diff --git a/packages/babel-core/src/config/validation/options.js b/packages/babel-core/src/config/validation/options.js
index b48ad22a941b..dbfaa1b4856f 100644
--- a/packages/babel-core/src/config/validation/options.js
+++ b/packages/babel-core/src/config/validation/options.js
@@ -332,6 +332,7 @@ type EnvPath = $ReadOnly<{
 export type NestingPath = RootPath | OverridesPath | EnvPath;
 
 export const assumptionsNames = new Set<string>([
+  "ignoreFunctionLength",
   "ignoreToPrimitiveHint",
   "mutableTemplateObject",
   "setClassMethods",
diff --git a/packages/babel-plugin-proposal-object-rest-spread/src/index.js b/packages/babel-plugin-proposal-object-rest-spread/src/index.js
index e1f9b7d51016..5fdc599b655f 100644
--- a/packages/babel-plugin-proposal-object-rest-spread/src/index.js
+++ b/packages/babel-plugin-proposal-object-rest-spread/src/index.js
@@ -23,6 +23,9 @@ export default declare((api, opts) => {
     throw new Error(".loose must be a boolean, or undefined");
   }
 
+  const ignoreFunctionLength =
+    api.assumption("ignoreFunctionLength") ?? opts.loose;
+
   function getExtendsHelper(file) {
     return useBuiltIns
       ? t.memberExpression(t.identifier("Object"), t.identifier("assign"))
@@ -275,7 +278,7 @@ export default declare((api, opts) => {
             idx >= i - 1 || paramsWithRestElement.has(idx);
           convertFunctionParams(
             path,
-            loose,
+            ignoreFunctionLength,
             shouldTransformParam,
             replaceRestElement,
           );
diff --git a/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/assumption-ignoreFunctionLength/options.json b/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/assumption-ignoreFunctionLength/options.json
new file mode 100644
index 000000000000..ec966b0b43ba
--- /dev/null
+++ b/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/assumption-ignoreFunctionLength/options.json
@@ -0,0 +1,8 @@
+{
+  "plugins": [
+    "proposal-object-rest-spread"
+  ],
+  "assumptions": {
+    "ignoreFunctionLength": true
+  }
+}
diff --git a/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/assumption-ignoreFunctionLength/parameters-object-rest-used-in-default/input.js b/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/assumption-ignoreFunctionLength/parameters-object-rest-used-in-default/input.js
new file mode 100644
index 000000000000..000e99bcf250
--- /dev/null
+++ b/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/assumption-ignoreFunctionLength/parameters-object-rest-used-in-default/input.js
@@ -0,0 +1,3 @@
+({...R}, a = R) => {}
+({...R}, e, c = 2, a = R, f = q) => { let q; }
+({...R}, a = f(R)) => {}
diff --git a/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/assumption-ignoreFunctionLength/parameters-object-rest-used-in-default/output.js b/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/assumption-ignoreFunctionLength/parameters-object-rest-used-in-default/output.js
new file mode 100644
index 000000000000..84e28e863e5b
--- /dev/null
+++ b/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/assumption-ignoreFunctionLength/parameters-object-rest-used-in-default/output.js
@@ -0,0 +1,33 @@
+function _extends() { _extends = Object.assign || function (target) { for (var i = 1; i < arguments.length; i++) { var source = arguments[i]; for (var key in source) { if (Object.prototype.hasOwnProperty.call(source, key)) { target[key] = source[key]; } } } return target; }; return _extends.apply(this, arguments); }
+
+(_ref, a) => {
+  let R = _extends({}, _ref);
+
+  if (a === void 0) {
+    a = R;
+  }
+};
+
+(_ref2, e, c = 2, a, f) => {
+  let R = _extends({}, _ref2);
+
+  if (a === void 0) {
+    a = R;
+  }
+
+  if (f === void 0) {
+    f = q;
+  }
+
+  return function () {
+    let q;
+  }();
+};
+
+(_ref3, a) => {
+  let R = _extends({}, _ref3);
+
+  if (a === void 0) {
+    a = f(R);
+  }
+};
diff --git a/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-rest/parameters-extra/input.js b/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-rest/parameters-object-rest-used-in-default/input.js
similarity index 100%
rename from packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-rest/parameters-extra/input.js
rename to packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-rest/parameters-object-rest-used-in-default/input.js
diff --git a/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-rest/parameters-extra/output.js b/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-rest/parameters-object-rest-used-in-default/output.js
similarity index 100%
rename from packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-rest/parameters-extra/output.js
rename to packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-rest/parameters-object-rest-used-in-default/output.js
diff --git a/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-spread-loose/parameters-object-rest-used-in-default/input.js b/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-spread-loose/parameters-object-rest-used-in-default/input.js
new file mode 100644
index 000000000000..000e99bcf250
--- /dev/null
+++ b/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-spread-loose/parameters-object-rest-used-in-default/input.js
@@ -0,0 +1,3 @@
+({...R}, a = R) => {}
+({...R}, e, c = 2, a = R, f = q) => { let q; }
+({...R}, a = f(R)) => {}
diff --git a/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-spread-loose/parameters-object-rest-used-in-default/output.js b/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-spread-loose/parameters-object-rest-used-in-default/output.js
new file mode 100644
index 000000000000..84e28e863e5b
--- /dev/null
+++ b/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-spread-loose/parameters-object-rest-used-in-default/output.js
@@ -0,0 +1,33 @@
+function _extends() { _extends = Object.assign || function (target) { for (var i = 1; i < arguments.length; i++) { var source = arguments[i]; for (var key in source) { if (Object.prototype.hasOwnProperty.call(source, key)) { target[key] = source[key]; } } } return target; }; return _extends.apply(this, arguments); }
+
+(_ref, a) => {
+  let R = _extends({}, _ref);
+
+  if (a === void 0) {
+    a = R;
+  }
+};
+
+(_ref2, e, c = 2, a, f) => {
+  let R = _extends({}, _ref2);
+
+  if (a === void 0) {
+    a = R;
+  }
+
+  if (f === void 0) {
+    f = q;
+  }
+
+  return function () {
+    let q;
+  }();
+};
+
+(_ref3, a) => {
+  let R = _extends({}, _ref3);
+
+  if (a === void 0) {
+    a = f(R);
+  }
+};
diff --git a/packages/babel-plugin-transform-parameters/src/index.js b/packages/babel-plugin-transform-parameters/src/index.js
index 1f5cb7184b04..91816d56e908 100644
--- a/packages/babel-plugin-transform-parameters/src/index.js
+++ b/packages/babel-plugin-transform-parameters/src/index.js
@@ -6,7 +6,9 @@ export { convertFunctionParams };
 export default declare((api, options) => {
   api.assertVersion(7);
 
-  const { loose } = options;
+  const ignoreFunctionLength =
+    api.assumption("ignoreFunctionLength") ?? options.loose;
+
   return {
     name: "transform-parameters",
 
@@ -23,7 +25,10 @@ export default declare((api, options) => {
         }
 
         const convertedRest = convertFunctionRest(path);
-        const convertedParams = convertFunctionParams(path, loose);
+        const convertedParams = convertFunctionParams(
+          path,
+          ignoreFunctionLength,
+        );
 
         if (convertedRest || convertedParams) {
           // Manually reprocess this scope to ensure that the moved params are updated.
diff --git a/packages/babel-plugin-transform-parameters/src/params.js b/packages/babel-plugin-transform-parameters/src/params.js
index 4608a225af45..58629183c35f 100644
--- a/packages/babel-plugin-transform-parameters/src/params.js
+++ b/packages/babel-plugin-transform-parameters/src/params.js
@@ -44,7 +44,7 @@ const iifeVisitor = {
 // last 2 parameters are optional -- they are used by proposal-object-rest-spread/src/index.js
 export default function convertFunctionParams(
   path,
-  loose,
+  ignoreFunctionLength,
   shouldTransformParam,
   replaceRestElement,
 ) {
@@ -123,7 +123,10 @@ export default function convertFunctionParams(
     }
 
     const paramIsAssignmentPattern = param.isAssignmentPattern();
-    if (paramIsAssignmentPattern && (loose || node.kind === "set")) {
+    if (
+      paramIsAssignmentPattern &&
+      (ignoreFunctionLength || node.kind === "set")
+    ) {
       const left = param.get("left");
       const right = param.get("right");
 
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/complex-assignment/input.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/complex-assignment/input.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/complex-assignment/input.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/complex-assignment/input.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/complex-assignment/output.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/complex-assignment/output.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/complex-assignment/output.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/complex-assignment/output.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-array-destructuring/exec.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-array-destructuring/exec.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-array-destructuring/exec.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-array-destructuring/exec.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-array-destructuring/input.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-array-destructuring/input.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-array-destructuring/input.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-array-destructuring/input.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-array-destructuring/output.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-array-destructuring/output.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-array-destructuring/output.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-array-destructuring/output.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-assignment-with-types/input.mjs b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-assignment-with-types/input.mjs
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-assignment-with-types/input.mjs
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-assignment-with-types/input.mjs
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-assignment-with-types/options.json b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-assignment-with-types/options.json
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-assignment-with-types/options.json
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-assignment-with-types/options.json
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-assignment-with-types/output.mjs b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-assignment-with-types/output.mjs
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-assignment-with-types/output.mjs
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-assignment-with-types/output.mjs
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-earlier-params/exec.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-earlier-params/exec.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-earlier-params/exec.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-earlier-params/exec.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-iife-1128/exec.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-iife-1128/exec.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-iife-1128/exec.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-iife-1128/exec.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-iife-4253/exec.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-iife-4253/exec.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-iife-4253/exec.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-iife-4253/exec.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-iife-self/exec.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-iife-self/exec.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-iife-self/exec.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-iife-self/exec.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-multiple/input.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-multiple/input.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-multiple/input.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-multiple/input.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-multiple/output.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-multiple/output.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-multiple/output.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-multiple/output.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-object-destructuring/exec.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-object-destructuring/exec.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-object-destructuring/exec.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-object-destructuring/exec.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-rest-mix/input.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-rest-mix/input.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-rest-mix/input.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-rest-mix/input.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-rest-mix/output.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-rest-mix/output.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-rest-mix/output.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-rest-mix/output.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-rest/exec.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-rest/exec.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-rest/exec.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-rest/exec.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-single/input.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-single/input.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-single/input.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-single/input.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-single/output.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-single/output.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/default-single/output.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/default-single/output.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/destructuring-rest/input.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/destructuring-rest/input.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/destructuring-rest/input.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/destructuring-rest/input.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/destructuring-rest/output.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/destructuring-rest/output.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/destructuring-rest/output.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/destructuring-rest/output.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/options.json b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/options.json
similarity index 62%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/options.json
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/options.json
index aa8f6cf317dd..f8c2c7a9fafe 100644
--- a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/options.json
+++ b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/options.json
@@ -3,12 +3,14 @@
     "proposal-class-properties",
     "external-helpers",
     "syntax-flow",
-    ["transform-parameters", { "loose": true }],
+    "transform-parameters",
     "transform-block-scoping",
     "transform-spread",
     "transform-classes",
     "transform-destructuring",
-    "transform-arrow-functions",
-    "transform-for-of"
-  ]
+    "transform-arrow-functions"
+  ],
+  "assumptions": {
+    "ignoreFunctionLength": true
+  }
 }
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/overwrite-undefined/exec.js b/packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/overwrite-undefined/exec.js
similarity index 100%
rename from packages/babel-plugin-transform-parameters/test/fixtures/use-loose-option/overwrite-undefined/exec.js
rename to packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/overwrite-undefined/exec.js
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/loose/ignoreFunctionLength/input.js b/packages/babel-plugin-transform-parameters/test/fixtures/loose/ignoreFunctionLength/input.js
new file mode 100644
index 000000000000..b91cfb244914
--- /dev/null
+++ b/packages/babel-plugin-transform-parameters/test/fixtures/loose/ignoreFunctionLength/input.js
@@ -0,0 +1,3 @@
+function fn(a, b = c()) {
+  return b;
+}
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/loose/ignoreFunctionLength/output.js b/packages/babel-plugin-transform-parameters/test/fixtures/loose/ignoreFunctionLength/output.js
new file mode 100644
index 000000000000..6c1471d59a4f
--- /dev/null
+++ b/packages/babel-plugin-transform-parameters/test/fixtures/loose/ignoreFunctionLength/output.js
@@ -0,0 +1,7 @@
+function fn(a, b) {
+  if (b === void 0) {
+    b = c();
+  }
+
+  return b;
+}
diff --git a/packages/babel-plugin-transform-parameters/test/fixtures/loose/options.json b/packages/babel-plugin-transform-parameters/test/fixtures/loose/options.json
new file mode 100644
index 000000000000..0d49296cfac0
--- /dev/null
+++ b/packages/babel-plugin-transform-parameters/test/fixtures/loose/options.json
@@ -0,0 +1,5 @@
+{
+  "plugins": [
+    ["transform-parameters", { "loose": true }]
+  ]
+}