From db8922b7afda06a85517d4cb57febb852e551956 Mon Sep 17 00:00:00 2001
From: Justin Ridgewell <justin@ridgewell.name>
Date: Thu, 12 Mar 2020 03:08:15 -0400
Subject: [PATCH 1/2] Add stricter Optional Chain node validation

Optional chains cannot start with `optional: false`. Somewhere in the `object`/`callee` tree, there must be an optional with `optional: true`.
---
 .../src/definitions/experimental.js           |  9 +++++--
 packages/babel-types/src/definitions/utils.js | 27 +++++++++++++++++++
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/packages/babel-types/src/definitions/experimental.js b/packages/babel-types/src/definitions/experimental.js
index a77350105a5b..22f302a9eb5e 100644
--- a/packages/babel-types/src/definitions/experimental.js
+++ b/packages/babel-types/src/definitions/experimental.js
@@ -1,5 +1,6 @@
 // @flow
 import defineType, {
+  assertOptionalChainStart,
   assertEach,
   assertNodeType,
   assertValueType,
@@ -104,7 +105,9 @@ defineType("OptionalMemberExpression", {
       default: false,
     },
     optional: {
-      validate: assertValueType("boolean"),
+      validate: !process.env.BABEL_TYPES_8_BREAKING
+        ? assertValueType("boolean")
+        : chain(assertValueType("boolean"), assertOptionalChainStart()),
     },
   },
 });
@@ -150,7 +153,9 @@ defineType("OptionalCallExpression", {
       ),
     },
     optional: {
-      validate: assertValueType("boolean"),
+      validate: !process.env.BABEL_TYPES_8_BREAKING
+        ? assertValueType("boolean")
+        : chain(assertValueType("boolean"), assertOptionalChainStart()),
     },
     typeArguments: {
       validate: assertNodeType("TypeParameterInstantiation"),
diff --git a/packages/babel-types/src/definitions/utils.js b/packages/babel-types/src/definitions/utils.js
index cc3a347cca4a..76e8456fe8aa 100644
--- a/packages/babel-types/src/definitions/utils.js
+++ b/packages/babel-types/src/definitions/utils.js
@@ -186,6 +186,33 @@ export function assertShape(shape: { [string]: FieldOptions }): Validator {
   return validate;
 }
 
+export function assertOptionalChainStart(): Validator {
+  function validate(node) {
+    for (let current = node; current; ) {
+      const { type } = current;
+      if (type === "OptionalCallExpression") {
+        if (current.optional) return;
+        current = current.callee;
+        continue;
+      }
+
+      if (type === "OptionalMemberExpression") {
+        if (current.optional) return;
+        current = current.object;
+        continue;
+      }
+
+      break;
+    }
+
+    throw new TypeError(
+      `Non-optional ${node.type} must chain from an optional OptionalMemberExpression or OptionalCallExpression`,
+    );
+  }
+
+  return validate;
+}
+
 export function chain(...fns: Array<Validator>): Validator {
   function validate(...args) {
     for (const fn of fns) {

From b1c4c1ec8afe802f5142fc5c2d981b6ad5844613 Mon Sep 17 00:00:00 2001
From: Justin Ridgewell <justin@ridgewell.name>
Date: Fri, 13 Mar 2020 01:59:57 -0400
Subject: [PATCH 2/2] Print current node's type when finding an error.

---
 packages/babel-types/src/definitions/utils.js | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/packages/babel-types/src/definitions/utils.js b/packages/babel-types/src/definitions/utils.js
index 76e8456fe8aa..6fc112be19a2 100644
--- a/packages/babel-types/src/definitions/utils.js
+++ b/packages/babel-types/src/definitions/utils.js
@@ -188,7 +188,8 @@ export function assertShape(shape: { [string]: FieldOptions }): Validator {
 
 export function assertOptionalChainStart(): Validator {
   function validate(node) {
-    for (let current = node; current; ) {
+    let current = node;
+    while (node) {
       const { type } = current;
       if (type === "OptionalCallExpression") {
         if (current.optional) return;
@@ -206,7 +207,7 @@ export function assertOptionalChainStart(): Validator {
     }
 
     throw new TypeError(
-      `Non-optional ${node.type} must chain from an optional OptionalMemberExpression or OptionalCallExpression`,
+      `Non-optional ${node.type} must chain from an optional OptionalMemberExpression or OptionalCallExpression. Found chain from ${current?.type}`,
     );
   }