fix corner case in collapse_vars (#4605)

fixes #4604

lib/compress.js

@@ -1942,6 +1942,7 @@ merge(Compressor.prototype, {
                     var def = node.definition();
                     return (in_try || def.scope.resolve() !== scope) && !can_drop_symbol(node);
                 }
+                if (node instanceof AST_Template) return node.tag && !is_raw_tag(compressor, node.tag);
                 if (node instanceof AST_This) return symbol_in_lvalues(node, parent);
                 if (node instanceof AST_VarDef) {
                     if (check_destructured(node.name)) return true;

test/compress/templates.js

@@ -198,3 +198,27 @@ unsafe_side_effects: {
     expect_stdout: "foo"
     node_version: ">=4"
 }
+
+issue_4604: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = 0, log = console.log;
+        a = "FAIL";
+        (function() {
+            a = "PASS";
+        })``;
+        log(a);
+    }
+    expect: {
+        var a = 0, log = console.log;
+        a = "FAIL";
+        (function() {
+            a = "PASS";
+        })``;
+        log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}