build: fix a target that does not compile, and advice that discards a build tree (fixes #604, fixes #592)

CHANGELOG.md

@@ -241,6 +241,40 @@ API surface).
 
 ### Fixed
 
+- **`bank_gui_qml_tests` did not compile.** The target compiles
+  `examples/common/testkit/testkit_main.cpp`, whose `#include
+  <testkit/log_level.hpp>` resolves only against the repository's `tests/`
+  directory, and it reached that directory through nothing: it links
+  `Catch2::Catch2` directly, where its sibling `bank_gui_tests` links
+  `morph_test_main` (which carries the include path publicly, via
+  `morph_test_log_level`). Reproduced on `26bfdb8f` with the `linux-everything`
+  preset, the only configuration that switches `MORPH_BUILD_BANK_GUI` on:
+  `fatal error: 'testkit/log_level.hpp' file not found`. The target now also
+  links `morph_test_log_level` — the same split, for the same reason, that
+  `morph_qt_tests` already uses for a suite that owns its own `main`. It
+  builds, its 32 assertions pass offscreen, and `--log-level` (the helper that
+  header declares) is live on it. No CI job configures
+  `MORPH_BUILD_BANK_GUI`, so this fix is not yet gated by anything; morph#605
+  covers that separately. morph#604.
+
+- **The compiler-cache module told you to throw away your build directory, and
+  for a preset it did not even work.** When a launcher is already pinned in the
+  CMake cache, `cmake/CompileCache.cmake` advised "reconfigure with `--fresh`
+  to let this module choose instead". Reconfiguring the *same* directory with
+  `-UCMAKE_C_COMPILER_LAUNCHER -UCMAKE_CXX_COMPILER_LAUNCHER` removes exactly
+  the two entries the module's guard reads, reaches the same selection, and
+  leaves the build directory standing; `--fresh` deletes `CMakeFiles/` and,
+  unless the generator is re-passed, drops `CMAKE_GENERATOR` back to the
+  platform default (measured: a `-G Ninja` tree came back `Unix Makefiles`
+  with a stale `build.ninja` beside the new `Makefile`). And where the pin
+  comes from a **preset** — one of the three sources the message itself
+  names — `--fresh` clears nothing at all, because the preset re-applies its
+  `cacheVariables` on the very reconfigure `--fresh` triggers: the tree is
+  gone and the launcher is still pinned. The message now names `-U` first and
+  keeps `--fresh` with its actual cost and its actual limits. Behaviour is
+  unchanged; only the advice is. Measured on an isolated harness that does
+  nothing but `include()` the module. morph#592.
+
 - **`equation()` no longer walks a shared derivation once per path.**
   `EquationRenderer::assignLabels` was the one traversal without a visited set,
   so a node reachable by *k* displayed paths was walked *k* times. Since the

cmake/CompileCache.cmake

@@ -58,9 +58,36 @@ if(DEFINED CMAKE_CXX_COMPILER_LAUNCHER OR DEFINED CMAKE_C_COMPILER_LAUNCHER)
     # A build tree configured before this module existed carries the launcher of
     # the day in its cache, and would keep it forever without a word about why
     # the selection below never runs.
+    #
+    # -U before --fresh, because the guard above reads exactly two cache entries
+    # and -U deletes exactly those two: the reconfigure then falls through to
+    # the selection below without discarding the build directory. --fresh
+    # reaches the same place for a -D or an older configure, but it deletes
+    # CMakeFiles/ and, unless the generator is re-passed, drops CMAKE_GENERATOR
+    # back to the platform default on the way -- a `cmake -S . -B build --fresh`
+    # on a tree configured `-G Ninja` comes back a Makefile tree with a stale
+    # build.ninja still sitting in it.
+    #
+    # What -U saves is the configure state -- the populated _deps tree, the
+    # generator, generated sources, cached find_ results -- and not, note, the
+    # compiling: a launcher is part of every C/C++ compile command, so the next
+    # build recompiles those TUs whichever route got here. That is the honest
+    # size of it.
+    #
+    # The case that makes the ordering matter rather than merely cheaper is the
+    # "a preset" one this very message names, where --fresh does not work at
+    # all: the preset re-applies its cacheVariables on the reconfigure that
+    # --fresh triggers, so the launcher comes back pinned and the tree is gone
+    # for nothing. -U clears it there too, until the preset is next run.
+    # All of the above measured on an isolated harness, morph#592.
     if(DEFINED CACHE{CMAKE_CXX_COMPILER_LAUNCHER} OR DEFINED CACHE{CMAKE_C_COMPILER_LAUNCHER})
         message(STATUS "[cache] That value comes from the CMake cache (a -D, a preset, or an older configure); "
-                       "reconfigure with --fresh to let this module choose instead.")
+                       "to let this module choose instead, reconfigure the same build directory with "
+                       "-UCMAKE_C_COMPILER_LAUNCHER -UCMAKE_CXX_COMPILER_LAUNCHER, which clears exactly those "
+                       "two entries and leaves the build directory standing. (--fresh clears them too when they "
+                       "came from a -D or an older configure, at the price of the whole build directory; when "
+                       "they come from a preset it clears nothing, because the preset re-applies them on the "
+                       "same reconfigure.)")
     endif()
     return()
 endif()

examples/bank/CMakeLists.txt

@@ -275,6 +275,14 @@ if(MORPH_BUILD_TESTS)
                 ${PROJECT_SOURCE_DIR}/examples/common)
             target_link_libraries(bank_gui_qml_tests PRIVATE
                 bank_gui_lib Catch2::Catch2
+                # Our own main() owns the QGuiApplication, so this cannot link
+                # morph_test_main the way bank_gui_tests above does -- that
+                # library carries a `main` of its own. morph_test_log_level is
+                # the half of it that is wanted here: the --log-level helper
+                # testkit_main.cpp calls, and the tests/ include directory that
+                # puts <testkit/log_level.hpp> on the path. Same split, for the
+                # same reason, as morph_qt_tests (tests/qt/CMakeLists.txt).
+                morph_test_log_level
                 Qt6::Gui Qt6::Qml Qt6::Quick Qt6::QuickControls2)
             target_compile_features(bank_gui_qml_tests PRIVATE cxx_std_23)
             target_compile_definitions(bank_gui_qml_tests PRIVATE