LibJS: Don't treat '?.' followed by decimal digit as QuestionMarkPeriod

From the spec: https://tc39.es/ecma262/#sec-punctuators

    OptionalChainingPunctuator ::
        ?. [lookahead ∉ DecimalDigit]

We were missing the lookahead and therefore incorrectly treating any
'?.' as TokenType::QuestionMarkPeriod.

Fixes #4409.

2 files changed, 11 insertions, 4 deletions

diff --git a/Libraries/LibJS/Lexer.cpp b/Libraries/LibJS/Lexer.cpp
index 047c5437e9..89758ded1c 100644
--- a/Libraries/LibJS/Lexer.cpp
+++ b/Libraries/LibJS/Lexer.cpp
@@ -598,10 +598,13 @@ Token Lexer::next()
             auto two_chars_view = m_source.substring_view(m_position - 1, 2);
             auto it = s_two_char_tokens.find(two_chars_view.hash(), [&](auto& entry) { return entry.key == two_chars_view; });
             if (it != s_two_char_tokens.end()) {
-                found_two_char_token = true;
-                consume();
-                consume();
-                token_type = it->value;
+                // OptionalChainingPunctuator :: ?. [lookahead ∉ DecimalDigit]
+                if (!(it->value == TokenType::QuestionMarkPeriod && m_position + 1 < m_source.length() && isdigit(m_source[m_position + 1]))) {
+                    found_two_char_token = true;
+                    consume();
+                    consume();
+                    token_type = it->value;
+                }
             }
         }
 
diff --git a/Libraries/LibJS/Tests/operators/ternary-basic.js b/Libraries/LibJS/Tests/operators/ternary-basic.js
index 164d01e534..c0f68fdf83 100644
--- a/Libraries/LibJS/Tests/operators/ternary-basic.js
+++ b/Libraries/LibJS/Tests/operators/ternary-basic.js
@@ -14,3 +14,7 @@ test("object values", () => {
     expect(o.f ? true : false).toBeTrue();
     expect(1 ? o.f : null).toBeTrue();
 });
+
+test("issue #4409, '?.' followed by decimal digit", () => {
+    expect("return false?.1:.2").toEvalTo(0.2);
+});