Since #225, tools logs a warning when a schema applies a directive in a way its definition doesn't allow, and still builds the schema:

directive @uppercase on OBJECT
directive @auth(role: String!) on FIELD_DEFINITION
directive @cached(ttl: Int) on FIELD_DEFINITION

type Query {
    motto: String @uppercase
    secret: String @auth
    members: [Member!] @cached @cached(ttl: 10)
}

Directive @uppercase is applied to field 'Query.motto', but it's declared on OBJECT, not FIELD_DEFINITION.
Directive @auth is applied to field 'Query.secret' without its required argument 'role'.
Directive @cached isn't repeatable but is applied more than once to field 'Query.members'.

graphql-java's SchemaGenerator rejects all three, and the spec doesn't allow them either. But they always built with tools, so failing on them has to wait for a major release.

What to change

• replace the warnings from SchemaParser#directiveProblems with a SchemaError that has the same messages
• probably do the same for the warning in SchemaParser#directivesOf, when a type extension applies a non-repeatable directive the type already has. It's the same mistake, it just drops the repeat today
• turn DirectiveTest.should warn about invalid applied directives and keep them into a test for the error
• mention it in the 15.0.0 release notes