Can __STDC_VERSION__ be trusted to guarantee full C standard support?

Goal

Determine whether a program can rely on the __STDC_VERSION__ macro to detect that a compiler fully implements a target C standard and safely fall back when an upgrade fails.

Constraints and uncertainty

• The macro expands to an integer encoding the advertised standard version (e.g., 202311L for C23).
• Compilers may define the macro even when they only partially implement the standard.
• Command‑line flags (-std=) or a preceding #define can override the value, masking real capabilities.
• Library features such as <stdatomic.h> or <threads.h> are not reflected by the macro.

Open questions

1. Does a matching __STDC_VERSION__ value guarantee that all language and library features of that standard are available?
2. What practical checks (e.g., __has_include, feature‑test macros) should supplement the macro to avoid false positives during a failed upgrade?