Constraint Violation Diagnostics

What this C++ constraint-violation question tests

This is an easy question that probes your understanding of how modern C++ compilers enforce type constraints and handle violations. It rewards familiarity with compiler error messages and the ability to diagnose why valid-looking code fails to compile.

To approach problems like this, you need to read the code carefully, reason about what the compiler expects at each point, and predict where a type mismatch or constraint failure will surface. This skill is essential in production C++ environments, where compilation errors are your first line of defense against subtle type bugs.

• Template instantiation and type deduction
• Compiler error messages and diagnostics
• Type constraints and SFINAE (substitution failure is not an error)
• Const-correctness and reference semantics