From 8affe9b6d9892fd567187057237f3d028adec825 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nicol=C3=B2=20Ribaudo?= <nicolo.ribaudo@gmail.com>
Date: Thu, 10 Dec 2020 12:58:27 +0100
Subject: [PATCH] Make unknown assumptions default to `undefined`

---
 packages/babel-core/src/config/helpers/config-api.js |  4 ++--
 packages/babel-core/src/config/partial.js            |  5 +----
 packages/babel-core/src/config/validation/options.js |  1 -
 packages/babel-core/test/assumptions.js              | 10 ++++------
 packages/babel-core/test/config-chain.js             |  2 +-
 packages/babel-helper-plugin-utils/src/index.js      |  4 +---
 6 files changed, 9 insertions(+), 17 deletions(-)

diff --git a/packages/babel-core/src/config/helpers/config-api.js b/packages/babel-core/src/config/helpers/config-api.js
index a10ccbbde274..24dfedc9f817 100644
--- a/packages/babel-core/src/config/helpers/config-api.js
+++ b/packages/babel-core/src/config/helpers/config-api.js
@@ -26,7 +26,7 @@ type CallerFactory = ((CallerMetadata | void) => mixed) => SimpleType;
 
 type TargetsFunction = () => Targets;
 
-type AssumptionFunction = (name: string) => boolean;
+type AssumptionFunction = (name: string) => boolean | void;
 
 export type ConfigAPI = {|
   version: string,
@@ -94,7 +94,7 @@ export function makePresetAPI<SideChannel: Context.SimplePreset>(
 export function makePluginAPI<SideChannel: Context.SimplePlugin>(
   cache: CacheConfigurator<SideChannel>,
 ): PluginAPI {
-  const assumption = name => cache.using(data => !!data.assumptions[name]);
+  const assumption = name => cache.using(data => data.assumptions[name]);
 
   return { ...makePresetAPI(cache), assumption };
 }
diff --git a/packages/babel-core/src/config/partial.js b/packages/babel-core/src/config/partial.js
index ca610dce3587..5d74daa00ec0 100644
--- a/packages/babel-core/src/config/partial.js
+++ b/packages/babel-core/src/config/partial.js
@@ -118,10 +118,7 @@ export default function* loadPrivatePartialConfig(
   if (!configChain) return null;
 
   const merged: ValidatedOptions = {
-    // TODO(Babel 8): everything should default to false. Remove this object.
-    assumptions: {
-      newableArrowFunctions: true,
-    },
+    assumptions: {},
   };
   configChain.options.forEach(opts => {
     mergeOptions((merged: any), opts);
diff --git a/packages/babel-core/src/config/validation/options.js b/packages/babel-core/src/config/validation/options.js
index 507b183fa07a..c5e99846b974 100644
--- a/packages/babel-core/src/config/validation/options.js
+++ b/packages/babel-core/src/config/validation/options.js
@@ -333,7 +333,6 @@ export type NestingPath = RootPath | OverridesPath | EnvPath;
 
 export const assumptionsNames = new Set<string>([
   "mutableTemplateObject",
-  "newableArrowFunctions",
   "setPublicClassFields",
 ]);
 
diff --git a/packages/babel-core/test/assumptions.js b/packages/babel-core/test/assumptions.js
index ad23adae50a7..3f6e58005eb1 100644
--- a/packages/babel-core/test/assumptions.js
+++ b/packages/babel-core/test/assumptions.js
@@ -39,8 +39,6 @@ describe("assumptions", () => {
     ).toEqual({
       setPublicClassFields: true,
       mutableTemplateObject: true,
-      // This is enabled by default
-      newableArrowFunctions: true,
     });
   });
 
@@ -58,9 +56,9 @@ describe("assumptions", () => {
         api => {
           setPublicClassFields = api.assumption("setPublicClassFields");
 
-          // Unknown assumptions default to "false" and don't throw, so
-          // that plugins can keep compat with older @babel/core versions
-          // when they introduce support for a new assumption.
+          // Unknown assumptions don't throw, so that plugins can keep compat
+          // with older @babel/core versions when they introduce support for
+          // a new assumption.
           unknownAssumption = api.assumption("unknownAssumption");
 
           return {};
@@ -69,7 +67,7 @@ describe("assumptions", () => {
     });
 
     expect(setPublicClassFields).toBe(true);
-    expect(unknownAssumption).toBe(false);
+    expect(unknownAssumption).toBe(undefined);
   });
 
   it("cannot be queried from presets", () => {
diff --git a/packages/babel-core/test/config-chain.js b/packages/babel-core/test/config-chain.js
index d99a6da63068..f710d347ed9e 100644
--- a/packages/babel-core/test/config-chain.js
+++ b/packages/babel-core/test/config-chain.js
@@ -985,7 +985,7 @@ describe("buildConfigChain", function () {
       presets: [],
       cloneInputAst: true,
       targets: {},
-      assumptions: { newableArrowFunctions: true },
+      assumptions: {},
     });
     const realEnv = process.env.NODE_ENV;
     const realBabelEnv = process.env.BABEL_ENV;
diff --git a/packages/babel-helper-plugin-utils/src/index.js b/packages/babel-helper-plugin-utils/src/index.js
index 8f90cae836e7..5347c2470fa7 100644
--- a/packages/babel-helper-plugin-utils/src/index.js
+++ b/packages/babel-helper-plugin-utils/src/index.js
@@ -28,9 +28,7 @@ const apiPolyfills = {
   },
   // This is supported starting from Babel 7.13
   // TODO(Babel 8): Remove this polyfill
-  assumption: () => () => {
-    return false;
-  },
+  assumption: () => () => {},
 };
 
 function copyApiObject(api) {