fix escape analysis for AST_Yield

fixes #2565

lib/compress.js

@@ -681,7 +681,8 @@ merge(Compressor.prototype, {
             if (parent instanceof AST_Assign && parent.operator == "=" && node === parent.right
                 || parent instanceof AST_Call && node !== parent.expression
                 || parent instanceof AST_Exit && node === parent.value && node.scope !== d.scope
-                || parent instanceof AST_VarDef && node === parent.value) {
+                || parent instanceof AST_VarDef && node === parent.value
+                || parent instanceof AST_Yield && node === parent.value && node.scope !== d.scope) {
                 d.escaped = true;
                 return;
             } else if (parent instanceof AST_Array

test/compress/reduce_vars.js

@@ -5148,3 +5148,42 @@ issue_2560_6: {
     }
     expect_stdout: "PASS"
 }
+
+escape_yield: {
+    options = {
+        reduce_funcs: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        function main() {
+            var thing = gen.next().value;
+            if (thing !== (thing = gen.next().value))
+                console.log("FAIL");
+            else
+                console.log("PASS");
+        }
+        function foo() {}
+        function* baz(s) {
+            for (;;) yield foo;
+        }
+        var gen = baz();
+        main();
+    }
+    expect: {
+        function foo() {}
+        var gen = function*(s) {
+            for (;;) yield foo;
+        }();
+        (function() {
+            var thing = gen.next().value;
+            if (thing !== (thing = gen.next().value))
+                console.log("FAIL");
+            else
+                console.log("PASS");
+        })();
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}