fix corner cases in unused (#3519)

lib/compress.js

@@ -3738,7 +3738,7 @@ merge(Compressor.prototype, {
                         head.push(def);
                     } else {
                         var value = def.value
-                            && (sym.references.length != 1 || !sym.replaced)
+                            && !def.value.single_use
                             && def.value.drop_side_effect_free(compressor);
                         if (value) {
                             AST_Node.warn("Side effects in initialization of unused variable {name} [{file}:{line},{col}]", template(def.name));
@@ -6158,6 +6158,7 @@ merge(Compressor.prototype, {
             if (single_use && fixed) {
                 def.single_use = false;
                 fixed._squeezed = true;
+                fixed.single_use = true;
                 if (fixed instanceof AST_Defun) {
                     fixed = make_node(AST_Function, fixed, fixed);
                     fixed.name = make_node(AST_SymbolLambda, fixed.name, fixed.name);

test/compress/drop-unused.js

@@ -2103,7 +2103,7 @@ issue_3497: {
     expect_stdout: "undefined"
 }
 
-issue_3515: {
+issue_3515_1: {
     options = {
         collapse_vars: true,
         reduce_vars: true,
@@ -2127,3 +2127,62 @@ issue_3515: {
     }
     expect_stdout: "1"
 }
+
+issue_3515_2: {
+    options = {
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = "FAIL";
+        function f() {
+            typeof b === "number";
+            delete a;
+        }
+        var b = f(a = "PASS");
+        console.log(a);
+    }
+    expect: {
+        var a = "FAIL";
+        function f() {
+            delete a;
+        }
+        f(a = "PASS");
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+}
+
+issue_3515_3: {
+    options = {
+        collapse_vars: true,
+        unused: true,
+    }
+    input: {
+        var c = "FAIL";
+        (function() {
+            function f() {
+                c = "PASS";
+            }
+            var a = f();
+            var a = function g(b) {
+                b && (b.p = this);
+            }(a);
+        })();
+        console.log(c);
+    }
+    expect: {
+        var c = "FAIL";
+        (function() {
+            function f() {
+                c = "PASS";
+            }
+            (function(b) {
+                b && (b.p = this);
+            })(f());
+        })();
+        console.log(c);
+    }
+    expect_stdout: "PASS"
+}