From my CMakeError.log file:

Determining if the function _alloca exists failed with the following output:
Change Dir: D:/workarea/test.rel/CMakeFiles/CMakeTmp

Run Build Command(s):D:/Python38-32/Scripts/ninja.exe cmTC_44264 && [1/2] Building C object CMakeFiles\cmTC_44264.dir\CheckFunctionExists.c.obj

FAILED: CMakeFiles/cmTC_44264.dir/CheckFunctionExists.c.obj

D:\LLVM\bin\clang-cl.exe /nologo /DWIN32 /D_WINDOWS /W3 -DCHECK_FUNCTION_EXISTS=_alloca -Werror=unguarded-availability-new /MDd /Zi /Ob0 /Od /RTC1 /showIncludes /FoCMakeFiles\cmTC_44264.dir\CheckFunctionExists.c.obj /FdCMakeFiles\cmTC_44264.dir\ -c D:\CMake\share\cmake-3.18\Modules\CheckFunctionExists.c

D:\CMake\share\cmake-3.18\Modules\CheckFunctionExists.c(7,3): error: conflicting types for '_alloca'
CHECK_FUNCTION_EXISTS(void);
^
(6,31): note: expanded from here
#define CHECK_FUNCTION_EXISTS _alloca
^
D:\CMake\share\cmake-3.18\Modules\CheckFunctionExists.c(7,3): *note: '_alloca' is a builtin with type 'void (unsigned long long)'
(6,31): note: expanded from here
#define CHECK_FUNCTION_EXISTS _alloca
^
D:\CMake\share\cmake-3.18\Modules\CheckFunctionExists.c(17,25): error: too few arguments to function call, expected 1, have 0
CHECK_FUNCTION_EXISTS();

2 errors generated.
ninja: build stopped: subcommand failed.

I see that it is present within Microsoft's include directories/headers: https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/alloca?view=vs-2019

And this function shouldn't be searched, on Windows, since it never had to exist (within any header file):

Determining if the function __alloca exists failed with the following output:
Change Dir: D:/workarea/test.rel/CMakeFiles/CMakeTmp

Run Build Command(s):D:/Python38-32/Scripts/ninja.exe cmTC_8b6e0 && [1/2] Building C object CMakeFiles\cmTC_8b6e0.dir\CheckFunctionExists.c.obj

[2/2] Linking C executable cmTC_8b6e0.exe

FAILED: cmTC_8b6e0.exe

cmd.exe /C "cd . && D:\CMake\bin\cmake.exe -E vs_link_exe --intdir=CMakeFiles\cmTC_8b6e0.dir --rc="C:\Program Files (x86)\WINDOW1\10\bin\1001901.0\x64\rc.exe" --mt="C:\Program Files (x86)\WINDOW1\10\bin\1001901.0\x64\mt.exe" --manifests -- D:\LLVM\bin\lld-link.exe /nologo CMakeFiles\cmTC_8b6e0.dir\CheckFunctionExists.c.obj /out:cmTC_8b6e0.exe /implib:cmTC_8b6e0.lib /pdb:cmTC_8b6e0.pdb /version:0.0 /machine:x64 /debug /INCREMENTAL /subsystem:console kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib && cd ."

LINK Pass 1: command "D:\LLVM\bin\lld-link.exe /nologo CMakeFiles\cmTC_8b6e0.dir\CheckFunctionExists.c.obj /out:cmTC_8b6e0.exe /implib:cmTC_8b6e0.lib /pdb:cmTC_8b6e0.pdb /version:0.0 /machine:x64 /debug /INCREMENTAL /subsystem:console kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib /MANIFEST /MANIFESTFILE:CMakeFiles\cmTC_8b6e0.dir/intermediate.manifest CMakeFiles\cmTC_8b6e0.dir/manifest.res" failed (exit code 1) with the following output:
lld-link: error: undefined symbol: __alloca

referenced by D:\CMake\share\cmake-3.18\Modules\CheckFunctionExists.c:17

          CMakeFiles\cmTC_8b6e0.dir\CheckFunctionExists.c.obj:(main)

ninja: build stopped: subcommand failed.

What did you expect to see?

-- Looking for _alloca
-- Looking for _alloca - found
-- Looking for __alloca
-- Looking for __alloca - not found

or, if being more precise:

-- Looking for _alloca
-- Looking for _alloca - found

What did you see instead?

-- Looking for _alloca
-- Looking for _alloca - not found
-- Looking for __alloca
-- Looking for __alloca - not found

Ivan