Repository: conan-io/conan

Issue:
CMake helper requires compiler during package
To help us debug your issue please explain:

Hi everyone!

I'm trying give some support to taocpp/PEGTL and I would like to use CMake helper to install the project. The project is header-only, thus it installs the license, headers and pegtl-config.cmake.

However, when the package method runs over Windows, I receive the follow message:

    conans.errors.ConanExceptionInUserConanfileMethod: pegtl/2.5.2@taocpp/testing: Error in package() method, line 22
	cmake = CMake(self)
	ConanException: You must specify compiler, compiler.version and arch in your settings to use a CMake generator. You can also declare the env variable CONAN_CMAKE_GENERATOR.

When I run the same recipe on Linux (Travis CI), I have no problems.

Appveyor error log: https://ci.appveyor.com/project/uilianries/pegtl/build/feature/conan-9/job/tnwvkd69gvkgsb6x#L250

The current PR uses just self.copy, but my intention is revert to follow state, using CMake:

https://github.com/uilianries/PEGTL/blob/dc4011991624faedf67fd390e8f53737f562a897/conanfile.py

Current PR for PEGTL: https://github.com/taocpp/PEGTL/pull/112

I'm curious if CONAN_CMAKE_GENERATOR is required by Conan or is just a CMake limitation.

Conan Version: 1.4.3

- [X] I've read the [CONTRIBUTING guide](https://raw.githubusercontent.com/conan-io/conan/develop/.github/CONTRIBUTING.md).
- [X] I've specified the Conan version, operating system version and any tool that can be relevant.
- [X] I've explained the steps to reproduce the error or the motivation/use case of the question/suggestion.



Discussion:

Patch:
diff --git a/conans/client/build/cmake.py b/conans/client/build/cmake.py
index 8237a716b..1703d4345 100644
--- a/conans/client/build/cmake.py
+++ b/conans/client/build/cmake.py
@@ -117,9 +117,8 @@ class CMake(object):
 
         if not self._compiler or not self._compiler_version or not self._arch:
             if self._os_build == "Windows":
-                raise ConanException("You must specify compiler, compiler.version and arch in "
-                                     "your settings to use a CMake generator. You can also declare "
-                                     "the env variable CONAN_CMAKE_GENERATOR.")
+                # Not enough settings to set a generator in Windows
+                return None
             return "Unix Makefiles"
 
         if self._compiler == "Visual Studio":
@@ -243,11 +242,10 @@ class CMake(object):
 
     @property
     def command_line(self):
-        args = [
-            '-G "%s"' % self.generator,
-            self.flags,
-            '-Wno-dev'
-        ]
+        args = ['-G "%s"' % self.generator] if self.generator else []
+        args.append(self.flags)
+        args.append('-Wno-dev')
+
         if self.toolset:
             args.append('-T "%s"' % self.toolset)
         return join_arguments(args)
@@ -422,7 +420,7 @@ class CMake(object):
         if target is not None:
             args = ["--target", target] + args
 
-        if self.parallel:
+        if self.generator and self.parallel:
             if "Makefiles" in self.generator and "NMake" not in self.generator:
                 if "--" not in args:
                     args.append("--")


Test patch:
diff --git a/conans/test/build_helpers/cmake_test.py b/conans/test/build_helpers/cmake_test.py
index 09aab1631..4b5d400a9 100644
--- a/conans/test/build_helpers/cmake_test.py
+++ b/conans/test/build_helpers/cmake_test.py
@@ -820,9 +820,8 @@ build_type: [ Release]
         cmake = instance_with_os_build("Macos")
         self.assertEquals(cmake.generator, "Unix Makefiles")
 
-        with self.assertRaisesRegexp(ConanException, "You must specify compiler, "
-                                                     "compiler.version and arch"):
-            instance_with_os_build("Windows")
+        cmake = instance_with_os_build("Windows")
+        self.assertEquals(cmake.generator, None)
 
         with tools.environment_append({"CONAN_CMAKE_GENERATOR": "MyCoolGenerator"}):
             cmake = instance_with_os_build("Windows")
diff --git a/conans/test/integration/cmake_flags_test.py b/conans/test/integration/cmake_flags_test.py
index 89d3308b1..56988dc54 100644
--- a/conans/test/integration/cmake_flags_test.py
+++ b/conans/test/integration/cmake_flags_test.py
@@ -254,13 +254,7 @@ class MyLib(ConanFile):
             client = TestClient()
             client.save({"conanfile.py": conanfile % settings_line})
             client.run("install .")
-            error = client.run("build .", ignore_error=True)
-            if platform.system() == "Windows":
-                self.assertTrue(error)
-                self.assertIn("You must specify compiler, compiler.version and arch in "
-                              "your settings to use a CMake generator", client.user_io.out,)
-            else:
-                self.assertFalse(error)
+            client.run("build .")
 
     def cmake_shared_flag_test(self):
         conanfile = """
diff --git a/conans/test/integration/cmake_install_package_test.py b/conans/test/integration/cmake_install_package_test.py
index ecd2304f9..f7c222c2b 100644
--- a/conans/test/integration/cmake_install_package_test.py
+++ b/conans/test/integration/cmake_install_package_test.py
@@ -72,3 +72,14 @@ cmake_minimum_required(VERSION 2.8.12)
         client.run("create . user/channel")
         self.assertIn("Test/0.1@user/channel (test package): Content: my header h!!",
                       client.user_io.out)
+
+        # Test also recipe without settings
+        new_conanfile =\
+            conanfile.replace("settings = \"os\", \"compiler\", \"build_type\", \"arch\"", "")
+        self.assertNotIn("settings = \"os\", \"compiler\", \"build_type\", \"arch\"", new_conanfile)
+        client.save({"conanfile.py": new_conanfile})
+
+        client.run("remove * --force")
+        client.run("create . user/channel")
+        self.assertIn("Test/0.1@user/channel (test package): Content: my header h!!",
+                      client.user_io.out)