Allow sibling matches to use it

Author: rattrayalex

src/index.js

@@ -1009,7 +1009,7 @@ export default function (babel) {
     }, null);
   }
 
-  function prependDeclaration(path, id, init, kind = "const") {
+  function insertDeclarationBefore(path, id, init, kind = "const") {
     path.insertBefore(t.variableDeclaration(kind, [
       t.variableDeclarator(id, init)
     ]));
@@ -1581,19 +1581,23 @@ export default function (babel) {
         const { discriminant } = path.node;
         const alias = path.node.alias || t.identifier("it");
 
-        let argRef;
-        if (t.isIdentifier(discriminant) && !containsAliasReference(path, alias)) {
-          argRef = discriminant;
-        } else {
-          argRef = alias;
-          if (path.scope.hasOwnBinding(argRef.name)) {
-            throw path.buildCodeFrameError("Cannot re-use `it` binding in the same scope. Try a new shorthand name (eg; `match foo as x:`).");
-          }
-          prependDeclaration(path, argRef, discriminant);
-        }
-
+        const argRef = t.isIdentifier(discriminant) && !containsAliasReference(path, alias)
+          ? discriminant
+          : alias;
         const matchBody = transformMatchCases(argRef, path.get("cases"));
+
         path.replaceWith(matchBody);
+
+        // insert eg; `const it = foo()` above the body,
+        // possibly wrapping both in an anonymous block.
+        if (argRef !== discriminant) {
+          if (path.scope.hasOwnBinding(alias.name)) {
+            // wrap in anonymous block
+            path.replaceWith(t.blockStatement([path.node]));
+            path = path.get("body.0");
+          }
+          insertDeclarationBefore(path, argRef, discriminant);
+        }
       },
 
     });

test/fixtures/match/illegal-sibling-no-as/options.json

@@ -0,0 +1,9 @@
+f() ->
+  match foo():
+    | 1:
+      "ok"
+    | 2:
+      bar()
+      match baz():
+        | 3:
+          qux()

test/fixtures/match/implicit-return/actual.js

@@ -0,0 +1,14 @@
+function f() {
+  const it = foo();
+
+  if (it === 1) {
+    return "ok";
+  } else if (it === 2) {
+    bar();
+    const it = baz();
+
+    if (it === 3) {
+      return qux();
+    }
+  }
+}

test/fixtures/match/implicit-return/expected.js

@@ -0,0 +1,6 @@
+f() ->
+  match foo():
+    | 1: "ok"
+
+  match bar():
+    | 2: "oops"

test/fixtures/match/sibling-no-as-implicit-returns/actual.js

@@ -0,0 +1,15 @@
+function f() {
+  const it = foo();
+
+  if (it === 1) {
+      "ok";
+    }
+
+  {
+    const it = bar();
+
+    if (it === 2) {
+        return "oops";
+      }
+  }
+}

test/fixtures/match/sibling-no-as-implicit-returns/expected.js

@@ -0,0 +1,13 @@
+const it = foo();
+
+if (it === 1) {
+    "ok";
+  }
+
+{
+  const it = bar();
+
+  if (it === 2) {
+      "oops";
+    }
+}