fix corner case in inline (#4160)

fixes #4159

lib/compress.js

@@ -4793,15 +4793,15 @@ merge(Compressor.prototype, {
                     for (var a = node.argnames, i = a.length; --i >= 0;) {
                         var sym = a[i];
                         var def = sym.definition();
-                        if (!(def.id in in_use_ids)) {
+                        if (def.id in in_use_ids) {
+                            trim = false;
+                            if (indexOf_assign(def, sym) < 0) sym.__unused = null;
+                        } else {
                             sym.__unused = true;
                             if (trim) {
                                 log(sym, "Dropping unused function argument {name}");
                                 a.pop();
                             }
-                        } else {
-                            trim = false;
-                            if (indexOf_assign(def, sym) < 0) sym.__unused = null;
                         }
                     }
                     fns_with_marked_args.push(node);
@@ -7047,9 +7047,10 @@ merge(Compressor.prototype, {
                     value: null
                 }));
             }
+            if (!value) return;
             var sym = make_node(AST_SymbolRef, name, name);
             def.references.push(sym);
-            if (value) expressions.push(make_node(AST_Assign, self, {
+            expressions.push(make_node(AST_Assign, self, {
                 operator: "=",
                 left: sym,
                 right: value
@@ -7070,9 +7071,14 @@ merge(Compressor.prototype, {
                     var symbol = make_node(AST_SymbolVar, name, name);
                     name.definition().orig.push(symbol);
                     if (!value && in_loop) value = make_node(AST_Undefined, self);
+                    if ("__unused" in name) {
+                        append_var(decls, expressions, symbol);
+                        if (value) expressions.push(value);
+                    } else {
                         append_var(decls, expressions, symbol, value);
                     }
                 }
+            }
             decls.reverse();
             expressions.reverse();
         }

test/compress/functions.js

@@ -4808,3 +4808,24 @@ issue_4155: {
         "function",
     ]
 }
+
+issue_4159: {
+    options = {
+        collapse_vars: true,
+        inline: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = 42, c = function(b) {
+            (b = a) && console.log(a++, b);
+        }(c = a);
+    }
+    expect: {
+        var a = 42;
+        (b = a) && console.log(a++, b);
+        var b;
+    }
+    expect_stdout: "42 42"
+}