diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 6af29fd..1b1cf14 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -3,7 +3,7 @@ on: [push, pull_request]
 
 jobs:
   linux:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     if: github.event_name != 'pull_request' ||
       github.event.pull_request.head.repo.full_name !=
       github.event.pull_request.base.repo.full_name
diff --git a/CHANGELOG.md b/CHANGELOG.md
index ee4c34b..b715c92 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,6 +23,7 @@ The format is based on [Keep a Changelog], and this project adheres to
 - Include URL to README in usage if manpage is not being built.
 - Turn into a Nix Flake.
 - Improve serializer to be more robust in end-of-stream conditions.
+- Bump requirements to require at least GCC version 9.
 
 ### Removed
 - Badges (eg. LGTM and build status) in README and Hydra.
diff --git a/INSTALL.adoc b/INSTALL.adoc
index 9219de6..506365b 100644
--- a/INSTALL.adoc
+++ b/INSTALL.adoc
@@ -11,8 +11,8 @@ toc::[]
 * https://github.com/jbeder/yaml-cpp[yaml-cpp], at least version 0.5.0
   Requirement will be removed in *ip2unix* version 3, since the YAML rule file
   format is deprecated.
-* {cpp} compiler supporting {cpp}17 (https://gcc.gnu.org/[GNU G++] version 7.0
-  onwards).
+* {cpp} compiler supporting {cpp}17 (eg. https://gcc.gnu.org/[GNU C++ compiler]
+  version 9.0 or https://clang.llvm.org/[Clang] version 7 or newer).
 * https://www.python.org/[Python] 3, at least version 3.6 is needed for running
   the integration tests.
 
@@ -145,11 +145,11 @@ $ sudo pacman -S python-pytest
 $ meson build
 ---------------------------------------------------------------------
 
-If you want to specify a different compiler executable, eg. `g++-7`:
+If you want to specify a different compiler executable, eg. `g++-9`:
 
 [source,sh-session]
 ---------------------------------------------------------------------
-$ CXX=g++-7 meson build
+$ CXX=g++-9 meson build
 ---------------------------------------------------------------------
 
 Compile:
diff --git a/flake.nix b/flake.nix
index bee7787..acf3c4b 100644
--- a/flake.nix
+++ b/flake.nix
@@ -322,7 +322,7 @@
         };
 
         gcc = {
-          minVersion = "7";
+          minVersion = "9";
           matchAttr = builtins.match "gcc([0-9]+)Stdenv";
           getVersion = attr: attr.cc.version;
           getStdenv = lib.id;