Fix cmake build --config XXXX for multi-config with hardcoded configurations

conan/tools/cmake/cmake.py

@@ -51,6 +51,10 @@ def __init__(self, conanfile):
 
         self._cmake_program = conanfile.conf.get("tools.cmake:cmake_program", default="cmake")
 
+    @property
+    def is_multi_configuration(self):
+        return is_multi_configuration(self._generator)
+
     def configure(self, variables=None, build_script_folder=None, cli_args=None,
                   stdout=None, stderr=None):
         """
@@ -117,18 +121,28 @@ def configure(self, variables=None, build_script_folder=None, cli_args=None,
         with chdir(self, build_folder):
             self._conanfile.run(command, stdout=stdout, stderr=stderr)
 
-    def _build(self, build_type=None, target=None, cli_args=None, build_tool_args=None, env="",
-               stdout=None, stderr=None):
-        bf = self._conanfile.build_folder
+    def _config_arg(self, build_type):
+        """ computes the '--config Release' arg when necessary, or warn or error if wrong
+        """
         is_multi = is_multi_configuration(self._generator)
         if build_type and not is_multi:
             self._conanfile.output.error("Don't specify 'build_type' at build time for "
                                          "single-config build systems")
+        if not build_type:
+            try:
+                build_type = self._conanfile.settings.build_type  # declared, but can be None
+                if not build_type:
+                    raise ConanException("CMake: build_type setting should be defined.")
+            except ConanException:
+                if is_multi:
+                    raise ConanException("CMake: build_type setting should be defined.")
+        build_config = "--config {}".format(build_type) if build_type and is_multi else ""
+        return build_config
 
-        bt = build_type or self._conanfile.settings.get_safe("build_type")
-        if not bt:
-            raise ConanException("build_type setting should be defined.")
-        build_config = "--config {}".format(bt) if bt and is_multi else ""
+    def _build(self, build_type=None, target=None, cli_args=None, build_tool_args=None, env="",
+               stdout=None, stderr=None):
+        bf = self._conanfile.build_folder
+        build_config = self._config_arg(build_type)
 
         args = []
         if target is not None:
@@ -191,11 +205,7 @@ def install(self, build_type=None, component=None, cli_args=None, stdout=None, s
         self._conanfile.output.info("Running CMake.install()")
         mkdir(self._conanfile, self._conanfile.package_folder)
 
-        bt = build_type or self._conanfile.settings.get_safe("build_type")
-        if not bt:
-            raise ConanException("build_type setting should be defined.")
-        is_multi = is_multi_configuration(self._generator)
-        build_config = "--config {}".format(bt) if bt and is_multi else ""
+        build_config = self._config_arg(build_type)
 
         pkg_folder = '"{}"'.format(self._conanfile.package_folder.replace("\\", "/"))
         build_folder = '"{}"'.format(self._conanfile.build_folder)

conan/tools/cmake/toolchain/toolchain.py

@@ -13,6 +13,7 @@
     AndroidSystemBlock, AppleSystemBlock, FPicBlock, ArchitectureBlock, GLibCXXBlock, VSRuntimeBlock, \
     CppStdBlock, ParallelBlock, CMakeFlagsInitBlock, TryCompileBlock, FindFiles, PkgConfigBlock, \
     SkipRPath, SharedLibBock, OutputDirsBlock, ExtraFlagsBlock, CompilersBlock, LinkerScriptsBlock
+from conan.tools.cmake.utils import is_multi_configuration
 from conan.tools.env import VirtualBuildEnv, VirtualRunEnv
 from conan.tools.intel import IntelCC
 from conan.tools.microsoft import VCVars
@@ -190,6 +191,10 @@ def content(self):
         content = relativize_generated_file(content, self._conanfile, "${CMAKE_CURRENT_LIST_DIR}")
         return content
 
+    @property
+    def is_multi_configuration(self):
+        return is_multi_configuration(self.generator)
+
     def _find_cmake_exe(self):
         for req in self._conanfile.dependencies.direct_build.values():
             if req.ref.name == "cmake":

conans/test/functional/toolchains/cmake/test_cmake_toolchain.py

@@ -1429,6 +1429,51 @@ def build(self):
     assert "-- MYVAR1 MYVALUE1!!" in client.out
 
 
+def test_no_build_type():
+    client = TestClient()
+
+    conanfile = textwrap.dedent("""
+        from conan import ConanFile
+        from conan.tools.cmake import CMake, CMakeToolchain
+
+        class AppConan(ConanFile):
+            name = "pkg"
+            version = "1.0"
+            settings = "os", "compiler", "arch"
+            exports_sources = "CMakeLists.txt"
+
+            def layout(self):
+                self.folders.build = "build"
+
+            def generate(self):
+                tc = CMakeToolchain(self)
+                if not tc.is_multi_configuration:
+                    tc.cache_variables["CMAKE_BUILD_TYPE"] = "Release"
+                tc.generate()
+
+            def build(self):
+                cmake = CMake(self)
+                cmake.configure()
+                build_type = "Release" if cmake.is_multi_configuration else None
+                cmake.build(build_type=build_type)
+
+            def package(self):
+                cmake = CMake(self)
+                build_type = "Release" if cmake.is_multi_configuration else None
+                cmake.install(build_type=build_type)
+        """)
+
+    cmake = """
+        cmake_minimum_required(VERSION 3.15)
+        project(pkg LANGUAGES NONE)
+    """
+
+    client.save({"conanfile.py": conanfile,
+                 "CMakeLists.txt": cmake})
+    client.run("create .")
+    assert "Don't specify 'build_type' at build time" not in client.out
+
+
 @pytest.mark.tool("cmake", "3.19")
 def test_redirect_stdout():
     client = TestClient()