JDK-8291643 changed diagnostics to print types without type annotations, unless they opt-in to including the type annotations. To do that, it introduced a trivial wrapper around the annotated type, JCDiagnostic.AnnotatedType, to avoid the stripping.

That wrapper bypasses logic in RichDiagnosticFormatter that preprocesses arguments, including types. The missing preprocessing for types wrapped in JCDiagnostic.AnnotatedType causes crashes when the types are printed:

import java.lang.annotation.ElementType;
import java.lang.annotation.Target;

public class T {
  @Target(ElementType.TYPE_USE)
  @interface TA {}

  static class A {
    static class B<T> {}
  }

  <T> @TA A.B<T> f() {}
}

$ javac T.java
java.lang.AssertionError: Missing type variable in where clause: T
        at jdk.compiler/com.sun.tools.javac.util.RichDiagnosticFormatter.unique(RichDiagnosticFormatter.java:249)
        at jdk.compiler/com.sun.tools.javac.util.RichDiagnosticFormatter$RichPrinter.visitTypeVar(RichDiagnosticFormatter.java:400)
        at jdk.compiler/com.sun.tools.javac.util.RichDiagnosticFormatter$RichPrinter.visitTypeVar(RichDiagnosticFormatter.java:342)
        at jdk.compiler/com.sun.tools.javac.code.Type$TypeVar.accept(Type.java:1709)