accept empty build_type in case of CMake single configuration

conan/tools/cmake/cmake.py

@@ -118,11 +118,10 @@ def _build(self, build_type=None, target=None, cli_args=None, build_tool_args=No
         bf = self._conanfile.build_folder
         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")
-
+            self._conanfile.output.warning("Specifying 'build_type' does not have "
+                                           "any effect for single-config build systems")
         bt = build_type or self._conanfile.settings.get_safe("build_type")
-        if not bt:
+        if not bt and is_multi:
             raise ConanException("build_type setting should be defined.")
         build_config = "--config {}".format(bt) if bt and is_multi else ""
 
@@ -180,9 +179,9 @@ def install(self, build_type=None, component=None, cli_args=None):
         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)
+        if not bt and is_multi:
+            raise ConanException("build_type setting should be defined.")
         build_config = "--config {}".format(bt) if bt and is_multi else ""
 
         pkg_folder = '"{}"'.format(self._conanfile.package_folder.replace("\\", "/"))

conans/test/integration/toolchains/cmake/test_cmake.py

@@ -31,3 +31,52 @@ def run(self, *args, **kwargs):
     assert "-something" in client.out
     assert "--testverbose" in client.out
     assert "-testok" in client.out
+
+def test_build_type_single_config_warn():
+    client = TestClient()
+
+    conanfile = textwrap.dedent("""
+        from conan import ConanFile
+        from conan.tools.cmake import CMake
+        class Pkg(ConanFile):
+            name = "pkg"
+            version = "0.1"
+            settings = "os", "compiler", "arch"
+            generators = "CMakeToolchain"
+            def build(self):
+                cmake = CMake(self)
+                cmake.configure()
+                cmake.build(build_type="Release")
+
+            def run(self, *args, **kwargs):
+                self.output.info("MYRUN: {}".format(*args))
+            """)
+    client.save({"conanfile.py": conanfile})
+    client.run("create . ")
+    assert "Specifying 'build_type' does not have " + \
+           "any effect for single-config build systems" in client.out
+    assert "Created package" in client.out
+
+def test_build_type_empty():
+    client = TestClient()
+
+    conanfile = textwrap.dedent("""
+        from conan import ConanFile
+        from conan.tools.cmake import CMake
+        class Pkg(ConanFile):
+            name = "pkg"
+            version = "0.1"
+            settings = "os", "compiler", "arch"
+            generators = "CMakeToolchain"
+            def build(self):
+                cmake = CMake(self)
+                cmake.configure()
+                cmake.build()
+                cmake.install()
+
+            def run(self, *args, **kwargs):
+                self.output.info("MYRUN: {}".format(*args))
+            """)
+    client.save({"conanfile.py": conanfile})
+    client.run("create . ")
+    assert "Created package" in client.out