Projects using Indy don't compile after upgrade to 10.2 Tokyo

Question

After uninstalling Delphi 10 Seattle, removing all directories containing 'Embarcadero', then installing Delphi 10.2 Tokyo, my projects using Indy no longer compile:

Could not compile used unit 'IdSSLOpenSSLHeaders'

• Nothing changed in the projects' search path (these are only relative to the project)
• Tools/Delphi Options/Library/Browsing path contains $(BDS)\source\Indy10\Protocol (and Core and System), and IdSSLOpenSSLHeaders.pas is in c:\Program Files (x86)\Embarcadero\Studio\19.0\source\Indy10\Protocols\
• Adding IdSSLOpenSSLHeaders to the uses clause does not help (12 other Indy file names were added automatically because I used design time components)
• There is a TIdSSLIOHandlerSocketOpenSSL component on the form
• There is nothing relating to Indy in the Tools/Delphi Options/Library/Library Path
• IdSSLOpenSSLHeaders.dcu etc. are in the c:\Program Files (x86)\Embarcadero\Studio\19.0\lib\win32\release\ folder
• The editor shows red wigglies under IdSSLOpenSSL in the Uses clause, with hint 'Cannot resolve unit name'
• There are no old id*.dcu files in unexpected places on the disk

I found two workarounds that I'm both not particularly happy with:

1. The Indy files are not part of the project; explicitly adding IdSSLOpenSSLHeaders.pas to the project makes it compile again, but I'm not happy about that: IdSSLOpenSSLHeaders.pas now compiles in that source directory, leaving a .dcu file there.
   Besides, all the other Indy files are not part of the project.

2. Copying IdSSLOpenSSLHeaders.pas and IdCompilerDefines.inc to my projects' directory

Is there a better solution that I'm overlooking?

I am trying to create a small project to reproduce the issue, but so far this compiles???

Answer 1

Found it.

There were two projects involved, with some common files. Whichever one of the two I tried to build, I got the error.

The solution was to delete all .dcu files local to both projects.

Don't ask me why, but it seems there was some (order?)^* dependency that building either one was unable to resolve.
After this, the order in which I build does not matter.

Note: I was thinking about deleting my question because this is such a weird and unreproducable issue (a test program I was gradually complicating kept compiled without errors).
Instead I decided to self-answer; who knows who this may help in the future.

^{* While trying, I even tried changing the order of the file names in the Uses statement, but that did not help.}