From 722382d3c91987a56cf78fff51f967568707ca04 Mon Sep 17 00:00:00 2001
From: "Klein, Thorsten (GDE-EDS9)" <thorsten.klein@bshg.com>
Date: Mon, 18 Sep 2023 21:32:39 +0200
Subject: [PATCH] accept empty build_type in case of CMake single configuration

---
 conan/tools/cmake/cmake.py                    | 11 ++---
 .../toolchains/cmake/test_cmake.py            | 49 +++++++++++++++++++
 2 files changed, 54 insertions(+), 6 deletions(-)

diff --git a/conan/tools/cmake/cmake.py b/conan/tools/cmake/cmake.py
index 55ddbd116be..e56572d2456 100644
--- a/conan/tools/cmake/cmake.py
+++ b/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("\\", "/"))
diff --git a/conans/test/integration/toolchains/cmake/test_cmake.py b/conans/test/integration/toolchains/cmake/test_cmake.py
index 86ae90d270d..7f819257fa4 100644
--- a/conans/test/integration/toolchains/cmake/test_cmake.py
+++ b/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