Accept names that start with a keyword

modules/core/src/main/scala/parser.scala

@@ -348,7 +348,7 @@ object GraphQLParser {
       }
 
     def InlineFragment(n: Int): Parser[Ast.Selection.InlineFragment] =
-      ((TypeCondition.? ~ Directives).with1 ~ SelectionSetN(n)).map {
+      ((TypeCondition.backtrack.? ~ Directives).with1 ~ SelectionSetN(n)).map {
         case ((cond, dirs), sel) => Ast.Selection.InlineFragment(cond, dirs, sel)
       }
 
@@ -371,7 +371,7 @@ object GraphQLParser {
       (Name <* punctuation(":")) ~ Value
 
     lazy val FragmentName: Parser[Ast.Name] =
-      not(string("on")).with1 *> Name
+      not(string("on") <* not(charIn(nameSubsequent))).with1 *> Name
 
     lazy val FragmentDefinition: Parser[Ast.FragmentDefinition] =
       ((keyword("fragment") *> FragmentName) ~ TypeCondition ~ Directives ~ SelectionSet).map {
@@ -382,11 +382,12 @@ object GraphQLParser {
       keyword("on") *> NamedType
 
     lazy val NullValue: Parser[Ast.Value.NullValue.type] =
-      keyword("null").as(Ast.Value.NullValue)
+      keyword("null").backtrack.as(Ast.Value.NullValue)
 
     lazy val EnumValue: Parser[Ast.Value.EnumValue] =
-      (not(string("true") | string("false") | string("null")).with1 *> Name)
-        .map(Ast.Value.EnumValue.apply)
+      (not(
+        (string("true") | string("false") | string("null")) <* not(
+          charIn(nameSubsequent))).with1 *> Name).map(Ast.Value.EnumValue.apply)
 
     def ListValue(n: Int): Parser[Ast.Value.ListValue] =
       token(
@@ -413,7 +414,8 @@ object GraphQLParser {
     }
 
     lazy val BooleanValue: Parser[Ast.Value.BooleanValue] =
-      token(booleanLiteral).map(Ast.Value.BooleanValue.apply)
+      token((booleanLiteral <* not(charIn(nameSubsequent))).backtrack)
+        .map(Ast.Value.BooleanValue.apply)
 
     def ObjectField(n: Int): Parser[(Ast.Name, Ast.Value)] =
       (Name <* punctuation(":")) ~ ValueN(n)

modules/core/src/test/scala/parser/ParserSuite.scala

@@ -980,6 +980,85 @@ final class ParserSuite extends CatsEffectSuite {
     assertEquals(parser.parseText("query { # inner\n x } # a\n# b"), Result(expected))
   }
 
+  test("fragment name that starts with 'on'") {
+    val query = """
+      query { x { ...onlyFriends } }
+      fragment onlyFriends on X { name }
+    """
+
+    val expected =
+      List(
+        Operation(
+          Query,
+          None,
+          Nil,
+          Nil,
+          List(
+            Field(None, Name("x"), Nil, Nil, List(FragmentSpread(Name("onlyFriends"), Nil))))),
+        FragmentDefinition(
+          Name("onlyFriends"),
+          Named(Name("X")),
+          Nil,
+          List(Field(None, Name("name"), Nil, Nil, Nil)))
+      )
+
+    assertEquals(parser.parseText(query), Result(expected))
+  }
+
+  test("fragment name 'on' is still rejected") {
+    assert(parser.parseText("fragment on on X { name }").hasValue == false)
+  }
+
+  test("enum values that start with 'true', 'false' or 'null'") {
+    val query = "query { x(a: trueStory, b: falseAlarm, c: nullable) }"
+
+    val expected =
+      Operation(
+        Query,
+        None,
+        Nil,
+        Nil,
+        List(
+          Field(
+            None,
+            Name("x"),
+            List(
+              (Name("a"), EnumValue(Name("trueStory"))),
+              (Name("b"), EnumValue(Name("falseAlarm"))),
+              (Name("c"), EnumValue(Name("nullable")))
+            ),
+            Nil,
+            Nil
+          ))
+      )
+
+    assertEquals(parser.parseText(query), Result(List(expected)))
+  }
+
+  test("bare keywords are still literals, not enum values") {
+    val query = "query { x(a: true, b: false, c: null) }"
+
+    val expected =
+      Operation(
+        Query,
+        None,
+        Nil,
+        Nil,
+        List(
+          Field(
+            None,
+            Name("x"),
+            List(
+              (Name("a"), BooleanValue(true)),
+              (Name("b"), BooleanValue(false)),
+              (Name("c"), NullValue)
+            ),
+            Nil,
+            Nil)))
+
+    assertEquals(parser.parseText(query), Result(List(expected)))
+  }
+
   def mkParser(
       maxSelectionDepth: Int = GraphQLParser.defaultConfig.maxSelectionDepth,
       maxSelectionWidth: Int = GraphQLParser.defaultConfig.maxSelectionWidth,