`brew upgrade <package>`: "<package> cannot be built with any available compilers"

Question

For example, I tried to run brew upgrade ruby-build.

That failed with error:

CompilerSelectionError: ruby-build cannot be built with any available compilers.
Install GNU's GCC
  brew install gcc

When I ran brew install gcc , I got:

Warning: Building gcc from source:
  The bottle needs the Xcode CLT to be installed.
xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun
Error: An exception occurred within a child process:
  CompilerSelectionError: gcc cannot be built with any available compilers.
Install GNU's GCC
  brew install gcc

How can I upgrade ruby-build via brew?

Answer 1

I ran this command and it now works:

xcode-select --install