Fix block comment at EOF without trailing newline (#1828)

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

@@ -625,11 +625,11 @@ public final class Lexer {
     while (lookahead != EOF) {
       if (prev == '*' && lookahead == '/') {
         nextChar();
-        break;
+        return;
       }
       prev = nextChar();
     }
-    if (lookahead == EOF) throw unexpectedEndOfFile();
+    throw unexpectedEndOfFile();
   }
 
   private void lexHexNumber() {

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

@@ -113,6 +113,52 @@ class LexerTest {
     assertThat(thrown.message).contains("Whitespace")
   }
 
+  @Test
+  fun `block comment at EOF without trailing newline`() {
+    // A block comment that is the last thing in the file, with no trailing
+    // newline, must lex correctly and not be mistaken for an unterminated comment.
+    val lexer = Lexer("/*\nbar\n*/")
+    assertThat(lexer.next()).isEqualTo(Token.BLOCK_COMMENT)
+    assertThat(lexer.next()).isEqualTo(Token.EOF)
+  }
+
+  @Test
+  fun `block comment after code at EOF without trailing newline`() {
+    val lexer = Lexer("foo = 123\n/*\nbar\n*/")
+    assertThat(lexer.next()).isEqualTo(Token.IDENTIFIER)
+    assertThat(lexer.next()).isEqualTo(Token.ASSIGN)
+    assertThat(lexer.next()).isEqualTo(Token.INT)
+    assertThat(lexer.next()).isEqualTo(Token.BLOCK_COMMENT)
+    assertThat(lexer.next()).isEqualTo(Token.EOF)
+  }
+
+  @Test
+  fun `empty block comment at EOF without trailing newline`() {
+    val lexer = Lexer("/**/")
+    assertThat(lexer.next()).isEqualTo(Token.BLOCK_COMMENT)
+    assertThat(lexer.next()).isEqualTo(Token.EOF)
+  }
+
+  @Test
+  fun `plain code at EOF without trailing newline`() {
+    // Documents that code whose final line has no trailing newline lexes
+    // cleanly and terminates with EOF rather than erroring.
+    val lexer = Lexer("foo = 123")
+    assertThat(lexer.next()).isEqualTo(Token.IDENTIFIER)
+    assertThat(lexer.next()).isEqualTo(Token.ASSIGN)
+    assertThat(lexer.next()).isEqualTo(Token.INT)
+    assertThat(lexer.next()).isEqualTo(Token.EOF)
+  }
+
+  @Test
+  fun `unterminated block comment fails`() {
+    val thrown = assertThrows<ParserError> { Lexer("/* bar").next() }
+    assertThat(thrown).hasMessageContaining("Unexpected end of file")
+
+    val justOpener = assertThrows<ParserError> { Lexer("/*").next() }
+    assertThat(justOpener).hasMessageContaining("Unexpected end of file")
+  }
+
   @Test
   fun acceptsAllUnicodeCodepointsInComments() {
     // Test valid Unicode codepoints can appear literally