我尝试使用以下方法将gcc-6升级到gcc-8：
brew upgrade gcc
然后我得到了
Error: An exception occured within a build process:
RuntimeError: The requested Homebrew GCC was not installed. You must:
brew install gcc@6
但是，我已经通过自制软件安装了gcc-6。当我检查版本时，可以看到
gcc-6 (Homebrew gcc 6.2.0 --without-multilib) 6.2.0
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
即使我输入了提示brew install gcc@6的错误消息，我仍然得到：
brew install gcc@6
==> Installing dependencies for gcc@6: gmp, isl, mpfr, libmpc
==> Installing gcc@6 dependency: gmp
/usr/local/Homebrew/Library/Homebrew/utils/fork.rb:49:in `write': Broken pipe (Errno::EPIPE)
from /usr/local/Homebrew/Library/Homebrew/utils/fork.rb:49:in `puts'
from /usr/local/Homebrew/Library/Homebrew/utils/fork.rb:49:in `rescue in block (3 levels) in safe_fork'
from /usr/local/Homebrew/Library/Homebrew/utils/fork.rb:31:in `block (3 levels) in safe_fork'
from /usr/local/Homebrew/Library/Homebrew/utils/fork.rb:30:in `fork'
from /usr/local/Homebrew/Library/Homebrew/utils/fork.rb:30:in `block (2 levels) in safe_fork'
from /usr/local/Homebrew/Library/Homebrew/utils/fork.rb:27:in `open'
from /usr/local/Homebrew/Library/Homebrew/utils/fork.rb:27:in `block in safe_fork'
from /usr/local/Homebrew/Library/Homebrew/vendor/portable-ruby/2.3.7/lib/ruby/2.3.0/tmpdir.rb:89:in `mktmpdir'
from /usr/local/Homebrew/Library/Homebrew/utils/fork.rb:26:in `safe_fork'
from /usr/local/Homebrew/Library/Homebrew/formula_installer.rb:719:in `build'
from /usr/local/Homebrew/Library/Homebrew/formula_installer.rb:311:in `install'
from /usr/local/Homebrew/Library/Homebrew/formula_installer.rb:572:in `install_dependency'
from /usr/local/Homebrew/Library/Homebrew/formula_installer.rb:534:in `block in install_dependencies'
from /usr/local/Homebrew/Library/Homebrew/formula_installer.rb:534:in `each'
from /usr/local/Homebrew/Library/Homebrew/formula_installer.rb:534:in `install_dependencies'
from /usr/local/Homebrew/Library/Homebrew/formula_installer.rb:251:in `install'
from /usr/local/Homebrew/Library/Homebrew/cmd/install.rb:321:in `install_formula'
from /usr/local/Homebrew/Library/Homebrew/cmd/install.rb:253:in `block in install'
from /usr/local/Homebrew/Library/Homebrew/cmd/install.rb:251:in `each'
from /usr/local/Homebrew/Library/Homebrew/cmd/install.rb:251:in `install'
from /usr/local/Homebrew/Library/Homebrew/brew.rb:89:in `<main>'
Error: An exception occured within a build process:
RuntimeError: The requested Homebrew GCC was not installed. You must:
brew install gcc@6
这是递归的。有人知道吗？