.value).length > 1,
+    );
+
+    if (hasDuplicateEnumValues) {
+      const enumValueMap = enumValues.reduce<Record<string, EnumValue[]>>(
+        (acc, val) => {
+          if (!acc[val.value]) {
+            acc[val.value] = [];
+          }
+          acc[val.value].push(val);
+          return acc;
+        },
+        {},
+      );
+      for (const duplicateValue of Object.keys(enumValueMap)) {
+        if (enumValueMap[duplicateValue].length > 1) {
+          const err = JsiiDiagnostic.JSII_1004_DUPLICATE_ENUM_VALUE.create(
+            enumValueMap[duplicateValue][0].decl!,
+            duplicateValue,
+            enumValueMap[duplicateValue].map((v) => v.name),
+          );
+          for (let i = 1; i < enumValueMap[duplicateValue].length; i++) {
+            err.addRelatedInformation(
+              enumValueMap[duplicateValue][i].decl!,
+              'The conflicting declaration is here',
+            );
+          }
+          this._diagnostics.push(err);
+        }
+      }
+    }
+  }
+
   /**
    * Return docs for a symbol
    */

packages/jsii/lib/jsii-diagnostic.ts

@@ -109,7 +109,7 @@ export class Code<
    *
    * @param code            the numeric code for the diagnostic
    * @param name            the symbolic name for the diagnostic
-   * @param defaultCategory the default category this diagnostic ransk in
+   * @param defaultCategory the default category this diagnostic ranks in
    * @param formatter       a message formatter for easy creation of diagnostics
    */
   private constructor(
@@ -292,6 +292,15 @@ export class JsiiDiagnostic implements ts.Diagnostic {
     name: 'typescript-restrictions/unsupported-type',
   });
 
+  public static readonly JSII_1004_DUPLICATE_ENUM_VALUE = Code.error({
+    code: 1004,
+    formatter: (enumValue: string, enumMemberNames: string[]) =>
+      `Value ${enumValue} is used for multiple enum values: ${enumMemberNames.join(
+        ', ',
+      )}`,
+    name: 'typescript-restrictions/duplicate-enum-value',
+  });
+
   //////////////////////////////////////////////////////////////////////////////
   // 2000 => 2999 -- RESERVED
 

packages/jsii/test/enums.test.ts

@@ -57,3 +57,16 @@ test('enums can have a mix of letters and number', () => {
     { name: 'IB3M' },
   ]);
 });
+
+test('enums with the same assigned value should fail', () => {
+  expect(() =>
+    sourceToAssemblyHelper(`
+    export enum Foo {
+      BAR = 'Bar',
+      BAR_DUPE = 'Bar',
+      BAZ = 'Baz',
+      BAZ_DUPE = 'Baz',
+    }
+  `),
+  ).toThrow(/There were compiler errors/);
+});

packages/jsii/test/negatives/neg.enum-duplicate-values.ts

@@ -0,0 +1,6 @@
+export enum Foo {
+  FOO = 'foo',
+  FOO_DUPLICATE = 'foo',
+  BAR = 'bar',
+  BAR_COPY = 'bar',
+}