Add trailing lookahead restriction to throw expressions

.gitattributes

@@ -0,0 +1,3 @@
+index.html  -diff merge=ours
+spec.js     -diff merge=ours
+spec.css    -diff merge=ours

.github/workflows/build.yml

@@ -0,0 +1,18 @@
+name: Publish Spec to gh-pages
+on:
+  push:
+    branches: [ main ]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - run: npm install --legacy-peer-deps
+      - run: npm run compile
+      - name: Deploy
+        uses: JamesIves/github-pages-deploy-action@4.1.4
+        with:
+          branch: gh-pages
+          folder: docs
+          clean-exclude: |
+            pr

.github/workflows/pr.yml

@@ -0,0 +1,29 @@
+name: Publish PR to gh-pages/pr/
+on:
+  pull_request:
+    branches: [ main ]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    if: ${{ github.event.number }}
+    steps:
+      - uses: actions/checkout@v2
+      - run: npm install --legacy-peer-deps
+      - run: npm run compile
+      - name: Deploy
+        uses: JamesIves/github-pages-deploy-action@4.1.4
+        with:
+          branch: gh-pages
+          folder: docs
+          target-folder: pr/${{ github.event.number }}/
+      - id: get-preview-url
+        name: Get preview url
+        run: echo "::set-output name=preview-url::https://tc39.es/$(basename $GITHUB_REPOSITORY)/pr/${{ github.event.number }}"
+        shell: bash
+      - name: Post Preview Comment
+        uses: phulsechinmay/rewritable-pr-comment@v0.3.0
+        with:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          COMMENT_IDENTIFIER: tc39_pr_preview_comment
+          message: |
+            A preview of this PR can be found at ${{ steps.get-preview-url.outputs.preview-url }}.

.gitignore

@@ -1,2 +1,4 @@
 node_modules
-~$*.pptx
+coverage
+docs
+~$*.pptx

.vscode/extensions.json

@@ -1,6 +1,6 @@
 {
 	"recommendations": [
-		"rbuckton.grammarkdown-language",
+		"rbuckton.grammarkdown-vscode",
 		"rbuckton.ecmarkup-vscode"
 	]
 }

.vscode/settings.json

@@ -2,7 +2,16 @@
     "[markdown]": {
         "files.trimTrailingWhitespace": false
     },
+    "[html]": {
+        "editor.insertSpaces": true,
+        "editor.tabSize": 2,
+    },
+    "[ecmarkup]": {
+        "editor.insertSpaces": true,
+        "editor.tabSize": 2,
+    },
     "files.associations": {
-        "*.html": "ecmarkup"
+        // "*.html": "ecmarkup",
+        "*.emu": "ecmarkup"
     }
 }

README.md

@@ -49,14 +49,52 @@ A `throw` expression *does not* replace a `throw` statement due to the differenc
 in the precedence of their values. To maintain the precedence of the `throw` statement,
 we must add a lookahead restriction to `ExpressionStatement` to avoid ambiguity.
 
+Due to the difference in precedence between a `throw` expression and a _ThrowStatement_, certain operators to the right
+of the expression would parse differently between the two which could cause ambiguity and confusion:
+
+```js
+throw a ? b : c; // evaluates 'a' and throws either 'b' or 'c'
+(throw a ? b : c); // without restriction would throw 'a', so `?` is forbidden
+
+throw a, b; // evaluates 'a', throws 'b'
+(throw a, b); // would throw 'a', not 'b', so `,` is forbidden
+
+throw a && b; // throws 'a' if 'a' is falsy, otherwise throws 'b'
+(throw a && b); // would always throw 'a', so `&&` is forbidden
+
+throw a || b; // throws 'a' if 'a' is truthy, otherwise throws 'b'
+(throw a || b); // would always throw 'a', so `||` is forbidden
+
+// ... etc.
+```
+
+As a result, all binary operators and the `?` operator are forbidden to the right of a `throw` expression. To use these
+operators inside of a `throw` expression, the expression must be surrounded with parentheses:
+
+```js
+(throw (a, b)); // evaluates 'a', throws 'b'
+(throw (a ? b : c)); // evaluates 'a' and throws either 'b' or 'c'
+```
+
+However, we do not forbid `:` so that a `throw` expression can still be easily used in a ternary:
+
+```js
+const x = a ? throw b : c; // if 'a' then throw 'b', else evaluate 'c'
+```
+
 # Grammar
 
-```grammarkdown
-UnaryExpression[Yield, Await]:
-  `throw` UnaryExpression[?Yield, ?Await]
+```diff grammarkdown
+++ThrowExpressionInvalidPunctuator : one of
+  `,` `<` `>` `<=` `>=` `==` `!=` `===` `!==` `+` `-` `*` `/` `%` `**` `<<` `>>` `>>>` `&` `|` `^` `&&` `||` `??`
+  `=` `+=` `-=` `*=` `%=` `**=` `<<=` `>>=` `>>>=` `&=` `|=` `^=` `&&=` `||=` `??=` `?`
+
+  UnaryExpression[Yield, Await] :
+++  `throw` UnaryExpression[?Yield, ?Await] [lookahead ∉ ThrowExpressionInvalidPunctuator]
 
-ExpressionStatement[Yield, Await]:
-  [lookahead ∉ {`{`, `function`, `async` [no |LineTerminator| here] `function`, `class`, `let [`, `throw`}] Expression[+In, ?Yield, ?Await] `;`
+  ExpressionStatement[Yield, Await] :
+--  [lookahead ∉ {`{`, `function`, `async` [no |LineTerminator| here] `function`, `class`, `let [`}] Expression[+In, ?Yield, ?Await] `;`
+++  [lookahead ∉ {`{`, `function`, `async` [no |LineTerminator| here] `function`, `class`, `let [`, `throw`}] Expression[+In, ?Yield, ?Await] `;`
 ```
 
 # Other Notes