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.hasBinding(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 +1,16 @@
+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

@@ -3,16 +3,20 @@ function _hasProps(obj) { if (obj == null) return false; if (typeof obj !== "obj
 const it = foo();
 
 if (it === 1) {
-  const it = it;
+  {
+    const it = it;
 
-  if (it === 2) {
-      it;
-    }
+    if (it === 2) {
+        it;
+      }
+  }
 } else if (_hasProps(it, "x")) {
   const { x } = it;
-  const it = x;
+  {
+    const it = x;
 
-  if (it === 2) {
-      it;
-    }
+    if (it === 2) {
+        it;
+      }
+  }
 }

test/fixtures/match/nested-it/expected.js

@@ -0,0 +1,6 @@
+it = "good"
+f() ->
+  match "bad":
+    | true: true
+
+  it

test/fixtures/match/shadow-it/actual.js

@@ -0,0 +1,8 @@
+it = "good"
+f() ->
+  match "bad":
+    | true: true
+
+  it
+
+assert.equal("good", f())

test/fixtures/match/shadow-it/exec.js

@@ -0,0 +1,12 @@
+const it = "good";
+
+function f() {
+  {
+    const it = "bad";
+
+    if (it === true) {
+        true;
+      }
+  }
+  return it;
+}

test/fixtures/match/shadow-it/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";
+      }
+  }
+}