What do you mean by this? Can you explain how it uses more memory?

which is the purpose of using constexpr? (no hate, i don't know its uses :c)

lol

I think a best practice is to always use a Named function instead of an Arrow function for easy error location and handling.

Yep, in the tree that is described even in the example, this solution returns the incorrect value "15" instead of "-51".

Why is this low in Best Practices?

But will it work on RISC-V test machine? ;)

oh, ok. thanks for answering

For fun.

why replace function with a constant?

added

Missing useful assertion messages

Okay, thanks ~ please check it now

Hello folks ...
Well who ever spoke of performance is absolutely right ...
The "classic" switch will do the job much more efficiently than any other implementation version.
I like the elegacy of the code above also but performance wise is nowhere near the simple "switch" statement solution.
I run a benchmark test (using the BenchmarkDotNet v0.13.7 package) for the above (renamed as AmIAfraid02) implementation.
The last one (AmIAfraid03) is the one using the "switch" statement.

See the summary for yourselves:

// * Summary *

BenchmarkDotNet v0.13.7, Windows 10 (10.0.19044.3086/21H2/November2021Update)
Intel Core i5-7500 CPU 3.40GHz (Kaby Lake), 1 CPU, 4 logical and 4 physical cores
.NET SDK 7.0.400
[Host] : .NET 6.0.21 (6.0.2123.36311), X64 RyuJIT AVX2
.NET 7.0 : .NET 7.0.10 (7.0.1023.36312), X64 RyuJIT AVX2

Job=.NET 7.0 Runtime=.NET 7.0

// * Hints *
Outliers
BenchmarkAmIAfraidMethod.AmIAfraid02: .NET 7.0 -> 4 outliers were removed (218.49 ns..220.67 ns)
// * Config Issues *

// * Warnings *
Configuration
Summary -> The exporter BenchmarkDotNet.Exporters.MarkdownExporter is already present in configuration. There may be unexpected results.
Summary -> The exporter BenchmarkDotNet.Exporters.HtmlExporter is already present in configuration. There may be unexpected results.
Summary -> The exporter BenchmarkDotNet.Exporters.Csv.CsvExporter is already present in configuration. There may be unexpected results.

// * Legends *
Mean : Arithmetic mean of all measurements
Error : Half of 99.9% confidence interval
StdDev : Standard deviation of all measurements
Ratio : Mean of the ratio distribution ([Current]/[Baseline])
RatioSD : Standard deviation of the ratio distribution ([Current]/[Baseline])
1 ns : 1 Nanosecond (0.000000001 sec)

// ***** BenchmarkRunner: End *****
Run time: 00:00:59 (59.8 sec), executed benchmarks: 3

Global total time: 00:01:05 (65.47 sec), executed benchmarks: 3
// * Artifacts cleanup *
Artifacts cleanup is finished