Constexpr string: make it work for older libstdc++

.github/actions/install-hemelb-deps/action.yml

@@ -33,7 +33,7 @@ runs:
         libctemplate-dev
         ${{ inputs.extra_apt }}
 
-    - uses: actions/cache@v2
+    - uses: actions/cache@v3
       id: deps-cache
       with:
         path: ${{ env.deps_install_prefix }}

.github/actions/setup-compilers/action.yml

@@ -0,0 +1,30 @@
+name: 'Set CC and CXX env vars for the configured compiler'
+
+inputs:
+  compiler:
+    description: 'Use $suite-$version'
+    required: true
+
+runs:
+  using: 'composite'
+  steps:
+    - name: Set the env vars with some bash
+      shell: bash
+      run: |
+        set -ex
+        input=${{ inputs.compiler }}
+        split=(${input//-/ })
+        suite=${split[0]}
+        version=${split[1]}
+        case $suite in
+          gnu)
+            echo "CC=gcc-$version" >> $GITHUB_ENV
+            echo "CXX=g++-$version" >> $GITHUB_ENV
+            ;;
+          llvm)
+            echo "CC=clang-$version" >> $GITHUB_ENV
+            echo "CXX=clang++-$version" >> $GITHUB_ENV
+            echo "CXXFLAGS=-stdlib=libc++" >> $GITHUB_ENV
+            echo "LDFLAGS=-stdlib=libc++" >> $GITHUB_ENV
+            ;;
+        esac

.github/workflows/gmy-tool.yml

@@ -44,6 +44,7 @@ jobs:
     runs-on: ubuntu-22.04
 
     strategy:
+      fail-fast: false
       matrix:
         python-version:
           - '3.8'

.github/workflows/main-app.yml

@@ -24,9 +24,6 @@ env:
   hemelb_install_prefix: ${{github.workspace}}/install
   # VMs have 2 cores, double as compilation often IO bound
   CMAKE_BUILD_PARALLEL_LEVEL: 4
-  # Latest Gnu compiler on Ubuntu 22.04
-  CC: gcc-12
-  CXX: g++-12
 
 jobs:
   basic-checks:
@@ -46,9 +43,18 @@ jobs:
     name: Build in fluid-only mode
     runs-on: ubuntu-22.04
 
+    strategy:
+      fail-fast: false
+      matrix:
+        compiler: [gnu-11, gnu-12, gnu-13]
+
     steps:
     - uses: actions/checkout@v3
 
+    - uses: ./.github/actions/setup-compilers
+      with:
+        compiler: ${{ matrix.compiler }}
+
     - uses: ./.github/actions/install-hemelb-deps
       with:
         name: fluidonly
@@ -128,9 +134,18 @@ jobs:
     name: Build in RBC mode
     runs-on: ubuntu-22.04
 
+    strategy:
+      fail-fast: false
+      matrix:
+        compiler: [gnu-11, gnu-12, gnu-13]
+
     steps:
     - uses: actions/checkout@v3
 
+    - uses: ./.github/actions/setup-compilers
+      with:
+        compiler: ${{ matrix.compiler }}
+
     - uses: ./.github/actions/install-hemelb-deps
       with:
         name: rbc

.github/workflows/py-hemetools.yml

@@ -41,6 +41,7 @@ jobs:
     runs-on: ubuntu-22.04
 
     strategy:
+      fail-fast: false
       matrix:
         python-version:
           - '3.8'

Code/util/ct_string.h

@@ -8,6 +8,7 @@
 
 #include <cstddef>
 #include <string>
+#include <string_view>
 #include <type_traits>
 
 namespace hemelb {
@@ -33,6 +34,9 @@ namespace hemelb {
         constexpr std::string str() const {
             return {str_, N};
         }
+        constexpr std::string_view view() const {
+            return {str_, N};
+        }
 
         constexpr char const *c_str() const {
             return str_;
@@ -45,12 +49,12 @@ namespace hemelb {
 
     template <std::size_t M, std::size_t N>
     constexpr auto operator==(const ct_string<M>& left, const ct_string<N>& right) {
-        return left.str() == right.str();
+        return left.view() == right.view();
     }
 
     template <std::size_t N>
     constexpr auto operator==(const ct_string<N>& left, const char* right) {
-        return left.str() == std::string{right};
+        return left.view() == std::string_view{right};
     }
     template <std::size_t N>
     constexpr auto operator==(const char* left, const ct_string<N> right) {