How to upgrade gcc-5 to gcc-7 in Ubuntu 16.04?

Question

This is a follow up question to How to install gcc-7 or clang 4.0?.

Following the answers therein, I was able to install gcc-7.2 by:

sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt-get update
sudo apt-get install -y gcc-7

However, after the install, the gcc/g++ command still defaults to 5.4.0

gcc --version
gcc (Ubuntu 5.4.0-6ubuntu1~16.04.5) 5.4.0 20160609

Is it possible/safe to have gcc-7 as the default and remove the old gcc-5.x files entirely?

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Note that, different from the said duplicate, I'm looking to remove/replace gcc-5.x instead of setting a symlink.

Solution

To answer my own question after much trial and error, it seems not safe to remove/replace the older gcc (gcc-5), because it will break a lot of dependencies to the extent of wiping out the entire tool chain.

For example, when trying to remove/replace gcc-5 as said in the comments with

Run update-alternatives to set gcc-7 as the default gcc version and the type sudo dpkg -r gcc-5

This generates an error:

dpkg: dependency problems prevent removal of gcc-5:
 g++-5 depends on gcc-5 (= 5.4.1-2ubuntu1~16.04).
 g++ depends on gcc-5 (>= 5.3.1-3~).
 gcc depends on gcc-5 (>= 5.3.1-3~).
 gfortran-5 depends on gcc-5 (= 5.4.1-2ubuntu1~16.04).

When adding these dependencies to the list of sudo dpkg -r items, an increasing number of packages need to be removed. e.g.

 ...
 r-base-dev depends on g++.
 build-essential depends on g++ (>= 4:5.2).

I haven't tried to find the largest set of packages that will be removed as described in How to uninstall a .deb package? . But it's pretty clear that most of the useful packages in the Ubuntu will be gone by then.

Comments

[muru]

You cannot remove gcc because it depends on gcc-5, and packages may break if they depend on gcc.