Say C++17 everywhere the project describes itselfchore/cpp17-descriptions

The move to C++17 left a trail of older claims. The Runner's own
.clang-format still said c++20; the result is byte for byte the same under
either value. Premake and Xmake still compiled the library as C++20 and
the main library as C++14, which meant their CI jobs were not testing the
language the project promises. Both now use C++17 for every target.

The rest is text: the Meson pkg-config description claimed C++14, comments
in the CMake and Meson files explained requirements that no longer exist,
.clang-tidy gave C++14 as the reason for two exclusions, issue templates
asked for C++20 samples, and the example headers named -std=c++14 and
catch2-with-main. The release notes and the migration guide state the
C++17 minimum. Doxygen's brief called the project "Catch2's test API",
which presents it as Catch2. Meson declared only BSL-1.0; it now names both
licenses of the tree. The amalgamated files are regenerated.

3 files changed, 5 insertions, 7 deletions

diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index a5524a38..33bbc4b5 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -31,9 +31,8 @@ if(CATCH3_BUILD_SURROGATES)
   # Extracts all non-helper (e.g. Progmasoft/Catch3/All.hpp) headers from the
   # Catch3 target, and returns them through the argument.
   function(ExtractCatch3Headers OutArg)
-    # The engine's headers are the ones that come from Catch2. The other
-    # headers need C++20; the surrogates check the engine's headers at the
-    # language level of the build.
+    # The engine's headers are the ones that come from Catch2; the surrogates
+    # check them at the language level of the build.
     file(STRINGS "${CATCH_DIR}/tools/scripts/upstreamFiles.txt" upstreamFiles REGEX "^src/")
     get_target_property(targetSources Catch3 SOURCES)
     foreach(Source ${targetSources})
@@ -153,8 +152,7 @@ add_executable(SelfTest ${TEST_SOURCES} ${TEST_HEADERS})
 target_include_directories(SelfTest PRIVATE ${SELF_TEST_DIR})
 target_link_libraries(SelfTest PRIVATE Catch3WithMain)
 
-# Exercise the C++20 Progmasoft layer independently so the upstream Catch2
-# SelfTest continues to compile at its supported baseline language level.
+# Exercise the Progmasoft layer independently of the upstream Catch2 SelfTest.
 add_executable(ProgmasoftSelfTest
   Progmasoft/Generators.tests.cpp
   Progmasoft/Property.tests.cpp
diff --git a/tests/ExtraTests/CMakeLists.txt b/tests/ExtraTests/CMakeLists.txt
index e6488650..232b86dc 100644
--- a/tests/ExtraTests/CMakeLists.txt
+++ b/tests/ExtraTests/CMakeLists.txt
@@ -430,7 +430,7 @@ add_executable(AmalgamatedTestCompilation
   ${CATCH_DIR}/extras/catch_amalgamated_third_party.hpp
 )
 target_include_directories(AmalgamatedTestCompilation PRIVATE ${CATCH_DIR}/extras)
-# The amalgamated files hold the runner, which needs C++20.
+# The amalgamated files hold the runner, which needs C++17.
 target_compile_features(AmalgamatedTestCompilation PRIVATE cxx_std_17)
 
 add_test(NAME AmalgamatedFileTest COMMAND AmalgamatedTestCompilation --no-history)
diff --git a/tests/meson.build b/tests/meson.build
index 95556f06..6ec807d7 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -68,7 +68,7 @@ self_test = executable(
 
 test('SelfTest', self_test, args: ['--no-history'])
 
-# The Progmasoft tests need C++20; SelfTest keeps the C++ mode it has upstream.
+# The Progmasoft tests need C++17; SelfTest keeps the project's C++ mode.
 progmasoft_property_tests = executable(
   'ProgmasoftPropertyTests',
   [