[REQUEST] Better validation for CLI flags

[mcandre]

Is your feature request related to a problem? Please describe.

When I reconfigure Compiler Explorer to use a different compiler, then it silently ignores any errors relating to the build flags. For example, MSVC will continue to build unoptimized machine code when the flags feature "-O3". This is invalid syntax for MSVC. Because the problem is silently ignored, the Compiler Explorer user may continue to debug their application with the wrong impression of which settings are applied to the compiler.

Describe the solution you'd like

• Abort the build and report an error in the compilation log, when the compiler rejects the command line flags.

Describe alternatives you've considered

• Reset the flags each time the compiler is changed, even if the flags would be compatible between both compilers. This could get annoying though, for users who regularly test on multiple compilers.

[partouf]

I'm not sure we can just throw errors when flags are ignored (it's definitely not Silently ignored, there's always a warning in the output log) - but maybe it's indeed too hidden for the user.

Related issues to this:

• [REQUEST] Per-compiler options #1865
• [REQUEST] Save compiler options per compiler #2214
• MSVC compiler suggested options include /Ox but not /O2 when /O2 is what a user would most likely want #2065