fix corner case in join_vars & loops (#5757)

fixes #5756

lib/compress.js

@@ -4276,11 +4276,11 @@ Compressor.prototype.compress = function(node) {
                         prev.definitions = prev.definitions.concat(stat.definitions);
                         changed = true;
                     } else if (stat && prev instanceof AST_Let && stat.can_letify(compressor)) {
-                        prev.definitions = prev.definitions.concat(to_let(stat).definitions);
+                        prev.definitions = prev.definitions.concat(to_let(stat, block_scope).definitions);
                         changed = true;
                     } else if (prev && stat instanceof AST_Let && prev.can_letify(compressor)) {
                         defs = prev_defs;
-                        statements[j] = prev = to_let(prev);
+                        statements[j] = prev = to_let(prev, block_scope);
                         prev.definitions = prev.definitions.concat(stat.definitions);
                         changed = true;
                     } else if (defs && defs.TYPE == stat.TYPE && declarations_only(stat)) {
@@ -9394,7 +9394,7 @@ Compressor.prototype.compress = function(node) {
                 }, true)) {
                 self.init = to_var(self.init, self.resolve());
             } else if (self.init.can_letify(compressor, true)) {
-                self.init = to_let(self.init);
+                self.init = to_let(self.init, self);
             }
         }
         return self;
@@ -10208,13 +10208,18 @@ Compressor.prototype.compress = function(node) {
         }
     }
 
-    function to_let(stat) {
+    function to_let(stat, scope) {
         return make_node(AST_Let, stat, {
             definitions: stat.definitions.map(function(defn) {
                 return make_node(AST_VarDef, defn, {
                     name: defn.name.convert_symbol(AST_SymbolLet, function(name, node) {
                         var def = name.definition();
                         def.orig[def.orig.indexOf(node)] = name;
+                        for (var s = scope; s !== def.scope && (s = s.parent_scope);) {
+                            remove(s.enclosed, def);
+                        }
+                        def.scope = scope;
+                        scope.variables.set(def.name, def);
                     }),
                     value: defn.value,
                 });
@@ -10297,7 +10302,7 @@ Compressor.prototype.compress = function(node) {
     OPT(AST_Const, function(self, compressor) {
         if (!compressor.option("varify")) return self;
         if (self.can_varify(compressor)) return to_var(self, compressor.find_parent(AST_Scope));
-        if (self.can_letify(compressor)) return to_let(self);
+        if (self.can_letify(compressor)) return to_let(self, find_scope(compressor));
         return self;
     });
 

test/compress/let.js

@@ -2391,3 +2391,105 @@ issue_5745_2: {
     expect_stdout: "PASS"
     node_version: ">=4"
 }
+
+issue_5756_1: {
+    options = {
+        join_vars: true,
+        loops: true,
+        reduce_vars: true,
+        toplevel: true,
+    }
+    input: {
+        "use strict";
+        do {
+            function f() {
+                return b;
+            }
+            var a = "PASS".toString();
+            let b;
+            console.log(a);
+        } while (!console);
+    }
+    expect: {
+        "use strict";
+        do {
+            function f() {
+                return b;
+            }
+            let a = "PASS".toString(), b;
+            console.log(a);
+        } while (!console);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
+issue_5756_2: {
+    options = {
+        join_vars: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        "use strict";
+        function f() {
+            let a = console.log("PASS");
+            {
+                var b;
+                for (var c in b) {
+                    b;
+                    var c = function() {
+                        a;
+                    };
+                }
+            }
+        }
+        f();
+    }
+    expect: {
+        "use strict";
+        (function() {
+            let a = console.log("PASS"), b;
+            for (c in b) {
+                b;
+                var c = function() {
+                    a;
+                };
+            }
+        })();
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
+issue_5756_3: {
+    options = {
+        module: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+        varify: true,
+    }
+    input: {
+        "use strict";
+        console.log(f()());
+        function f() {
+            const a = "PASS";
+            return function() {
+                return a;
+            };
+        }
+    }
+    expect: {
+        "use strict";
+        console.log(function() {
+            let a = "PASS";
+            return function() {
+                return a;
+            };
+        }()());
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}