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Add an internal annotation for declarations that need to work with all AST levels, e.g. @AllAstLevels Most of JDT UI only uses the latest AST level from org.eclipse.jdt.internal.ui.javaeditor.ASTProvider.SHARED_AST_LEVEL. But we have a few APIs that take an AST or ASTNode, and those need to support older levels as well. See e.g. bug 435571 and the comment above StubUtility2#createConstructorStub(..).
To find potentially affected entry points, select API packages and search for type "org.eclipse.jdt.core.dom.*" in match locations "Method return types" and "Method parameter types".