Can't compile: #error "Unsupported Compiler!" /* Compiler defines not found */

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I'm trying to compile a new project. I've not added any additional code to main.cpp

My platformio.ini file is as follows:

[env:stm8sblack]
platform = ststm8
board = stm8sblack
framework = arduino

I'm using Visual Studio Code on debian buster The compiler output is:

Compiling .pio/build/stm8sblack/src/main.cpp.rel
In file included from /home/james/.platformio/packages/framework-arduinoststm8/cores/sduino/Arduino.h:32,
                 from src/main.cpp:1:
/home/james/.platformio/packages/framework-arduinoststm8/STM8S_StdPeriph_Driver/inc/stm8s.h:90:3: error: #error "Unsupported Compiler!"
  #error "Unsupported Compiler!"          /* Compiler defines not found */

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I've tried to make it use SDCC. I've also tried renaming main.cpp to main.c as others have suggested (and I'm not sure how I could have ended up with the wrong file extension), Neither of these actions have helped.

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Closed because it's a duplicate of #21