fix: disallow "true", "false", and "null" as enum values

The specification disallows these in the SDL grammar definition, and using one of these enum values would result in a schema that would throw validation errors on creation.

Fixes #3221.

src/language/__tests__/parser-test.ts

@@ -120,6 +120,21 @@ describe('Parser', () => {
     });
   });
 
+  it('does not allow "true", "false", or "null" as enum value', () => {
+    expectSyntaxError('enum Test { VALID, true }').to.deep.equal({
+      message: 'Syntax Error: Name "true" is reserved and cannot be used for an enum value.',
+      locations: [{ line: 1, column: 20 }],
+    });
+    expectSyntaxError('enum Test { VALID, false }').to.deep.equal({
+      message: 'Syntax Error: Name "false" is reserved and cannot be used for an enum value.',
+      locations: [{ line: 1, column: 20 }],
+    });
+    expectSyntaxError('enum Test { VALID, null }').to.deep.equal({
+      message: 'Syntax Error: Name "null" is reserved and cannot be used for an enum value.',
+      locations: [{ line: 1, column: 20 }],
+    });
+  });
+
   it('parses multi-byte characters', () => {
     // Note: \u0A0A could be naively interpreted as two line-feed chars.
     const ast = parse(`

src/language/parser.ts

@@ -1036,13 +1036,11 @@ export class Parser {
 
   /**
    * EnumValueDefinition : Description? EnumValue Directives[Const]?
-   *
-   * EnumValue : Name
    */
   parseEnumValueDefinition(): EnumValueDefinitionNode {
     const start = this._lexer.token;
     const description = this.parseDescription();
-    const name = this.parseName();
+    const name = this.parseEnumValueName();
     const directives = this.parseConstDirectives();
     return this.node<EnumValueDefinitionNode>(start, {
       kind: Kind.ENUM_VALUE_DEFINITION,
@@ -1052,6 +1050,23 @@ export class Parser {
     });
   }
 
+  /**
+   * EnumValue : Name but not `true`, `false` or `null`
+   */
+  parseEnumValueName(): NameNode {
+    if (this._lexer.token.value === 'true' ||
+      this._lexer.token.value === 'false' ||
+      this._lexer.token.value === 'null'
+    ) {
+      throw syntaxError(
+        this._lexer.source,
+        this._lexer.token.start,
+        `${getTokenDesc(this._lexer.token)} is reserved and cannot be used for an enum value.`,
+      );
+    }
+    return this.parseName();
+  }
+
   /**
    * InputObjectTypeDefinition :
    *   - Description? input Name Directives[Const]? InputFieldsDefinition?