Nicer error in the compatibility plugin with custom compilers (#13328)

* Nicer error when the compiler is not included in the supported_cppstd compilers

* Return an empty list, but warn the user about missing compatibility, which is the best of both worlds

* Add cppstd_compat test

* Fix GenConanfile import

* Fixup imports

conan/tools/build/cppstd.py

@@ -94,7 +94,7 @@ def default_cppstd(conanfile, compiler=None, compiler_version=None):
 
 def supported_cppstd(conanfile, compiler=None, compiler_version=None):
     """
-    Get the a list of supported ``compiler.cppstd`` for the "conanfile.settings.compiler" and
+    Get a list of supported ``compiler.cppstd`` for the "conanfile.settings.compiler" and
     "conanfile.settings.compiler_version" or for the parameters "compiler" and "compiler_version"
     if specified.
 

conans/client/graph/compatibility.py

@@ -24,6 +24,7 @@ def compatibility(conanfile):
 # This file was generated by Conan. Remove this comment if you edit this file or Conan
 # will destroy your changes.
 from conan.tools.build import supported_cppstd
+from conan.errors import ConanException
 
 
 def cppstd_compat(conanfile):
@@ -36,6 +37,9 @@ def cppstd_compat(conanfile):
         return []
     base = dict(conanfile.settings.values_list)
     cppstd_possible_values = supported_cppstd(conanfile)
+    if cppstd_possible_values is None:
+        conanfile.output.warning(f'No cppstd compatibility defined for compiler "{compiler}"')
+        return []
     ret = []
     for _cppstd in cppstd_possible_values:
         if _cppstd is None or _cppstd == cppstd:

conans/test/integration/extensions/test_cppstd_compat.py

@@ -0,0 +1,85 @@
+import textwrap
+
+from conans.test.assets.genconanfile import GenConanfile
+from conans.test.utils.tools import TestClient
+
+
+def test_compatible_cppstd():
+    c = TestClient()
+    conanfile = textwrap.dedent("""
+        from conan import ConanFile
+
+        class Pkg(ConanFile):
+            name = "pkg"
+            version = "0.1"
+            settings = "os", "compiler"
+
+            def package_info(self):
+                self.output.info("PackageInfo!: Cppstd version: %s!"
+                                 % self.settings.compiler.cppstd)
+        """)
+    profile = textwrap.dedent("""
+        [settings]
+        os = Linux
+        compiler=gcc
+        compiler.version=12
+        compiler.libcxx=libstdc++
+        """)
+    c.save({"conanfile.py": conanfile,
+            "myprofile": profile})
+    # Create package with cppstd 17
+    c.run("create .  -pr=myprofile -s compiler.cppstd=17")
+    package_id = "95dcfeb51c04968b4ee960ee393cf2c1ebcf7782"
+    assert f"pkg/0.1: Package '{package_id}' created" in c.out
+
+    # package can be used with a profile gcc cppstd20 falling back to 17
+    c.save({"conanfile.py": GenConanfile().with_require("pkg/0.1")})
+    c.run("install . -pr=myprofile -s compiler.cppstd=20")
+    assert f"Using compatible package '{package_id}'"
+    assert "pkg/0.1: PackageInfo!: Cppstd version: 17!" in c.out
+    c.assert_listed_binary({"pkg/0.1": (f"{package_id}", "Cache")})
+    assert "pkg/0.1: Already installed!" in c.out
+
+
+def test_compatible_cppstd_missing_compiler():
+    c = TestClient()
+    settings_user = textwrap.dedent("""
+    compiler:
+        mycompiler:
+            version: ["12"]
+            libcxx: ["libstdc++"]
+            cppstd: [15, 17, 20]
+    """)
+    conanfile = textwrap.dedent("""
+            from conan import ConanFile
+
+            class Pkg(ConanFile):
+                name = "pkg"
+                version = "0.1"
+                settings = "os", "compiler"
+
+                def package_info(self):
+                    self.output.info("PackageInfo!: Cppstd version: %s!"
+                                     % self.settings.compiler.cppstd)
+            """)
+    profile = textwrap.dedent("""
+            [settings]
+            os = Linux
+            compiler=mycompiler
+            compiler.version=12
+            compiler.libcxx=libstdc++
+            """)
+    c.save({"conanfile.py": conanfile,
+            "myprofile": profile})
+    c.save_home({"settings_user.yml": settings_user})
+    # Create package with cppstd 17
+    c.run("create .  -pr=myprofile -s compiler.cppstd=17")
+    package_id = "51a90090adb1cbd330a64b4c3b3b32af809af4f9"
+    assert f"pkg/0.1: Package '{package_id}' created" in c.out
+
+    # package can't be used with cppstd 20 and fallback to 17, because mycompiler is not known
+    # to the default cppstd_compat function. Ensure that it does not break and warns the user
+    c.save({"conanfile.py": GenConanfile().with_require("pkg/0.1")})
+    c.run("install . -pr=myprofile -s compiler.cppstd=20", assert_error=True)
+    assert "Missing binary: pkg/0.1:b4b07859713551e8aac612f8080888c58b4711ae" in c.out
+    assert 'pkg/0.1: WARN: No cppstd compatibility defined for compiler "mycompiler"' in c.out