fix corner case in collapse_vars (#4217)

fixes #4216

lib/compress.js

@@ -1501,9 +1501,9 @@ merge(Compressor.prototype, {
                 // Skip (non-executed) functions
                 if (node instanceof AST_Scope) return node;
                 // Stop upon collision with block-scoped variables
-                if (node.variables && !node.variables.all(function(def) {
+                if (!(node.variables && node.variables.all(function(def) {
                     return !lvalues.has(def.name);
-                })) {
+                }))) {
                     abort = true;
                     return node;
                 }

lib/output.js

@@ -1385,8 +1385,6 @@ function OutputStream(options) {
             make_block(stat, output);
         } else if (!stat || stat instanceof AST_EmptyStatement) {
             output.force_semicolon();
-        } else if (stat instanceof AST_Const) {
-            make_block(stat, output);
         } else {
             stat.print(output);
         }

test/compress/const.js

@@ -1009,3 +1009,26 @@ issue_4212_2: {
     }
     expect_stdout: true
 }
+
+issue_4216: {
+    options = {
+        collapse_vars: true,
+        conditionals: true,
+        dead_code: true,
+        evaluate: true,
+    }
+    input: {
+        if (a = 0) {
+            const a = 0;
+        }
+        console.log(typeof a);
+    }
+    expect: {
+        a = 0;
+        {
+            const a = void 0;
+        }
+        console.log(typeof a);
+    }
+    expect_stdout: true
+}