result code = -1
standard error = {
tests/diagnostics/interfaces/mutating-impl-of-non-mutating-req.slang(10): error 38100: type 'Counter' does not provide required interface member 'processValue'
struct Counter : IThing
                 ^~~~~~
tests/diagnostics/interfaces/mutating-impl-of-non-mutating-req.slang(7): note: see declaration of 'processValue'
}
standard output = {
}