Use TreeWalker for more accurate @const results and update tests

lib/scope.js

@@ -358,13 +358,31 @@ AST_Symbol.DEFMETHOD("global", function(){
 });
 
 AST_Symbol.DEFMETHOD("has_const_pragma", function() {
-    var token = this.scope.body[0] && this.scope.body[0].start;
-    var comments = token && token.comments_before;
-    if (comments && comments.length > 0) {
-        var last = comments[comments.length - 1];
-        return /@const/.test(last.value);
-    }
-    return false;
+    var symbol = this;
+    var symbol_has_pragma = false;
+    var pragma_found = false;
+    var found_symbol = false;
+    // Walk the current scope, looking for a comment with the @const pragma.
+    // If it exists, mark a bool that will remain true only for the next iteration.
+    // If the next iteration is this symbol, then we return true.
+    // Otherwise we stop descending and get out of here.
+    var tw = new TreeWalker(function(node, descend){
+        // This is our symbol. Was the pragma before this?
+        if (node.name === symbol) {
+            found_symbol = true;
+            symbol_has_pragma = pragma_found;
+        }
+
+        // Look for the /** @const */ pragma
+        var comments_before = node.start && node.start.comments_before;
+        var lastComment = comments_before && comments_before[comments_before.length - 1];
+        pragma_found = lastComment && /@const/.test(lastComment.value);
+
+        // no need to descend after finding our node
+        return found_symbol;
+    });
+    this.scope.walk(tw);
+    return symbol_has_pragma;
 })
 
 AST_Toplevel.DEFMETHOD("_default_mangler_options", function(options){

test/compress/dead-code.js

@@ -97,6 +97,7 @@ dead_code_const_declaration: {
         evaluate     : true
     };
     input: {
+        var unused;
         const CONST_FOO = false;
         if (CONST_FOO) {
             console.log("unreachable");
@@ -105,6 +106,7 @@ dead_code_const_declaration: {
         }
     }
     expect: {
+        var unused;
         const CONST_FOO = !1;
         var moo;
         function bar() {}
@@ -120,7 +122,8 @@ dead_code_const_annotation: {
         evaluate     : true
     };
     input: {
-        /** @const*/ var CONST_FOO_ANN = false;
+        var unused;
+        /** @const */ var CONST_FOO_ANN = false;
         if (CONST_FOO_ANN) {
             console.log("unreachable");
             var moo;
@@ -128,8 +131,52 @@ dead_code_const_annotation: {
         }
     }
     expect: {
+        var unused;
         var CONST_FOO_ANN = !1;
         var moo;
         function bar() {}
     }
 }
+
+dead_code_const_annotation_complex_scope: {
+    options = {
+        dead_code    : true,
+        loops        : true,
+        booleans     : true,
+        conditionals : true,
+        evaluate     : true
+    };
+    input: {
+        var unused_var;
+        /** @const */ var test = 'test';
+        /** @const */ var CONST_FOO_ANN = false;
+        var unused_var_2;
+        if (CONST_FOO_ANN) {
+            console.log("unreachable");
+            var moo;
+            function bar() {}
+        }
+        if (test === 'test') {
+            var beef = 'good';
+            /** @const */ var meat = 'beef';
+            var pork = 'bad';
+            if (meat === 'pork') {
+                console.log('also unreachable');
+            } else if (pork === 'good') {
+                console.log('reached, not const');
+            }
+        }
+    }
+    expect: {
+        var unused_var;
+        var test = 'test';
+        var CONST_FOO_ANN = !1;
+        var unused_var_2;
+        var moo;
+        function bar() {}
+        var beef = 'good';
+        var meat = 'beef';
+        var pork = 'bad';
+        'good' === pork && console.log('reached, not const');
+    }
+}