feat(minifier): fold simple literals passed to unary + (#8943)

Added support for folding `+[]` and `+/any-regex/`.

`+` uses `ToNumber` and `ToNumber` uses `ToPrimitive` for objects. `ToPrimitive` for `[]` returns `""`, for `/any-regex/` it returns `"/any-regex/"`.

**References**
- [Spec of unary `+`](https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#sec-unary-plus-operator-runtime-semantics-evaluation)
- [Spec of `ToPrimitive`](https://tc39.es/ecma262/multipage/abstract-operations.html#sec-toprimitive)

crates/oxc_ecmascript/src/constant_evaluation/mod.rs

@@ -145,7 +145,20 @@ pub trait ConstantEvaluation<'a>: MayHaveSideEffects {
             Expression::SequenceExpression(s) => {
                 s.expressions.last().and_then(|e| self.eval_to_number(e))
             }
+            // If the object is empty, `toString` / `valueOf` / `Symbol.toPrimitive` is not overridden.
+            // (assuming that those methods in Object.prototype are not modified)
+            // In that case, `ToPrimitive` returns `"[object Object]"`
             Expression::ObjectExpression(e) if e.properties.is_empty() => Some(f64::NAN),
+            // `ToPrimitive` for RegExp object returns `"/regexp/"`
+            Expression::RegExpLiteral(_) => Some(f64::NAN),
+            Expression::ArrayExpression(arr) => {
+                // If the array is empty, `ToPrimitive` returns `""`
+                if arr.elements.is_empty() {
+                    Some(0.0)
+                } else {
+                    None
+                }
+            }
             expr => {
                 use crate::ToNumber;
                 expr.to_number()

crates/oxc_minifier/tests/peephole/esbuild.rs

@@ -891,9 +891,9 @@ fn constant_evaluation_test() {
     test("x = !5", "x = !1;");
     test("x = typeof 5", "x = 'number';");
     test("x = +''", "x = 0;");
-    // test("x = +[]", "x = 0;");
+    test("x = +[]", "x = 0;");
     test("x = +{}", "x = NaN;");
-    // test("x = +/1/", "x = NaN;");
+    test("x = +/1/", "x = NaN;");
     test("x = +[1]", "x = +[1];");
     test("x = +'123'", "x = 123;");
     test("x = +'-123'", "x = -123;");