Correctly handle EOF after unmatched backtick (#1187)

pkl-parser/src/main/java/org/pkl/parser/Lexer.java

@@ -489,7 +489,7 @@ public class Lexer {
   }
 
   private void lexQuotedIdentifier() {
-    while (lookahead != '`' && lookahead != '\n' && lookahead != '\r') {
+    while (lookahead != '`' && lookahead != '\n' && lookahead != '\r' && lookahead != EOF) {
       nextChar();
     }
     if (lookahead == '`') {
@@ -705,6 +705,13 @@ public class Lexer {
   }
 
   private ParserError unexpectedChar(char got, String didYouMean) {
+    if (got == EOF) {
+      return unexpectedChar("EOF", didYouMean);
+    }
+    return lexError("unexpectedCharacter", got, didYouMean);
+  }
+
+  private ParserError unexpectedChar(String got, String didYouMean) {
     return lexError("unexpectedCharacter", got, didYouMean);
   }
 

pkl-parser/src/test/kotlin/org/pkl/parser/LexerTest.kt

@@ -17,6 +17,7 @@ package org.pkl.parser
 
 import org.assertj.core.api.Assertions.assertThat
 import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.assertThrows
 
 class LexerTest {
 
@@ -46,4 +47,10 @@ class LexerTest {
     assertThat(Lexer.maybeQuoteIdentifier("this")).isEqualTo("`this`")
     assertThat(Lexer.maybeQuoteIdentifier("😀")).isEqualTo("`😀`")
   }
+
+  @Test
+  fun lexSingleBacktick() {
+    val thrown = assertThrows<ParserError> { Lexer("`").next() }
+    assertThat(thrown).hasMessageContaining("Unexpected character `EOF`")
+  }
 }