Warn with loose

packages/babel-helper-create-class-features-plugin/src/index.js

@@ -42,6 +42,31 @@ export function createClassFeaturePlugin({
   const setPublicClassFields = api.assumption("setPublicClassFields");
   const privateFieldsAsProperties = api.assumption("privateFieldsAsProperties");
 
+  if (loose) {
+    const explicit = [];
+
+    if (setPublicClassFields !== undefined) {
+      explicit.push(`"setPublicClassFields"`);
+    }
+    if (privateFieldsAsProperties !== undefined) {
+      explicit.push(`"privateFieldsAsProperties"`);
+    }
+    if (explicit.length !== 0) {
+      console.warn(
+        `[${name}]: You are using the "loose: true" option and you are` +
+          ` explicitly setting a value for the ${explicit.join(" and ")}` +
+          ` assumption${explicit.length > 1 ? "s" : ""}. The "loose" option` +
+          ` can cause incompatibilities with the other class features` +
+          ` plugins, so it's recommended that you replace it with the` +
+          ` following top-level option:\n` +
+          `\t"assumptions": {\n` +
+          `\t\t"setPublicClassFields": true,\n` +
+          `\t\t"privateFieldsAsProperties": true\n` +
+          `\t}`,
+      );
+    }
+  }
+
   return {
     name,
     manipulateOptions,

packages/babel-helper-create-class-features-plugin/test/fixtures/plugin-proposal-class-properties/warn-loose-and-assumptions/input.js

@@ -0,0 +1,3 @@
+class A {
+  foo;
+}

packages/babel-helper-create-class-features-plugin/test/fixtures/plugin-proposal-class-properties/warn-loose-and-assumptions/options.json

@@ -0,0 +1,9 @@
+{
+  "validateLogs": true,
+  "plugins": [
+    ["proposal-class-properties", { "loose": true }]
+  ],
+  "assumptions": {
+    "setPublicClassFields": true
+  }
+}

packages/babel-helper-create-class-features-plugin/test/fixtures/plugin-proposal-class-properties/warn-loose-and-assumptions/output.js

@@ -0,0 +1,6 @@
+class A {
+  constructor() {
+    this.foo = void 0;
+  }
+
+}

packages/babel-helper-create-class-features-plugin/test/fixtures/plugin-proposal-class-properties/warn-loose-and-assumptions/stderr.txt

@@ -0,0 +1,5 @@
+[proposal-class-properties]: You are using the "loose: true" option and you are explicitly setting a value for the "setPublicClassFields" assumption. The "loose" option can cause incompatibilities with the other class features plugins, so it's recommended that you replace it with the following top-level option:
+	"assumptions": {
+		"setPublicClassFields": true,
+		"privateFieldsAsProperties": true
+	}