fix corner case in collapse_vars & reduce_vars (#4748)

fixes #4747

lib/compress.js

@@ -5188,6 +5188,12 @@ merge(Compressor.prototype, {
         fn.rest = null;
     }
 
+    OPT(AST_Lambda, function(self, compressor) {
+        drop_rest_farg(self, compressor);
+        self.body = tighten_body(self.body, compressor);
+        return self;
+    });
+
     function opt_arrow(self, compressor) {
         if (!compressor.option("arrows")) return self;
         drop_rest_farg(self, compressor);
@@ -5210,12 +5216,6 @@ merge(Compressor.prototype, {
     OPT(AST_Arrow, opt_arrow);
     OPT(AST_AsyncArrow, opt_arrow);
 
-    OPT(AST_Defun, function(self, compressor) {
-        drop_rest_farg(self, compressor);
-        self.body = tighten_body(self.body, compressor);
-        return self;
-    });
-
     OPT(AST_Function, function(self, compressor) {
         drop_rest_farg(self, compressor);
         self.body = tighten_body(self.body, compressor);
@@ -10389,7 +10389,13 @@ merge(Compressor.prototype, {
                         }
                     }));
                 } else {
-                    value = fixed.optimize(compressor);
+                    if (fixed instanceof AST_Scope) {
+                        compressor.push(fixed);
+                        value = fixed.optimize(compressor);
+                        compressor.pop();
+                    } else {
+                        value = fixed.optimize(compressor);
+                    }
                     if (value === fixed) value = value.transform(new TreeTransformer(function(node, descend) {
                         if (node instanceof AST_Scope) return node;
                         node = node.clone();

test/compress/awaits.js

@@ -1379,3 +1379,32 @@ issue_4738_3: {
     expect_stdout: "PASS"
     node_version: ">=8"
 }
+
+issue_4747: {
+    options = {
+        collapse_vars: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        console.log(function(a) {
+            async function f() {
+                a = "PASS";
+                null.p += "PASS";
+            }
+            f();
+            return a;
+        }("FAIL"));
+    }
+    expect: {
+        console.log(function(a) {
+            (async function() {
+                a = "PASS";
+                null.p += "PASS";
+            })();
+            return a;
+        }("FAIL"));
+    }
+    expect_stdout: "PASS"
+    node_version: ">=8"
+}