[Docs]Constrained example with num_phyiscal_cores doesn't work

[Ivo-Balbaert]

Where is the problem?

https://docs.modular.com/mojo/stdlib/builtin/constrained/constrained/

What can we do better?

The code on this page gives a compile error (see also #3683)

from sys.info import num_physical_cores

def main():
   alias cores_to_use = 2
   multicore_check[cores_to_use]()

def multicore_check[cores: Int]():
   constrained[
       cores <= num_physical_cores(),
       "build failed: not enough cores"
   ]()
   constrained[
       cores >= 2,
       "at least two cores are required"
   ]()

The error is:

open-source/mojo/stdlib/stdlib/builtin/_startup.mojo:114:4: error: call expansion failed
open-source/mojo/stdlib/stdlib/builtin/_startup.mojo:97:4: note: function instantiation failed
open-source/mojo/stdlib/stdlib/builtin/_startup.mojo:109:57: note: call expansion failed
open-source/mojo/stdlib/stdlib/builtin/_startup.mojo:69:4: note: function instantiation failed
open-source/mojo/stdlib/stdlib/builtin/_startup.mojo:85:18: note: call expansion failed
open-source/mojo/stdlib/stdlib/builtin/_startup.mojo:106:8: note: function instantiation failed
/home/ivo/mojo_book/using_functions/test.mojo:6:34: note: call expansion failed
multicore_checkcores_to_use
^
/home/ivo/mojo_book/using_functions/test.mojo:9:5: note: function instantiation failed
def multicore_checkcores: Int:
^
/home/ivo/mojo_book/using_functions/test.mojo:12:6: note: call expansion failed
]()
^
open-source/mojo/stdlib/stdlib/builtin/constrained.mojo:56:6: note: failed to compile-time evaluate function call
open-source/mojo/stdlib/stdlib/sys/info.mojo:789:4: note: failed to interpret function @StdLib::sys::info::num_physical_cores()
open-source/mojo/stdlib/stdlib/sys/info.mojo:796:73: note: call expansion failed
open-source/mojo/stdlib/stdlib/sys/ffi.mojo:624:6: note: failed to interpret operation pop.external_call{func: "KGEN_CompilerRT_NumPhysicalCores" : !kgen.string, funcAttrs: ["willreturn"], memory: #llvm.memory_effects<other = none, argMem = none, inaccessibleMem = none>, resAttrs: [{llvm.noundef}]}(#kgen.pack<> : !kgen.pack<[]>)
open-source/mojo/stdlib/stdlib/sys/info.mojo:796:73: note: call expansion failed
open-source/mojo/stdlib/stdlib/sys/ffi.mojo:624:6: note: unable to interpret call to unknown external function: KGEN_CompilerRT_NumPhysicalCores
mojo: error: failed to run the pass manager

Anything else?

No response

[ematejska]

Reproduced. Thanks for filing.