fix corner case in conditionals (#4599)

fixes #4598

Author: alexlamsl

lib/compress.js

@@ -4913,10 +4913,10 @@ merge(Compressor.prototype, {
         return self;
     });
 
-    function trim_block(node) {
+    function trim_block(node, in_list) {
         switch (node.body.length) {
           case 0:
-            return make_node(AST_EmptyStatement, node);
+            return in_list ? List.skip : make_node(AST_EmptyStatement, node);
           case 1:
             var stat = node.body[0];
             if (!(stat instanceof AST_Const || stat instanceof AST_Let)) return stat;
@@ -5983,12 +5983,8 @@ merge(Compressor.prototype, {
                 return node;
             }
         }, function(node, in_list) {
-            if (node instanceof AST_BlockStatement) switch (node.body.length) {
-              case 0:
-                return in_list ? List.skip : make_node(AST_EmptyStatement, node);
-              case 1:
-                var stat = node.body[0];
-                if (!(stat instanceof AST_Const || stat instanceof AST_Let)) return stat;
+            if (node instanceof AST_BlockStatement) {
+                return trim_block(node, in_list);
             } else if (node instanceof AST_For) {
                 // Certain combination of unused name + side effect leads to invalid AST:
                 //    https://github.com/mishoo/UglifyJS/issues/44
@@ -7515,7 +7511,7 @@ merge(Compressor.prototype, {
                 var exprs = [];
                 for (var i = 0; i < stat.body.length; i++) {
                     var line = stat.body[i];
-                    if (line instanceof AST_Defun) {
+                    if (is_defun(line)) {
                         defuns.push(line);
                     } else if (line instanceof AST_EmptyStatement) {
                         continue;
@@ -7532,7 +7528,7 @@ merge(Compressor.prototype, {
                 }
                 return exprs;
             }
-            if (stat instanceof AST_Defun) {
+            if (is_defun(stat)) {
                 defuns.push(stat);
                 return [];
             }

test/compress/awaits.js

@@ -1047,3 +1047,20 @@ issue_4595: {
     expect_stdout: "0"
     node_version: ">=8"
 }
+
+issue_4598: {
+    options = {
+        conditionals: true,
+    }
+    input: {
+        if (console.log("PASS")) {
+            async function f() {}
+        }
+    }
+    expect: {
+        async function f() {}
+        console.log("PASS");
+    }
+    expect_stdout: "PASS"
+    node_version: ">=8"
+}