Why adding the linker flag `-save-temps` resolves the clang-8 error: "unable to make temporary file"?

Question

I am using Windows + Cygwin + Eclipse + LLVM toolchain to build a C/C++ project.

The files compile fine with clang C compiler, but linking with clang++ fails with multiple error: unable to make temporary file: No such file or directory.

clang++ -o test "source1.bc" "source2.bc" "source3.bc" 
clang-8: error: unable to make temporary file: No such file or directory
clang-8: error: unable to make temporary file: No such file or directory
clang-8: error: unable to make temporary file: No such file or directory

Linker generates one error per each object file.

When I add the -save-temps option to linker, it works OK!

What could be the problem?

Adding --verbose gives:

clang version 8.0.1 (tags/RELEASE_801/final) Target: x86_64-unknown-windows-cygnus
Thread model: posix
InstalledDir: /usr/bin
clang-8: error: unable to make temporary file: No such file or directory
clang-8: error: unable to make temporary file: No such file or directory
clang-8: error: unable to make temporary file: No such file or directory

I have checked the TMP and TEMP variables - everything seems OK. I tried changing the TMP and TEMP to include trailing slash /, I also tried the whole path with backslashes \ or forward slashes / - nothing helped.

Here is a more verbose outputs.

First, here is the outputs with flags -save-temps and -###:

 "/usr/bin/clang-8" "-cc1" "-triple" "x86_64-unknown-windows-cygnus" "-S" "-save-temps=cwd" "-disable-free" "-disable-llvm-verifier" "-discard-value-names" "-main-file-name" "my_files\\my_file1.bc" "-mrelocation-model" "pic" "-pic-level" "2" "-mthread-model" "posix" "-fmath-errno" "-masm-verbose" "-mconstructor-aliases" "-munwind-tables" "-target-cpu" "x86-64" "-dwarf-column-info" "-debugger-tuning=gdb" "-momit-leaf-frame-pointer" "-v" "-resource-dir" "/usr/lib/clang/8.0.1" "-fdebug-compilation-dir" "/cygdrive/c/my_projects/prj1/Debug" "-ferror-limit" "19" "-fmessage-length" "0" "-fobjc-runtime=gcc" "-fseh-exceptions" "-fdiagnostics-show-option" "-o" "my_files\\my_file1.s" "-x" "ir" "my_files\\my_file1.bc" "-faddrsig"

Here is the output without -save-temps, just -###:

 "/usr/bin/clang-8" "-cc1" "-triple" "x86_64-unknown-windows-cygnus" "-emit-obj" "-mrelax-all" "-disable-free" "-disable-llvm-verifier" "-discard-value-names" "-main-file-name" "my_files\\my_file1.bc" "-mrelocation-model" "pic" "-pic-level" "2" "-mthread-model" "posix" "-fmath-errno" "-masm-verbose" "-mconstructor-aliases" "-munwind-tables" "-target-cpu" "x86-64" "-dwarf-column-info" "-debugger-tuning=gdb" "-momit-leaf-frame-pointer" "-v" "-resource-dir" "/usr/lib/clang/8.0.1" "-fdebug-compilation-dir" "/cygdrive/c/my_projects/prj1/Debug" "-ferror-limit" "19" "-fmessage-length" "0" "-fobjc-runtime=gcc" "-fseh-exceptions" "-fdiagnostics-show-option" "-o" "" "-x" "ir" "my_files\\my_file1.bc" "-faddrsig"

So the "-o" "my_files\\my_file1.s" vs "-o" "" seems to be the difference. Is there an explanation?

Also, some more output with flags -save-temps, -###:

"/usr/bin/ld" "-m" "i386pep" "--wrap" "_Znwm" "--wrap" "_Znam" "--wrap" "_ZnwmRKSt9nothrow_t" "--wrap" "_ZnamRKSt9nothrow_t" "--wrap" "_ZdlPv" "--wrap" "_ZdaPv" "--wrap" "_ZdlPvRKSt9nothrow_t" "--wrap" "_ZdaPvKSt9nothrow_t" "-Bdynamic" "--tsaware" "-o" "myexecutable" "/usr/lib/crt0.o" "/usr/lib/gcc/x86_64-pc-cygwin/10/crtbegin.o" "-LC:/mylibraries" "-L/usr/lib/gcc/x86_64-pc-cygwin/10" "-L/usr/x86_64-pc-cygwin/lib" "-L/usr/lib" "-L/usr/lib/w32api" "my_files\\my_file1.o" "my_files\\my_file2.o" " my_files\\my_file3.o" "-lnaturedsp" "-lgcc_s" "-gcc" "-lcygwin" "-ladvapi32" "-lshell32" "-luser32" "-lkernel32" "/usr/lib/default-manifest.o" "/usr/lib/gcc/x86_64-pc-cygwin/10/crtend.o"

and with only -###:

"/usr/bin/ld" "-m" "i386pep" "--wrap" "_Znwm" "--wrap" "_Znam" "--wrap" "_ZnwmRKSt9nothrow_t" "--wrap" "_ZnamRKSt9nothrow_t" "--wrap" "_ZdlPv" "--wrap" "_ZdaPv" "--wrap" "_ZdlPvRKSt9nothrow_t" "--wrap" "_ZdaPvKSt9nothrow_t" "-Bdynamic" "--tsaware" "-o" "myexecutable" "/usr/lib/crt0.o" "/usr/lib/gcc/x86_64-pc-cygwin/10/crtbegin.o" "-LC:/mylibraries" "-L/usr/lib/gcc/x86_64-pc-cygwin/10" "-L/usr/x86_64-pc-cygwin/lib" "-L/usr/lib" "-L/usr/lib/w32api" "" "" "" "-lnaturedsp" "-lgcc_s" "-lgcc" "-lcygwin" "-ladvapi32" "-lshell32" "-luser32" "-lkernel32" "/usr/lib/default-manifest.o" "/usr/lib/gcc/x86_64-pc-cygwin/10/crtend.o"

So, again "my_files\\my_file1.o" "my_files\\my_file2.o" "my_files\\my_file3.o" are lost and substituted with empty strings "" "" "".

One more thing, as pointed out on the clang mailing list: "Neither the driver option -save-temps nor the linker option --save-temps (driver option -Wl,--save-temps) respects TMP/TEMP/TMPDIR. The temporary files are saved relative to the current working directory..."

So, it's not about temporary directory location.