enhance `side_effects` & `unused` (#4711)

3 files changed, 96 insertions, 15 deletions

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 2cc18d88..60db779c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,7 +7,7 @@ jobs:
   test:
     strategy:
       matrix:
-        node: [ '0.8', '0.10', '0.12', '4', '6', '8', '10', '12', latest ]
+        node: [ '0.8', '0.10', '0.12', '4', '6', '8', '10', '12', '14', latest ]
         os: [ ubuntu-latest, windows-latest ]
         script: [ compress, mocha, release/benchmark, release/jetstream ]
         exclude:
diff --git a/lib/compress.js b/lib/compress.js
index fb6d16f9..6b9d7e7c 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -420,12 +420,14 @@ merge(Compressor.prototype, {
         } while (sym = sym.parent_scope);
     }
 
-    function can_drop_symbol(ref, keep_lambda) {
+    function can_drop_symbol(ref, compressor, keep_lambda) {
         var def = ref.definition();
         if (ref.in_arg && is_funarg(def)) return false;
         return all(def.orig, function(sym) {
-            return !(sym instanceof AST_SymbolConst || sym instanceof AST_SymbolLet
-                || keep_lambda && sym instanceof AST_SymbolLambda);
+            if (sym instanceof AST_SymbolConst || sym instanceof AST_SymbolLet) {
+                return compressor && can_varify(compressor, sym);
+            }
+            return !(keep_lambda && sym instanceof AST_SymbolLambda);
         });
     }
 
@@ -599,6 +601,7 @@ merge(Compressor.prototype, {
         function is_immutable(value) {
             if (!value) return false;
             return value.is_constant()
+                || value instanceof AST_Class
                 || value instanceof AST_Lambda
                 || value instanceof AST_ObjectIdentity;
         }
@@ -3585,6 +3588,7 @@ merge(Compressor.prototype, {
                 return false;
             }
         });
+        def(AST_Class, return_false);
         def(AST_Conditional, function(compressor) {
             return this.consequent._dot_throw(compressor) || this.alternative._dot_throw(compressor);
         });
@@ -3593,7 +3597,7 @@ merge(Compressor.prototype, {
             if (!is_strict(compressor)) return false;
             var exp = this.expression;
             if (exp instanceof AST_SymbolRef) exp = exp.fixed_value();
-            return !(exp instanceof AST_Lambda && this.property == "prototype");
+            return !(this.property == "prototype" && (exp instanceof AST_Class || exp instanceof AST_Lambda));
         });
         def(AST_Lambda, return_false);
         def(AST_Null, return_true);
@@ -4869,7 +4873,7 @@ merge(Compressor.prototype, {
         });
         def(AST_SymbolDeclaration, return_false);
         def(AST_SymbolRef, function(compressor) {
-            return !this.is_declared(compressor) || !can_drop_symbol(this);
+            return !this.is_declared(compressor) || !can_drop_symbol(this, compressor);
         });
         def(AST_Template, function(compressor) {
             return this.tag && !is_raw_tag(compressor, this.tag) || any(this.expressions, compressor);
@@ -5698,7 +5702,7 @@ merge(Compressor.prototype, {
             var def = sym.definition();
             if (export_defaults[def.id]) return;
             if (compressor.exposed(def)) return;
-            if (!can_drop_symbol(sym, nested)) return;
+            if (!can_drop_symbol(sym, compressor, nested)) return;
             return sym;
         };
         var assign_in_use = Object.create(null);
@@ -7418,7 +7422,7 @@ merge(Compressor.prototype, {
             return make_sequence(this, [ expression, property ]);
         });
         def(AST_SymbolRef, function(compressor) {
-            return this.is_declared(compressor) && can_drop_symbol(this) ? null : this;
+            return this.is_declared(compressor) && can_drop_symbol(this, compressor) ? null : this;
         });
         def(AST_Template, function(compressor, first_in_statement) {
             if (this.tag && !is_raw_tag(compressor, this.tag)) return this;
@@ -7432,7 +7436,9 @@ merge(Compressor.prototype, {
                 this.write_only = !exp.has_side_effects(compressor);
                 return this;
             }
-            if (this.operator == "typeof" && exp instanceof AST_SymbolRef && can_drop_symbol(exp)) return null;
+            if (this.operator == "typeof" && exp instanceof AST_SymbolRef && can_drop_symbol(exp, compressor)) {
+                return null;
+            }
             var node = exp.drop_side_effect_free(compressor, first_in_statement);
             if (first_in_statement && node && is_iife_call(node)) {
                 if (node === exp && this.operator == "!") return this;
@@ -8237,13 +8243,16 @@ merge(Compressor.prototype, {
         });
     }
 
+    function can_varify(compressor, sym) {
+        if (!sym.fixed_value()) return false;
+        var def = sym.definition();
+        return is_safe_lexical(def) && !may_overlap(compressor, def);
+    }
+
     function varify(self, compressor) {
         return compressor.option("varify") && all(self.definitions, function(defn) {
             return !defn.name.match_symbol(function(node) {
-                if (node instanceof AST_SymbolDeclaration) {
-                    var def = node.definition();
-                    return !node.fixed_value() || !is_safe_lexical(def) || may_overlap(compressor, def);
-                }
+                if (node instanceof AST_SymbolDeclaration) return !can_varify(compressor, node);
             }, true);
         }) ? to_var(self) : self;
     }
@@ -9370,7 +9379,7 @@ merge(Compressor.prototype, {
                 // always true in booleans
                 AST_Node.warn("Boolean expression always true [{file}:{line},{col}]", self.start);
                 var exprs = [ make_node(AST_True, self) ];
-                if (!(exp instanceof AST_SymbolRef && can_drop_symbol(exp))) exprs.unshift(exp);
+                if (!(exp instanceof AST_SymbolRef && can_drop_symbol(exp, compressor))) exprs.unshift(exp);
                 return make_sequence(self, exprs).optimize(compressor);
             }
         }
diff --git a/test/compress/classes.js b/test/compress/classes.js
index dea0d5b0..f94731fb 100644
--- a/test/compress/classes.js
+++ b/test/compress/classes.js
@@ -125,7 +125,7 @@ private_methods: {
     }
     expect_exact: "(new class A{static*#f(){yield 3*A.#p}async #g(){for(var a of A.#f())return a*await 2}static get #p(){return 7}get q(){return this.#g()}}).q.then(console.log);"
     expect_stdout: "42"
-    node_version: ">=14"
+    node_version: ">=14.6"
 }
 
 await: {
@@ -573,6 +573,78 @@ computed_key_generator: {
     node_version: ">=4"
 }
 
+issue_805_1: {
+    options = {
+        inline: true,
+        passes: 2,
+        pure_getters: "strict",
+        reduce_vars: true,
+        sequences: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        "use strict";
+        (function(a) {
+            var unused = class {};
+            unused.prototype[a()] = 42;
+            (unused.prototype.bar = function() {
+                console.log("bar");
+            })();
+            return unused;
+        })(function() {
+            console.log("foo");
+            return "foo";
+        });
+    }
+    expect: {
+        "use strict";
+        console.log("foo"),
+        console.log("bar");
+    }
+    expect_stdout: [
+        "foo",
+        "bar",
+    ]
+    node_version: ">=4"
+}
+
+issue_805_2: {
+    options = {
+        inline: true,
+        passes: 2,
+        pure_getters: "strict",
+        reduce_vars: true,
+        sequences: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        "use strict";
+        (function(a) {
+            class unused {}
+            unused.prototype[a()] = 42;
+            (unused.prototype.bar = function() {
+                console.log("bar");
+            })();
+            return unused;
+        })(function() {
+            console.log("foo");
+            return "foo";
+        });
+    }
+    expect: {
+        "use strict";
+        console.log("foo"),
+        console.log("bar");
+    }
+    expect_stdout: [
+        "foo",
+        "bar",
+    ]
+    node_version: ">=4"
+}
+
 issue_4681: {
     options = {
         unused: true,