I would like to use the {fmt} library (https://fmt.dev) as a replacement for iostreams in my mbed-os6 projects. Unfortunately, I get cryptic compiler errors complaining about missing locale definitions.
> Compile: C:\Program Files (x86)\GNU Tools ARM Embedded\9 2020-q2-update\bin\arm-none-eabi-g++ -c -std=gnu++17 -fno-rtti -Wvla -Wno-register -Wall -Wextra -Wpedantic -Wno-unused-parameter -Wno-missing-field-initializers -fmessage-length=0 -fno-exceptions -ffunction-sections -fdata-sections -funsigned-char -MMD -fno-delete-null-pointer-checks -fomit-frame-pointer -Os -DNDEBUG -g -mcpu=cortex-m4 -mthumb -mfpu=fpv4-sp-d16 -mfloat-abi=softfp -DMBED_ROM_START=0x8000000 -DMBED_ROM_SIZE=0x80000 -DMBED_RAM_START=0x20000000 -DMBED_RAM_SIZE=0x20000 -D__MBED__=1 -DTARGET_LIKE_MBED -DDEVICE_PORTOUT=1 -DSTM32F446xx -DDEVICE_PORTINOUT=1 -DTARGET_RTOS_M4_M7 -DDEVICE_RTC=1 -DDEVICE_MPU=1 -DDEVICE_SERIAL_ASYNCH=1 -DTARGET_STM32F4 -D__CMSIS_RTOS -DHSE_VALUE=25000000ul -DLED1=MBED_CONF_APP_HEARTBEAT_LED -DTOOLCHAIN_GCC -DDEVICE_STDIO_MESSAGES=1 -DDEVICE_CAN=1 -DTARGET_CORTEX_M -DTARGET_LIKE_CORTEX_M4 -DTARGET_M4 -DDEVICE_SPI_ASYNCH=1 -DDEVICE_LPTICKER=1 -DDEVICE_PWMOUT=1 -DCOMPONENT_FLASHIAP=1 -DTARGET_DMCB_60009 -DUSE_FULL_LL_DRIVER -DTARGET_CORTEX -DDEVICE_I2C=1 -DTRANSACTION_QUEUE_SIZE_SPI=2 -D__CORTEX_M4 -D__MBED_CMSIS_RTOS_CM -DTARGET_MCU_STM32 -D__FPU_PRESENT=1 -DUSE_HAL_DRIVER -DDEVICE_PORTIN=1 -DTARGET_RELEASE -DTARGET_STM -DUSBHOST_OTHER -DDEVICE_SERIAL_FC=1 -DDEVICE_USTICKER=1 -DDEVICE_WATCHDOG=1 -DTARGET_NAME=DMCB_60009 -DTARGET_REV_A -DDEVICE_SLEEP=1 -DTOOLCHAIN_GCC_ARM -DDEVICE_RESET_REASON=1 -DDEVICE_SPI=1 -DUSB_STM_HAL -DDEVICE_INTERRUPTIN=1 -DTARGET_MCU_STM32F4 -DDEVICE_SPISLAVE=1 -DDEVICE_SERIAL=1 -DDEVICE_FLASH=1 -DARM_MATH_CM4 -DMBED_BUILD_TIMESTAMP=1621347711.8 @.\BUILD\DMCB_60009\GCC_ARM-RELEASE-BUILD\.includes_f4d498d36883c907a6ba67972c99ba21.txt -include .\BUILD\DMCB_60009\GCC_ARM-RELEASE-BUILD\mbed_config.h -MD -MF BUILD\DMCB_60009\GCC_ARM-RELEASE-BUILD\fmt-7.1.3\src\os.d -o BUILD\DMCB_60009\GCC_ARM-RELEASE-BUILD\fmt-7.1.3\src\os.o ../fmt-7.1.3\src\os.cc
> [Error] clocale@53,11: '::lconv' has not been declared
> [Error] clocale@54,11: '::setlocale' has not been declared
> [Error] clocale@55,11: '::localeconv' has not been declared
> [Error] c++locale.h@60,24: 'setlocale' is not a member of 'std'
> [Error] c++locale.h@60,34: 'LC_NUMERIC' was not declared in this scope
> [Error] c++locale.h@67,7: 'setlocale' is not a member of 'std'
> [Error] c++locale.h@83,7: 'setlocale' is not a member of 'std'
> [DEBUG] Return: 1
> [DEBUG] Output: In file included from c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\arm-none-eabi\thumb\v7e-m+fp\softfp\bits\c++locale.h:41,
> [DEBUG] Output: from c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\bits\localefwd.h:40,
> [DEBUG] Output: from c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\locale:38,
> [DEBUG] Output: from ../fmt-7.1.3/include/fmt/locale.h:11,
> [DEBUG] Output: from c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\clocale:42,
> [DEBUG] Output: from ../fmt-7.1.3/include/fmt/os.h:17,
> [DEBUG] Output: from ../fmt-7.1.3\src\os.cc:13:
> [DEBUG] Output: c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\clocale:53:11: error: '::lconv' has not been declared
> [DEBUG] Output: 53 | using ::lconv;
> [DEBUG] Output: | ^~~~~
> [DEBUG] Output: c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\clocale:54:11: error: '::setlocale' has not been declared
> [DEBUG] Output: 54 | using ::setlocale;
> [DEBUG] Output: | ^~~~~~~~~
> [DEBUG] Output: c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\clocale:55:11: error: '::localeconv' has not been declared
> [DEBUG] Output: 55 | using ::localeconv;
> [DEBUG] Output: | ^~~~~~~~~~
> [DEBUG] Output: In file included from c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\bits\localefwd.h:40,
> [DEBUG] Output: from c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\locale:38,
> [DEBUG] Output: from ../fmt-7.1.3/include/fmt/locale.h:11,
> [DEBUG] Output: from c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\clocale:42,
> [DEBUG] Output: from ../fmt-7.1.3/include/fmt/os.h:17,
> [DEBUG] Output: from ../fmt-7.1.3\src\os.cc:13:
> [DEBUG] Output: c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\arm-none-eabi\thumb\v7e-m+fp\softfp\bits\c++locale.h: In function 'int std::__convert_from_v(int* const&, char*, int, const char*, ...)':
> [DEBUG] Output: c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\arm-none-eabi\thumb\v7e-m+fp\softfp\bits\c++locale.h:60:24: error: 'setlocale' is not a member of 'std'
> [DEBUG] Output: 60 | char* __old = std::setlocale(LC_NUMERIC, 0);
> [DEBUG] Output: | ^~~~~~~~~
> [DEBUG] Output: c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\arm-none-eabi\thumb\v7e-m+fp\softfp\bits\c++locale.h:60:34: error: 'LC_NUMERIC' was not declared in this scope
> [DEBUG] Output: 60 | char* __old = std::setlocale(LC_NUMERIC, 0);
> [DEBUG] Output: | ^~~~~~~~~~
> [DEBUG] Output: c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\arm-none-eabi\thumb\v7e-m+fp\softfp\bits\c++locale.h:67:7: error: 'setlocale' is not a member of 'std'
> [DEBUG] Output: 67 | std::setlocale(LC_NUMERIC, "C");
> [DEBUG] Output: | ^~~~~~~~~
> [DEBUG] Output: c:\program files (x86)\gnu tools arm embedded\9 2020-q2-update\arm-none-eabi\include\c++\9.3.1\arm-none-eabi\thumb\v7e-m+fp\softfp\bits\c++locale.h:83:7: error: 'setlocale' is not a member of 'std'
> [DEBUG] Output: 83 | std::setlocale(LC_NUMERIC, __sav);
> [DEBUG] Output: | ^~~~~~~~~
Any hints would be appreciated.