SourcePositions.getStartPos() incorrectly returns -1 for enum constants. TreeInfo.java line 288 has:

	case(JCTree.VARDEF): {
	    JCVariableDecl node = (JCVariableDecl)tree;
	    if (node.mods.pos != Position.NOPOS) {
		return node.mods.pos;
	    } else {
		return getStartPos(node.vartype);
	    }

and you can see why it doesn't work for enum constants.
I'm lazy, so let me try to explain the problem one more time. If you still think you need a test case, I'll consider writing one.

The problem happens when I'm looking at the tree of enum constant declaration, like "ABC" of:

  enum E { ABC, DEF; }

Javac creates JCTree.JCVariableDecl node for 'ABC' (and 'DEF') Unlike other 'ordinary' field declarations or local variable declarations, this JCVariableDecl doesn't have the JCVariableDecl#vartype set. So when I try to access the start position of this JCVariableDecl, the following code runs:

	case(JCTree.VARDEF): {
	    JCVariableDecl node = (JCVariableDecl)tree;
	    if (node.mods.pos != Position.NOPOS) {
		return node.mods.pos;
	    } else {
		return getStartPos(node.vartype);
	    }

and I get NOPOS. The correct code should be: 

	case(JCTree.VARDEF): {
	    JCVariableDecl node = (JCVariableDecl)tree;
	    if (node.mods.pos != Position.NOPOS) {
		return node.mods.pos;
	    }
            if(node.vartype!=null) {
		return getStartPos(node.vartype);
	    }
            return node.sym.pos;
import com.sun.source.tree.CompilationUnitTree;
import com.sun.source.tree.Tree;
import com.sun.source.util.JavacTask;
import com.sun.source.util.SourcePositions;
import com.sun.source.util.TreeScanner;
import com.sun.source.util.Trees;
import com.sun.tools.javac.util.List;
import java.io.IOException;
import java.net.URI;
import javax.tools.JavaCompiler;
import javax.tools.JavaFileObject;
import javax.tools.SimpleJavaFileObject;
import javax.tools.ToolProvider;

public class T6472751 {
    static class MyFileObject extends SimpleJavaFileObject {
        public MyFileObject() {
            super(URI.create("myfo:/Test.java"), JavaFileObject.Kind.SOURCE);
        }
        public CharSequence getCharContent(boolean ignoreEncodingErrors) {
            return "public enum Test { ABC, DEF; }";
        }
    }
    static Trees trees;
    static SourcePositions positions;
    public static void main(String[] args) throws IOException {
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        JavacTask task = (JavacTask) compiler.getTask(null, null, null, null, null, List.of(new MyFileObject()));
        trees = Trees.instance(task);
        positions = trees.getSourcePositions();
        Iterable<? extends CompilationUnitTree> asts = task.parse();
        for (CompilationUnitTree ast : asts) {
            new MyVisitor().scan(ast, null);
        }
    }
    
    static class MyVisitor extends TreeScanner<Void,Void> {
        @Override
        public Void scan(Tree node, Void ignored) {
            if (node == null)
                return null;
            System.out.format("%s: %s%n", node.getKind(), positions.getStartPosition(null,node));
            return super.scan(node, ignored);
        }

    }
}