throw when overriding platform-specific base values

Author: vonovak

.claude/settings.local.json

@@ -10,6 +10,12 @@ import { EasJson } from '../types';
 
 jest.mock('fs');
 
+type DeepPartial<T> = T extends object
+  ? {
+      [P in keyof T]?: DeepPartial<T[P]>;
+    }
+  : T;
+
 beforeEach(async () => {
   vol.reset();
   await fs.mkdirp('/project');
@@ -936,13 +942,39 @@ test('invalid build profile with caching with both paths and customPaths - error
   }).rejects.toThrow(expectedError);
 });
 
-describe('extensions can disable cache which is present in base', () => {
-  type DeepPartial<T> = T extends object
-    ? {
-        [P in keyof T]?: DeepPartial<T[P]>;
-      }
-    : T;
+it.each([
+  {
+    testName: 'platform-specific setting from base can _not_ be overridden',
+    baseConfig: {
+      ios: {
+        prebuildCommand: 'ios prebuild',
+      },
+      android: {
+        prebuildCommand: 'android prebuild',
+      },
+    },
+  },
+])('$testName', async ({ baseConfig }) => {
+  await fs.writeJson('/project/eas.json', {
+    build: {
+      base: baseConfig,
+      extension1: {
+        extends: 'base',
+        prebuildCommand: 'new great prebuild',
+      },
+    },
+  } satisfies DeepPartial<EasJson>);
 
+  const accessor = EasJsonAccessor.fromProjectPath('/project');
+
+  await expect(() =>
+    EasJsonUtils.getBuildProfileAsync(accessor, Platform.ANDROID, 'extension1')
+  ).rejects.toThrow(
+    'Cannot override platform-specific base value "android prebuild" by value "new great prebuild" for key "prebuildCommand". Move the entry out of the "android" object, into the common properties, if you want to override it.'
+  );
+});
+
+describe('extensions can disable cache which is present in base', () => {
   it.each([
     {
       testName: 'platform-specific setting can be disabled',

packages/eas-json/src/__tests__/buildProfiles-test.ts

@@ -96,6 +96,22 @@ function mergeProfiles(
     } as CommonBuildProfile['cache'];
   }
 
+  for (const [key, newValue] of Object.entries(update ?? [])) {
+    for (const platform of ['android', 'ios']) {
+      // @ts-expect-error indexing issues
+      const existingValue = base?.[platform]?.[key];
+      if (existingValue !== undefined) {
+        throw new Error(
+          `Cannot override platform-specific base value ${JSON.stringify(
+            existingValue
+          )} by value ${JSON.stringify(
+            newValue
+          )} for key "${key}". Move the entry out of the "${platform}" object, into the common properties, if you want to override it.`
+        );
+      }
+    }
+  }
+
   if (base.android && update.android) {
     result.android = mergeProfiles(
       base.android as EasJsonBuildProfileResolved,