Flang does not support -fno-builtin

[arrowd]

When compiling some Fortran code (I unfortunately forgot what the project was called, but can find it if needed) I get:

flang-21: error: unknown argument '-fno-builtin'; did you mean '-Xclang -fno-builtin'?

I was asked to create a separate issue for this flag in #163550

[llvmbot]

@llvm/issue-subscribers-flang-driver

Author: Gleb Popov (arrowd)

When compiling some Fortran code (I unfortunately forgot what the project was called, but can find it if needed) I get: ``` flang-21: error: unknown argument '-fno-builtin'; did you mean '-Xclang -fno-builtin'? ``` I was asked to create a separate issue for this flag in #163550

[eugeneepshteyn]

@arrowd , I believe this option is only relevant to C/C++ code (specifically, for standard C library functions).

Even gfortran warns about it:

$ gfortran -c -fno-builtin hello.f90 
f951: Warning: command-line option ‘-fno-builtin’ is valid for C/C++/D/ObjC/ObjC++ but not for Fortran

So I think the best thing flang could do is to issue similar warning, instead of an error.

[arrowd]

Yes, making it a non-fatal warning would work for me too.