fix corner cases with export (#4743)

fixes #4742

lib/compress.js

@@ -3425,6 +3425,7 @@ merge(Compressor.prototype, {
                         if (parent instanceof AST_For && parent.init === node) return null;
                         return in_list ? List.skip : make_node(AST_EmptyStatement, node);
                     }
+                    if (node instanceof AST_ExportDeclaration) return node;
                     if (node instanceof AST_Scope) return node;
                     if (!is_statement(node)) return node;
                 }));
@@ -6167,7 +6168,9 @@ merge(Compressor.prototype, {
                         }
                         var old_def;
                         if (!value && !(node instanceof AST_Let)) {
-                            if (drop_sym && var_defs[sym.id] > 1) {
+                            if (parent instanceof AST_ExportDeclaration) {
+                                flush();
+                            } else if (drop_sym && var_defs[sym.id] > 1) {
                                 AST_Node.info("Dropping declaration of variable {name} [{file}:{line},{col}]", template(def.name));
                                 var_defs[sym.id]--;
                                 sym.eliminated++;

lib/scope.js

@@ -198,9 +198,10 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options) {
         } else if (node instanceof AST_SymbolConst) {
             var def = scope.def_variable(node);
             def.defun = defun;
-            def.exported = exported;
+            if (exported) def.exported = true;
         } else if (node instanceof AST_SymbolDefun) {
-            defun.def_function(node, tw.parent()).exported = exported;
+            var def = defun.def_function(node, tw.parent());
+            if (exported) def.exported = true;
             entangle(defun, scope);
         } else if (node instanceof AST_SymbolFunarg) {
             defun.def_variable(node);
@@ -209,9 +210,11 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options) {
             var def = defun.def_function(node, node.name == "arguments" ? undefined : defun);
             if (options.ie8) def.defun = defun.parent_scope.resolve();
         } else if (node instanceof AST_SymbolLet) {
-            scope.def_variable(node).exported = exported;
+            var def = scope.def_variable(node);
+            if (exported) def.exported = true;
         } else if (node instanceof AST_SymbolVar) {
-            defun.def_variable(node, node instanceof AST_SymbolImport ? undefined : null).exported = exported;
+            var def = defun.def_variable(node, node instanceof AST_SymbolImport ? undefined : null);
+            if (exported) def.exported = true;
             entangle(defun, scope);
         }
 

test/compress/exports.js

@@ -382,3 +382,62 @@ single_use_class_default: {
         A.prototype.p = "PASS";
     }
 }
+
+issue_4742_join_vars_1: {
+    options = {
+        join_vars: true,
+    }
+    input: {
+        var a = 42;
+        export var a;
+    }
+    expect: {
+        var a = 42;
+        export var a;
+    }
+}
+
+issue_4742_join_vars_2: {
+    options = {
+        join_vars: true,
+    }
+    input: {
+        export var a = "foo";
+        var b;
+        b = "bar";
+    }
+    expect: {
+        export var a = "foo";
+        var b, b = "bar";
+    }
+}
+
+issue_4742_unused_1: {
+    options = {
+        unused: true,
+    }
+    input: {
+        var a = 42;
+        export var a;
+    }
+    expect: {
+        var a = 42;
+        export var a;
+    }
+}
+
+issue_4742_unused_2: {
+    options = {
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        export var a = "foo";
+        var a = "bar";
+    }
+    expect: {
+        export var a = "foo";
+        a = "bar";
+    }
+}