fix corner cases in braces & sequences (#4221)

fixes #4220

lib/compress.js

@@ -2513,7 +2513,7 @@ merge(Compressor.prototype, {
                 var line = block.body[i];
                 if (line instanceof AST_Var && declarations_only(line)) {
                     decls.push(line);
-                } else if (stat) {
+                } else if (stat || line instanceof AST_Const) {
                     return false;
                 } else {
                     stat = line;

lib/output.js

@@ -990,7 +990,7 @@ function OutputStream(options) {
     /* -----[ if ]----- */
     function make_then(self, output) {
         var b = self.body;
-        if (output.option("braces")
+        if (output.option("braces") && !(b instanceof AST_Const)
             || output.option("ie8") && b instanceof AST_Do)
             return make_block(b, output);
         // The squeezer replaces "block"-s that contain only a single
@@ -1381,7 +1381,7 @@ function OutputStream(options) {
     });
 
     function force_statement(stat, output) {
-        if (output.option("braces")) {
+        if (output.option("braces") && !(stat instanceof AST_Const)) {
             make_block(stat, output);
         } else if (!stat || stat instanceof AST_EmptyStatement) {
             output.force_semicolon();

test/compress/const.js

@@ -1056,3 +1056,51 @@ issue_4216: {
     }
     expect_stdout: true
 }
+
+skip_braces: {
+    beautify = {
+        beautify: true,
+        braces: true,
+    }
+    input: {
+        if (console)
+            const a = 42;
+        else
+            const b = null;
+        console.log(typeof a, typeof b);
+    }
+    expect_exact: [
+        "if (console) const a = 42; else const b = null;",
+        "",
+        "console.log(typeof a, typeof b);",
+    ]
+    expect_stdout: true
+}
+
+issue_4220: {
+    options = {
+        collapse_vars: true,
+        conditionals: true,
+        sequences: true,
+        toplevel: true,
+    }
+    input: {
+        if (console) {
+            var o = console;
+            for (var k in o);
+        } else {
+            const a = 0;
+        }
+        console.log(typeof a);
+    }
+    expect: {
+        if (console) {
+            var o;
+            for (var k in console);
+        } else {
+            const a = 0;
+        }
+        console.log(typeof a);
+    }
+    expect_stdout: true
+}