CMake: Simplify default CMAKE_BUILD_TYPE logic

Previously we had four-ish locations where the CMAKE_BUILD_TYPE was
force set.
Twice in QtBuildInternalsExtra.cmake via
qt_internal_force_set_cmake_build_type_conditionally(), depending on
some conditions. This was executed right at
 find_package(Qt6 COMPONENTS BuildInternals)
time.

And twice in qt_internal_set_default_build_type() via
qt_build_repo_begin() / qt_prepare_standalone_project() that goes
through QtSetup.cmake. This was executed only if the relevant functions
were called, rather than directly at find_package() time.

The exact logic of which build type ended up being set was very
confusing.

Refactor the code to decide the build type in one single location
when qt_build_repo_begin() / qt_prepare_standalone_project() are
called, rather than directly at find_package() time.

The actual logic when we override the build type depends on many
factors:
- when an explicit CMAKE_BUILD_TYPE is given, honor it, unless it's
  a multi-config build
- when it's a multi-config build, don't set any CMAKE_BUILD_TYPE,
  use the value of CMAKE_CONFIGURATION_TYPES
- when it's a qtbase build, compute a default unless an explicit value
  was given
  - the default is Debug if FEATURE_developer_build is ON
  - otherwise the default is Release
- when it's a top-level build, only choose a build type for qtbase
- when it's another repo build, use the original build type unless
  another was given explicitly (including in a top-level build)
- when it's a standalone tests build
   - if qt is multi-config, the tests will be single config, due to
     various CI failure reasons, this hasn't changed
   - if qt is single config, use the original unless an explicit
     value was given
- when it's a single standalone test build, use the original unless
  an explicit value was given

To determine when an explicit CMAKE_BUILD_TYPE was given in contrast
to when it was default initialized, we now have one single function
that uses a few heuristics.
The heuristics are needed because we can't reliably determine an
explicitly given 'Debug' build on Windows, because CMake default
initializes to that.

The heuristics include:
- checking whether CMAKE_BUILD_TYPE_INIT is different from
  CMAKE_BUILD_TYPE
- checking what the CMAKE_BUILD_TYPE was before the first project()
  call when CMake default initializes
  - we save the previous value in the qt.toolchain.cmake file
  - also in QtAutoDetect during qtbase configuration
  - also when building the sqldrivers project
- honoring the value of QT_NO_FORCE_SET_CMAKE_BUILD_TYPE

As a result of the above changes, the build type will be set exactly
zero or one times, for a particular build directory.

Note that the configure script also has some logic on which
CMAKE_BUILD_TYPE / CMAKE_CONFIGURATION_TYPES to pass to CMake
depending on whether -debug / -release / -debug-and-release /
-force-debug-info were passed. But once the values are passed,
CMake will honor them.

Amends 48841c34d2e86a741ec9992b9704c0fa5973503c
Amends 8c912cddebe544010e7da3f87af5b21f3328d7ec

Pick-to: 6.7
Task-number: QTBUG-114958
Task-number: QTBUG-120436
Change-Id: I30db14d1e8e9ff9bd2d7ea1d2256cdeb9493ca0d
Reviewed-by:  Alexey Edelev <alexey.edelev@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

4 files changed, 122 insertions, 77 deletions

diff --git a/cmake/QtBuildHelpers.cmake b/cmake/QtBuildHelpers.cmake
index 26c2baa5aa..dc83954e48 100644
--- a/cmake/QtBuildHelpers.cmake
+++ b/cmake/QtBuildHelpers.cmake
@@ -381,13 +381,13 @@ macro(qt_internal_setup_build_and_global_variables)
     qt_internal_compute_features_from_possible_inputs()
 
     # Depends on qt_internal_compute_features_from_possible_inputs
-    qt_internal_set_default_build_type()
+    qt_internal_set_cmake_build_type()
 
     qt_internal_set_message_log_level(CMAKE_MESSAGE_LOG_LEVEL)
     qt_internal_unset_extra_build_internals_vars()
     qt_internal_get_generator_is_multi_config()
 
-    # Depends on qt_internal_set_default_build_type
+    # Depends on qt_internal_set_cmake_build_type
     qt_internal_setup_cmake_config_postfix()
 
     qt_internal_setup_position_independent_code()
diff --git a/cmake/QtBuildInternalsExtra.cmake.in b/cmake/QtBuildInternalsExtra.cmake.in
index c035a216f5..09db646507 100644
--- a/cmake/QtBuildInternalsExtra.cmake.in
+++ b/cmake/QtBuildInternalsExtra.cmake.in
@@ -132,43 +132,5 @@ if(NOT DEFINED QT_MAX_NEW_POLICY_CMAKE_VERSION)
     set(QT_MAX_NEW_POLICY_CMAKE_VERSION "@max_new_policy_version@")
 endif()
 
-get_property(__qt_internal_extras_is_multi_config GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
-
-# We want the same build type to be used when configuring all Qt repos or standalone
-# tests or single tests.
-# To do that, we need to force-set the CMAKE_BUILD_TYPE cache var because CMake itself
-# initializes it with the value of CMAKE_BUILD_TYPE_INIT at the start of project
-# configuration, so we need to override it.
-# Note the value of CMAKE_BUILD_TYPE_INIT is different based on the platform, most
-# Linux and macOS platforms will have it empty, but Windows platforms will have a value.
-#
-# We can't reliably differentiate between a value set on the command line by the user
-# and one set by CMake, so we use a few heuristics:
-# 1) When using a qt.toolchain.cmake file, we rely on the toolchain file to tell us
-#    if a value was set by the user at initial configure time. On a 2nd run there will
-#    always be a value in the cache, but at that point we've already set it to whatever it needs
-#    to be.
-# 2) If a toolchain file is not used, we rely on the value of the CMake internal
-#    CMAKE_BUILD_TYPE_INIT variable.
-#    This won't work reliably on Windows where CMAKE_BUILD_TYPE_INIT is non-empty.
-#
-# Both cases won't handle an empty "" config set by the user, but we claim that's an
-# unsupported config when building Qt.
-#
-# Allow an opt out when QT_NO_FORCE_SET_CMAKE_BUILD_TYPE is set.
-# Finally, don't set the variable if a multi-config generator is used. This can happen
-# when qtbase is built with a single config, but a test is built with a multi-config generator.
-function(qt_internal_force_set_cmake_build_type_conditionally value)
-    # STREQUAL check needs to be expanded variables because an undefined var is not equal to an
-    # empty defined var.
-    if("${CMAKE_BUILD_TYPE}" STREQUAL "${CMAKE_BUILD_TYPE_INIT}"
-        AND NOT __qt_toolchain_cmake_build_type_before_project_call
-        AND NOT QT_NO_FORCE_SET_CMAKE_BUILD_TYPE
-        AND NOT __qt_internal_extras_is_multi_config)
-        set(CMAKE_BUILD_TYPE "${value}" CACHE STRING "Choose the type of build." FORCE)
-        set(__qt_build_internals_cmake_build_type "${value}" PARENT_SCOPE)
-    endif()
-endfunction()
-
 # Extra set of exported variables
 @QT_EXTRA_BUILD_INTERNALS_VARS@
diff --git a/cmake/QtBuildOptionsHelpers.cmake b/cmake/QtBuildOptionsHelpers.cmake
index 1d5002bfd1..fbf2aa2b79 100644
--- a/cmake/QtBuildOptionsHelpers.cmake
+++ b/cmake/QtBuildOptionsHelpers.cmake
@@ -1,34 +1,129 @@
 # Copyright (C) 2023 The Qt Company Ltd.
 # SPDX-License-Identifier: BSD-3-Clause
 
-macro(qt_internal_set_default_build_type)
+# Try to detect if CMAKE_BUILD_TYPE is default initialized by CMake, or it was set by the user.
+#
+# CMake initializes CMAKE_BUILD_TYPE to the value of CMAKE_BUILD_TYPE_INIT during the first
+# project() call if CMAKE_BUILD_TYPE is empty.
+#
+# Unfortunately on most Windows platforms, it defaults to 'Debug', so we can't differentiate
+# between a 'Debug' value set on the command line by the user, a value set by the project, or if it
+# was default initialized.
+# We need to rely on heuristics to determine that.
+#
+# We try to check the value of CMAKE_BUILD_TYPE before the first project() call by inspecting
+# various variables:
+# 1) When using a qt.toolchain.cmake file, we rely on the toolchain file to tell us
+#    if a value was set by the user at initial configure time via the
+#    __qt_toolchain_cmake_build_type_before_project_call variable. On a 2nd run there will
+#    always be a value in the cache, but at that point we've already set it to whatever it needs
+#    to be.
+# 2) Whe configuring qtbase, a top-level qt, or a standalone project we rely on one of the following
+#    variables being set:
+#    - __qt_auto_detect_cmake_build_type_before_project_call (e.g for qtbase)
+#    - __qt_internal_standalone_project_cmake_build_type_before_project_call (e.g for sqldrivers)
+# 3) When using a multi-config generator, we assume that the CMAKE_BUILD_TYPE is not default
+#    initialized.
+# 4) The user can also force the build type to be considered non-default-initialized by setting
+#    QT_NO_FORCE_SET_CMAKE_BUILD_TYPE to TRUE. It has weird naming that doesn't quite correspond
+#    to the meaning, but it's been called like that for a while now and I'm hesitant to change
+#    the name in case it's used by various projects.
+#
+# The code doesn't handle an empty "" config set by the user, but we claim that's an
+# unsupported config when building Qt.
+function(qt_internal_is_cmake_build_type_default_initialized_heuristic out_var)
+    get_property(is_multi_config GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
+    get_cmake_property(aready_force_set _qt_build_internals_cmake_build_type_set)
+
+    if(
+        # Set by CMake's Platform/Windows-MSVC.cmake when CMAKE_BUILD_TYPE is empty
+        # The STREQUAL check needs to have expanded variables because an undefined var is not equal
+        # to an empty defined var.
+        "${CMAKE_BUILD_TYPE}" STREQUAL "${CMAKE_BUILD_TYPE_INIT}"
+
+        # Set by qt_internal_force_set_cmake_build_type()
+        AND aready_force_set MATCHES "NOTFOUND"
+
+        # Set by qt_auto_detect_cmake_build_type()
+        AND NOT __qt_auto_detect_cmake_build_type_before_project_call
+
+        # Set by sqldrivers project
+        AND NOT __qt_internal_standalone_project_cmake_build_type_before_project_call
+
+        # Set by qt.toolchain.cmake
+        AND NOT __qt_toolchain_cmake_build_type_before_project_call
+
+        # Set by user explicitily
+        AND NOT QT_NO_FORCE_SET_CMAKE_BUILD_TYPE
+
+        # Set in multi-config builds
+        AND NOT is_multi_config)
+
+        set(${out_var} TRUE PARENT_SCOPE)
+    else()
+        set(${out_var} FALSE PARENT_SCOPE)
+    endif()
+endfunction()
+
+function(qt_internal_force_set_cmake_build_type value)
+    cmake_parse_arguments(PARSE_ARGV 1 arg
+        "SHOW_MESSAGE"
+        ""
+        ""
+    )
+    _qt_internal_validate_all_args_are_parsed(arg)
+
+    set(CMAKE_BUILD_TYPE "${value}" CACHE STRING "Choose the type of build." FORCE)
+    set_property(CACHE CMAKE_BUILD_TYPE
+        PROPERTY STRINGS
+        "Debug" "Release" "MinSizeRel" "RelWithDebInfo") # Set the possible values for cmake-gui.
+    if(arg_SHOW_MESSAGE)
+        message(STATUS "Force setting build type to '${value}'.")
+    endif()
+    set_property(GLOBAL PROPERTY _qt_build_internals_cmake_build_type_set "${value}")
+endfunction()
+
+# Only override the build type if it was default initialized by CMake.
+function(qt_internal_force_set_cmake_build_type_if_cmake_default_initialized value)
+    qt_internal_is_cmake_build_type_default_initialized_heuristic(is_default_cmake_build_type)
+    if(is_default_cmake_build_type)
+        qt_internal_force_set_cmake_build_type("${value}" SHOW_MESSAGE)
+    endif()
+endfunction()
+
+function(qt_internal_set_cmake_build_type)
+    # When building standalone tests against a multi-config Qt, we want to configure the tests with
+    # the first multi-config configuration, rather than use CMake's default configuration.
+    # In the case of Windows, we definitely don't want it to default to Debug, because that causes
+    # issues in the CI.
+    if(QT_BUILD_STANDALONE_TESTS AND QT_MULTI_CONFIG_FIRST_CONFIG)
+        qt_internal_force_set_cmake_build_type_if_cmake_default_initialized(
+            "${QT_MULTI_CONFIG_FIRST_CONFIG}")
+
+    # We want the same build type to be used when configuring all Qt repos or standalone
+    # tests or single tests, so we reuse the initial build type set by qtbase.
+    # __qt_internal_initial_qt_cmake_build_type is saved in QtBuildInternalsExtra.cmake.in.
+    elseif(__qt_internal_initial_qt_cmake_build_type)
+        qt_internal_force_set_cmake_build_type_if_cmake_default_initialized(
+            "${__qt_internal_initial_qt_cmake_build_type}")
+
+    # Default to something sensible when configuring qtbase / top-level.
+    else()
+        qt_internal_set_qt_appropriate_default_cmake_build_type()
+    endif()
+endfunction()
+
+# Sets a default cmake build type for qtbase / top-level.
+macro(qt_internal_set_qt_appropriate_default_cmake_build_type)
     set(_default_build_type "Release")
     if(FEATURE_developer_build)
         set(_default_build_type "Debug")
     endif()
 
-    # Try to detect if an explicit CMAKE_BUILD_TYPE was set by the user.
-    # CMake sets CMAKE_BUILD_TYPE_INIT to Debug on most Windows platforms and doesn't set
-    # anything for UNIXes. CMake assigns CMAKE_BUILD_TYPE_INIT to CMAKE_BUILD_TYPE during
-    # the first project() call, if CMAKE_BUILD_TYPE had no previous value.
-    # We use extra information about the state of CMAKE_BUILD_TYPE before the first
-    # project() call that's set in QtAutoDetect.cmake or manually in a project via the
-    # __qt_internal_standalone_project_cmake_build_type_before_project_call variable (as done
-    # for the qtbase sqldrivers project).
-    # STREQUAL check needs to have expanded variables because an undefined var is not equal
-    # to an empty defined var.
-    # See also qt_internal_force_set_cmake_build_type_conditionally which is used
-    # to set the build type when building other repos or tests.
-    if("${CMAKE_BUILD_TYPE}" STREQUAL "${CMAKE_BUILD_TYPE_INIT}"
-        AND NOT __qt_auto_detect_cmake_build_type_before_project_call
-        AND NOT __qt_build_internals_cmake_build_type
-        AND NOT __qt_internal_standalone_project_cmake_build_type_before_project_call
-        AND NOT CMAKE_CONFIGURATION_TYPES)
-      message(STATUS "Setting build type to '${_default_build_type}' as none was specified.")
-      set(CMAKE_BUILD_TYPE "${_default_build_type}" CACHE STRING "Choose the type of build." FORCE)
-      set_property(CACHE CMAKE_BUILD_TYPE
-          PROPERTY STRINGS
-          "Debug" "Release" "MinSizeRel" "RelWithDebInfo") # Set the possible values for cmake-gui.
+    qt_internal_is_cmake_build_type_default_initialized_heuristic(is_default_cmake_build_type)
+    if(is_default_cmake_build_type)
+        qt_internal_force_set_cmake_build_type("${_default_build_type}")
+        message(STATUS "Setting build type to '${_default_build_type}' as none was specified.")
     elseif(CMAKE_CONFIGURATION_TYPES)
         message(STATUS "Building for multiple configurations: ${CMAKE_CONFIGURATION_TYPES}.")
         message(STATUS "Main configuration is: ${QT_MULTI_CONFIG_FIRST_CONFIG}.")
@@ -43,7 +138,7 @@ macro(qt_internal_set_default_build_type)
             )
         endif()
     else()
-        message(STATUS "CMAKE_BUILD_TYPE was set to: '${CMAKE_BUILD_TYPE}'")
+        message(STATUS "CMAKE_BUILD_TYPE was already explicitly set to: '${CMAKE_BUILD_TYPE}'")
     endif()
 endmacro()
 
diff --git a/cmake/QtPostProcessHelpers.cmake b/cmake/QtPostProcessHelpers.cmake
index b9a8d0d889..0384ee6d44 100644
--- a/cmake/QtPostProcessHelpers.cmake
+++ b/cmake/QtPostProcessHelpers.cmake
@@ -556,9 +556,8 @@ function(qt_generate_build_internals_extra_cmake_code)
         if(CMAKE_BUILD_TYPE)
             string(APPEND QT_EXTRA_BUILD_INTERNALS_VARS
                 "
+# Used by qt_internal_set_cmake_build_type.
 set(__qt_internal_initial_qt_cmake_build_type \"${CMAKE_BUILD_TYPE}\")
-qt_internal_force_set_cmake_build_type_conditionally(
-    \"\${__qt_internal_initial_qt_cmake_build_type}\")
 ")
         endif()
         if(CMAKE_CONFIGURATION_TYPES)
@@ -580,17 +579,6 @@ qt_internal_force_set_cmake_build_type_conditionally(
             string(APPEND QT_EXTRA_BUILD_INTERNALS_VARS
                 "\nset(QT_MULTI_CONFIG_FIRST_CONFIG \"${QT_MULTI_CONFIG_FIRST_CONFIG}\")\n")
         endif()
-        # When building standalone tests against a multi-config Qt, we want to choose the first
-        # configuration, rather than use CMake's default value.
-        # In the case of Windows, we definitely don't it to default to Debug, because that causes
-        # issues in the CI.
-        if(multi_config_specific)
-            string(APPEND QT_EXTRA_BUILD_INTERNALS_VARS "
-if(QT_BUILD_STANDALONE_TESTS)
-    qt_internal_force_set_cmake_build_type_conditionally(
-        \"\${QT_MULTI_CONFIG_FIRST_CONFIG}\")
-endif()\n")
-        endif()
 
         if(CMAKE_CROSS_CONFIGS)
             string(APPEND ninja_multi_config_specific