avm1: Correctly handle undefined delimiter in String.split()

core/src/avm1/globals/string.rs

@@ -258,16 +258,17 @@ fn split<'gc>(
     args: &[Value<'gc>],
 ) -> Result<Value<'gc>, Error<'gc>> {
     let this = Value::from(this).coerce_to_string(activation)?;
-    let delimiter = args
-        .get(0)
-        .unwrap_or(&Value::Undefined)
-        .coerce_to_string(activation)?;
     let limit = match args.get(1).unwrap_or(&Value::Undefined) {
         Value::Undefined => usize::MAX,
         limit => limit.coerce_to_i32(activation)?.max(0) as usize,
     };
+    if let Some(delimiter) = match args.get(0).unwrap_or(&Value::Undefined) {
+        &Value::Undefined => None,
+        v => Some(v.coerce_to_string(activation)?),
+    } {
         if delimiter.is_empty() {
-        // When using an empty delimiter, Str::split adds an extra beginning and trailing item, but Flash does not.
+            // When using an empty delimiter, Str::split adds an extra beginning and trailing item,
+            // but Flash does not.
             // e.g., split("foo", "") returns ["", "f", "o", "o", ""] in Rust but ["f, "o", "o"] in Flash.
             // Special case this to match Flash's behavior.
             Ok(ArrayObject::new(
@@ -288,6 +289,14 @@ fn split<'gc>(
             )
             .into())
         }
+    } else {
+        Ok(ArrayObject::new(
+            activation.context.gc_context,
+            activation.context.avm1.prototypes().array,
+            [this.into()]
+        )
+        .into())
+    }
 }
 
 fn substr<'gc>(

tests/tests/swfs/avm1/string_methods/output.txt

@@ -242,8 +242,8 @@ HELLO𝔄hello
 
 // split
 // s.split(",")
-5
+6
-A,,b,c,
+A,,b,undefined0,c,
 // s.split(",", 2)
 2
 A,
@@ -254,20 +254,23 @@ A,
 0
 
 // s.split(",", undefined)
-5
+6
-A,,b,c,
+A,,b,undefined0,c,
 // s.split(",", null)
 0
 
 // s.split("")
-7
+18
-A,,,,,b,,,c,,
+A,,,,,b,,,u,n,d,e,f,i,n,e,d,0,,,c,,
 // s.split(undefined)
 1
-A,,b,c,
+A,,b,undefined0,c,
+// s.split(undefined, 0)
+1
+A,,b,undefined0,c,
 // s.split()
 1
-A,,b,c,
+A,,b,undefined0,c,
 
 // toLowerCase
 // "teST😋".toLowerCase()