diff --git a/lib/ast.js b/lib/ast.js
index d117f320..2042332d 100644
--- a/lib/ast.js
+++ b/lib/ast.js
@@ -1292,12 +1292,15 @@ TreeWalker.prototype = {
             this.directives = Object.create(this.directives);
         } else if (node instanceof AST_Directive) {
             this.directives[node.value] = this.directives[node.value] ? "up" : true;
+        } else if (node instanceof AST_Class) {
+            this.directives = Object.create(this.directives);
+            this.directives["use strict"] = this.directives["use strict"] ? "up" : true;
         }
         this.stack.push(node);
     },
     pop: function(node) {
         this.stack.pop();
-        if (node instanceof AST_Lambda) {
+        if (node instanceof AST_Lambda || node instanceof AST_Class) {
             this.directives = Object.getPrototypeOf(this.directives);
         }
     },
diff --git a/lib/parse.js b/lib/parse.js
index 5093812f..7c9b1cd5 100644
--- a/lib/parse.js
+++ b/lib/parse.js
@@ -2039,6 +2039,9 @@ function parse($TEXT, options) {
     function class_(KindOfClass) {
         var start, method, class_name, extends_, a = [];
 
+        S.input.push_directives_stack(); // Push directive stack, but not scope stack
+        S.input.add_directive("use strict");
+
         if (S.token.type == "name" && S.token.value != "extends") {
             class_name = as_symbol(KindOfClass === AST_DefClass ? AST_SymbolDefClass : AST_SymbolClass);
         }
@@ -2063,6 +2066,8 @@ function parse($TEXT, options) {
             if (is("punc", ";")) { next(); }
         }
 
+        S.input.pop_directives_stack();
+
         next();
 
         return new KindOfClass({
diff --git a/test/compress/directives.js b/test/compress/directives.js
new file mode 100644
index 00000000..51587b95
--- /dev/null
+++ b/test/compress/directives.js
@@ -0,0 +1,10 @@
+class_directives_compression: {
+    input: {
+        class foo {
+            foo() {
+                "use strict";
+            }
+        }
+    }
+    expect_exact: "class foo{foo(){}}"
+}
diff --git a/test/compress/yield.js b/test/compress/yield.js
index c29ce926..4f28b647 100644
--- a/test/compress/yield.js
+++ b/test/compress/yield.js
@@ -120,7 +120,6 @@ yield_as_identifier_outside_strict_mode: {
         function foo(...yield){}
         try { new Error("") } catch (yield) {}
         var yield = "foo";
-        class yield {}
     }
     expect: {
         import yield from "bar";
@@ -137,7 +136,6 @@ yield_as_identifier_outside_strict_mode: {
         function foo(...yield){}
         try { new Error("") } catch (yield) {}
         var yield = "foo";
-        class yield {}
     }
 }
 
diff --git a/test/mocha/directives.js b/test/mocha/directives.js
index 82594758..f53ebc1a 100644
--- a/test/mocha/directives.js
+++ b/test/mocha/directives.js
@@ -62,10 +62,10 @@ describe("Directives", function() {
     it("Should know which strings are directive and which ones are not", function() {
         var test_directive = function(tokenizer, test) {
             test.directives.map(function(directive) {
-                assert.strictEqual(tokenizer.has_directive(directive), true, directive + " in " + test.input);
+                assert.strictEqual(tokenizer.has_directive(directive), true, "Didn't found directive `" + directive + "` at the end of `" + test.input + '`');
             });
             test.non_directives.map(function(fake_directive) {
-                assert.strictEqual(tokenizer.has_directive(fake_directive), false, fake_directive + " in " + test.input);
+                assert.strictEqual(tokenizer.has_directive(fake_directive), false, "Unexpectedly found directive `" + fake_directive + "` at the end of `" + test.input + '`');
             });
         }
 
@@ -156,6 +156,16 @@ describe("Directives", function() {
                 input: '"use strict";try{"use asm";',
                 directives: ["use strict"],
                 non_directives: ["use\nstrict", "use \nstrict", "use asm"]
+            },
+            {
+                input: 'class foo {',
+                directives: ["use strict"],
+                non_directives: ["use\nstrict", "use asm"]
+            },
+            {
+                input: 'class foo {}',
+                directives: [],
+                non_directives: ["use strict", "use asm", "use\nstrict"]
             }
         ];
 
@@ -367,4 +377,44 @@ describe("Directives", function() {
             );
         }
     });
+    it("Should be detect implicit usages of strict mode from tree walker", function() {
+        var tests = [
+            {
+                input: 'class foo {bar(){_check_}}',
+                directives: ["use strict"],
+                non_directives: ["use bar"]
+            },
+            {
+                input: 'class foo {bar(){}}_check_',
+                directives: [],
+                non_directives: ["use strict", "use bar"]
+            }
+        ];
+
+        var i = 0;
+        var checked;
+        var checkWalker = new uglify.TreeWalker(function(node, descend) {
+            if (node instanceof uglify.AST_Symbol && node.name === "_check_") {
+                checked = true;
+                for (var j = 0; j < tests[i].directives.length; j++) {
+                    assert.equal(checkWalker.has_directive(tests[i].directives[j]), true,
+                        "Did not found directive '" + tests[i].directives[j] +  "' in test " + tests[i].input)
+                }
+                for (var k = 0; k < tests[i].non_directives.length; k++) {
+                    assert.equal(checkWalker.has_directive(tests[i].non_directives[k]), undefined,
+                        "Found directive '" + tests[i].non_directives[k] +  "' in test " + tests[i].input)
+                }
+            }
+        });
+
+        for (; i < tests.length; i++) {
+            // Do tests - iterate the ast in each test - check only when _check_ occurs - fail when no _check_ has been found
+            checked = false;
+            var ast = uglify.parse(tests[i].input);
+            ast.walk(checkWalker);
+            if (!checked) {
+                throw "No _check_ symbol found in " + tests[i].input;
+            }
+        }
+    });
 });