test-poc for cppstd compatibility exact

conans/client/graph/compatibility.py

@@ -29,7 +29,8 @@ def compatibility(conanfile):
 
 def cppstd_compat(conanfile):
     # It will try to find packages with all the cppstd versions
-
+    if not getattr(conanfile, "compatibility_cppstd", True):
+        return []
     compiler = conanfile.settings.get_safe("compiler")
     compiler_version = conanfile.settings.get_safe("compiler.version")
     cppstd = conanfile.settings.get_safe("compiler.cppstd")

conans/test/integration/package_id/test_validate.py

@@ -4,7 +4,6 @@
 import textwrap
 import unittest
 
-import pytest
 
 from conan.cli.exit_codes import ERROR_INVALID_CONFIGURATION, ERROR_GENERAL
 from conans.client.graph.graph import BINARY_INVALID
@@ -661,6 +660,43 @@ def package_id(self):
                    assert_error=True)
         assert 'pkg/0.1: Invalid: I need at least cppstd=14 to be used' in client.out
 
+    def test_exact_cppstd(self):
+        """ Using the default cppstd_compat sometimes is not desired, and a recipe can
+        explicitly opt-out this default cppstd_compat behavior, if it knows its binaries
+        won't be binary compatible among them for different cppstd values
+        """
+        client = TestClient()
+        conanfile = textwrap.dedent("""
+            from conan import ConanFile
+            from conan.errors import ConanInvalidConfiguration
+            class Pkg(ConanFile):
+                name = "pkg"
+                version = "0.1"
+                settings = "compiler"
+
+                def compatibility(self):
+                    # It also works as class attribute, but more explicit here
+                    self.compatibility_cppstd = False
+            """)
+
+        client.save({"conanfile.py": conanfile})
+
+        settings = "-s compiler=gcc -s compiler.version=9 -s compiler.libcxx=libstdc++"
+        client.run(f"create . {settings} -s compiler.cppstd=17")
+        client.assert_listed_binary({"pkg/0.1": ("91faf062eb94767a31ff62a46767d3d5b41d1eff",
+                                                 "Build")})
+
+        # Install with cppstd=14 can NOT fallback to the previous one
+        client.run(f"install --requires=pkg/0.1 {settings} -s compiler.cppstd=14", assert_error=True)
+        assert "ERROR: Missing prebuilt package for 'pkg/0.1'" in client.out
+        assert "compiler.cppstd=14" in client.out
+        assert "compiler.cppstd=17" not in client.out
+        client.run(f"install --requires=pkg/0.1 {settings} -s compiler.cppstd=14 --build=missing")
+        client.assert_listed_binary({"pkg/0.1": ("36d978cbb4dc35906d0fd438732d5e17cd1e388d",
+                                                 "Build")})
+        assert "compiler.cppstd=14" in client.out
+        assert "compiler.cppstd=17" not in client.out
+
 
 class TestCompatibleSettingsTarget(unittest.TestCase):
     """ aims to be a very close to real use case of tool being used across different settings_target