Quick answer: Unity assembly tagged for editor-only failing to compile when run by Test Runner? Test Runner needs editor platform; restricted asmdefs need editor-only flag preserved.
Tagging an asmdef as iOS-only excludes it from Test Runner's editor scope; tests fail to find code.
Add Editor platform
asmdef > Platforms: include Editor + iOS. Test Runner sees the assembly; mobile-restricted code stays mobile-restricted.
Or split tests from production code
Tests in a separate asmdef; references production. Production stays platform-restricted; tests are editor-only.
Verify in Test Runner
Window > General > Test Runner. Listed tests; missing tests = asmdef restriction.
“asmdef platform flags include or exclude. Test Runner sees only included.”
If your tests fail to discover, the asmdef configuration is the first suspect. Audit; document.