Strict language feature enablement (StrictLanguageFeatures)

Warnings for unrecognized feature names in -enable-upcoming-feature or enable-experimental-feature.

Overview

By default, if an unrecognized feature name is specified with the -enable-upcoming-feature or -enable-experimental-feature flags, the compiler will ignore it without emitting a diagnostic since some projects must be simultaneously compatible with multiple versions of the language and toolchain. This can, however, lead to misspecified features. To diagnose these cases instead, enable StrictLanguageFeatures.