No SOURCES given to target: __CMAKE__CXX26 on ubuntu 25.04

Development
,
11

But the CMAKE_CXX_STANDARD_LIBRARY is set in CMakeDetermineComilerId.cmake:

  set(CMAKE_${lang}_STANDARD_LIBRARY "")
  if ("x${lang}" STREQUAL "xCXX" AND
      EXISTS "${CMAKE_CURRENT_FUNCTION_LIST_DIR}/${lang}-DetectStdlib.h" AND
      ("x${CMAKE_${lang}_COMPILER_ID}" STREQUAL "xClang" AND
       "x${CMAKE_${lang}_COMPILER_FRONTEND_VARIANT}" STREQUAL "xGNU") OR
      ("x${CMAKE_${lang}_COMPILER_ID}" STREQUAL "xGNU"))
    # See #20851 for a proper abstraction for this.
    execute_process(
      COMMAND "${CMAKE_${lang}_COMPILER}"
        ${CMAKE_${lang}_COMPILER_ID_ARG1}
        ${CMAKE_CXX_COMPILER_ID_FLAGS_LIST}
        -E
        -x c++-header
        "${CMAKE_CURRENT_FUNCTION_LIST_DIR}/${lang}-DetectStdlib.h"
        -o - # Write to stdout.
      OUTPUT_VARIABLE _lang_stdlib_out
      ERROR_VARIABLE _lang_stdlib_err
      RESULT_VARIABLE _lang_stdlib_res
      ERROR_STRIP_TRAILING_WHITESPACE)
    if (_lang_stdlib_res EQUAL 0)
      string(REGEX REPLACE ".*CMAKE-STDLIB-DETECT: (.+)\n.*" "\\1" "CMAKE_${lang}_STANDARD_LIBRARY" "${_lang_stdlib_out}")
    endif ()
  endif ()

default set to:
Modules/CMakeDetermineCompilerId.cmake(434): set(CMAKE_CXX_STANDARD_LIBRARY libstdc++ PARENT_SCOPE )

But this helps:

CXX=clang++-20 CXXFLAGS=-stdlib=libc++ cmake -G Ninja -S . -B build --trace-expand --trace-source=CMakeDetermineCompilerId.cmake --fresh