improve handling of declaration statements (#4980)

lib/compress.js

@@ -325,9 +325,12 @@ merge(Compressor.prototype, {
                 } else if (insert === "awaits" && node instanceof AST_Try) {
                     if (node.bfinally) return node;
                 }
-                var index = node.body.length - 1;
+                for (var index = node.body.length; --index >= 0;) {
-                if (index >= 0) {
+                    var stat = node.body[index];
-                    node.body[index] = node.body[index].transform(tt);
+                    if (!is_declaration(stat, true)) {
+                        node.body[index] = stat.transform(tt);
+                        break;
+                    }
                 }
             } else if (node instanceof AST_If) {
                 node.body = node.body.transform(tt);
@@ -1695,8 +1698,16 @@ merge(Compressor.prototype, {
         });
     }
 
-    function is_declaration(stat) {
+    function is_declaration(stat, lexical) {
-        return stat instanceof AST_Defun || stat instanceof AST_Var && declarations_only(stat);
+        if (stat instanceof AST_DefClass) return lexical && !stat.extends && all(stat.properties, function(prop) {
+            if (prop.key instanceof AST_Node) return false;
+            if (prop instanceof AST_ClassField && prop.static && prop.value) return false;
+            return true;
+        });
+        if (stat instanceof AST_Definitions) return (lexical || stat instanceof AST_Var) && declarations_only(stat);
+        if (stat instanceof AST_ExportDeclaration) return is_declaration(stat.body, lexical);
+        if (stat instanceof AST_ExportDefault) return is_declaration(stat.body, lexical);
+        return stat instanceof AST_LambdaDefinition;
     }
 
     function tighten_body(statements, compressor) {
@@ -3157,7 +3168,7 @@ merge(Compressor.prototype, {
                 var index = body.lastIndexOf(stat);
                 if (index < 0) return false;
                 while (++index < body.length) {
-                    if (!is_declaration(body[index])) return false;
+                    if (!is_declaration(body[index], true)) return false;
                 }
                 return true;
             }
@@ -7970,7 +7981,7 @@ merge(Compressor.prototype, {
                         self.condition,
                     ]);
                     body.splice(i, 1);
-                } else if (!is_declaration(stat)) {
+                } else if (!is_declaration(stat, true)) {
                     break;
                 }
             }

test/compress/functions.js

@@ -5236,7 +5236,7 @@ issue_4265: {
     expect: {
         function f() {
             return console, function() {
-                return console.log(a);
+                console.log(a);
                 var a;
             }(), 0;
         }

test/compress/let.js

@@ -569,6 +569,38 @@ loop_block_2: {
     node_version: ">=4"
 }
 
+do_break: {
+    options = {
+        loops: true,
+    }
+    input: {
+        "use strict";
+        try {
+            do {
+                if (a)
+                    break;
+                let a;
+            } while (!console);
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        "use strict";
+        try {
+            do {
+                if (a)
+                    break;
+                let a;
+            } while (!console);
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
 do_continue: {
     options = {
         loops: true,
@@ -629,6 +661,82 @@ dead_block_after_return: {
     node_version: ">=4"
 }
 
+if_return_1: {
+    options = {
+        if_return: true,
+    }
+    input: {
+        "use strict";
+        function f(a) {
+            function g() {
+                return b = "PASS";
+            }
+            if (a)
+                return g();
+            let b;
+            return g();
+        };
+        console.log(f());
+    }
+    expect: {
+        "use strict";
+        function f(a) {
+            function g() {
+                return b = "PASS";
+            }
+            if (a)
+                return g();
+            let b;
+            return g();
+        };
+        console.log(f());
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
+if_return_2: {
+    options = {
+        if_return: true,
+    }
+    input: {
+        "use strict";
+        function f(a) {
+            function g() {
+                return b = "FAIL";
+            }
+            if (a)
+                return g();
+            let b;
+            return g();
+        };
+        try {
+            console.log(f(42));
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        "use strict";
+        function f(a) {
+            function g() {
+                return b = "FAIL";
+            }
+            if (a)
+                return g();
+            let b;
+            return g();
+        };
+        try {
+            console.log(f(42));
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
 do_if_continue_1: {
     options = {
         if_return: true,