How can I tell vim to search a certain directory (as well as the current one) when trying to open a reference from the tags file?

Question

I have a tags file with two types of entries interspersed:

Integer_List    main.adb        19
Copy    a-cdlili.ads    118

It was generated by making an Ada project using Gnat Programming Studio, which uses gprbuild to build the project. Then, I run gnat xref obj/*.ali -v > src/tags to generate a tags file from the .ali files that describe the symbols in the source code. It works well for references in my own code, such as Integer_List above, but fails to find a-cdlili.ads and other references that are part of the Ada runtime, located in /opt/GNAT/2018/lib/gcc/x86_64-pc-linux-gnu/7.3.1/rts-native/adainclude/.

Is there any way to tell vim that tags may refer to files in that directory as well as the current one? Ideally, this only applies when an Ada file is open.

Answer 1

Gnat xref has a -f flag to output the full source directory. That solves the problem.