First off: recommended usage of CMake is to always specify CMAKE_BUILD_TYPE explicitly on the command line (if and only if using a single-configuration generator). Your use case deviates from this best practice, so treat this answer as "how you can do it," not necessarily as "how you should do it."

To address the first issue, you should be able to do this early in your CMakeList:

if(NOT CMAKE_BUILD_TYPE)
  set(CMAKE_BUILD_TYPE Release)
endif()

set(CMAKE_CXX_FLAGS "-Wall -Wextra")
set(CMAKE_CXX_FLAGS_DEBUG "-g")
set(CMAKE_CXX_FLAGS_RELEASE "-O3")

This will make sure that if you do not specify a build type at all, it will default to "Release" and thus CMAKE_CXX_FLAGS_RELEASE will be used.

The second one is harder to tackle. Variables passed from the command line (such as CMAKE_BUILD_TYPE=Debug) are cached by CMake and thus re-used in subsequent invocations (that is necessary, since CMake can re-trigger itself if you modify its inputs between builds).

The "proper" solution is to make the user switch the build type explicitly again, using cmake .. -DCMAKE_BUILD_TYPE=Release. If that is acceptable for you, do it.

I am currently unable to think of a way to make this "reset" implicit; I will expand the answer if I find one.

First off: recommended usage of CMake is to always specify CMAKE_BUILD_TYPE explicitly on the command line (if and only if using a single-configuration generator). Your use case deviates from this best practice, so treat this answer as "how you can do it," not necessarily as "how you should do it."

To address the first issue, you should be able to do this early in your CMakeList:

if(NOT CMAKE_BUILD_TYPE)
  set(CMAKE_BUILD_TYPE Release)
endif()

set(CMAKE_CXX_FLAGS "-Wall -Wextra")
set(CMAKE_CXX_FLAGS_DEBUG "-g")
set(CMAKE_CXX_FLAGS_RELEASE "-O3")

This will make sure that if you do not specify a build type at all, it will default to "Release" and thus CMAKE_CXX_FLAGS_RELEASE will be used.

The second one is harder to tackle. Variables passed from the command line (such as CMAKE_BUILD_TYPE=Debug) are cached by CMake and thus re-used in subsequent invocations (that is necessary, since CMake can re-trigger itself if you modify its inputs between builds).

The only solution is to make the user switch the build type explicitly again, using cmake .. -DCMAKE_BUILD_TYPE=Release.

Consider why this is necessary: as I said, CMake can re-trigger itself as part of a build if CMake's input (CMakeLists.txt files or their dependencies) has changed since last CMake ran. In such case, it will also be run without command-line arguments such as -DCMAKE_BUILD_TYPE=whatever, and will rely on the cache to supply the same value as last time. This scenario is indistinguishable from you manually running cmake .. without additional arguments.

I could provide a hacky solution to always reset CMAKE_BUILD_TYPE to Release if not specified explicitly on the command line. However, it would also mean that a buildsystem generated as Debug would get re-generated as Release if automatic re-generation happened. I am pretty sure that's not what you want.

To address the first issue, you should be able to do this early in your CMakeList:

if(NOT CMAKE_BUILD_TYPE)
  set(CMAKE_BUILD_TYPE Release)
endif()

set(CMAKE_CXX_FLAGS "-Wall -Wextra")
set(CMAKE_CXX_FLAGS_DEBUG "-g")
set(CMAKE_CXX_FLAGS_RELEASE "-O3")

This will make sure that if you do not specify a build type at all, it will default to "Release" and thus CMAKE_CXX_FLAGS_RELEASE will be used.

The second one is harder to tackle. Variables passed from the command line (such as CMAKE_BUILD_TYPE=Debug) are cached by CMake and thus re-used in subsequent invocations (that is necessary, since CMake can re-trigger itself if you modify its inputs between builds).

The "proper" solution is to make the user switch the build type explicitly again, using cmake .. -DCMAKE_BUILD_TYPE=Release. If that is acceptable for you, do it.

First off: recommended usage of CMake is to always specify CMAKE_BUILD_TYPE explicitly on the command line (if and only if using a single-configuration generator). Your use case deviates from this best practice, so treat this answer as "how you can do it," not necessarily as "how you should do it."

To address the first issue, you should be able to do this early in your CMakeList:

if(NOT CMAKE_BUILD_TYPE)
  set(CMAKE_BUILD_TYPE Release)
endif()

set(CMAKE_CXX_FLAGS "-Wall -Wextra")
set(CMAKE_CXX_FLAGS_DEBUG "-g")
set(CMAKE_CXX_FLAGS_RELEASE "-O3")

This will make sure that if you do not specify a build type at all, it will default to "Release" and thus CMAKE_CXX_FLAGS_RELEASE will be used.

The second one is harder to tackle. Variables passed from the command line (such as CMAKE_BUILD_TYPE=Debug) are cached by CMake and thus re-used in subsequent invocations (that is necessary, since CMake can re-trigger itself if you modify its inputs between builds).

The "proper" solution is to make the user switch the build type explicitly again, using cmake .. -DCMAKE_BUILD_TYPE=Release. If that is acceptable for you, do it.

I am currently unable to think of a way to make this "reset" implicit; I will expand the answer if I find one.