Ensure msvc fallback does not ignore 194 binaries (#16573)

* Ensure msvc fallback does not ignore already existing ones

* Reorder, so same compiler version is checked first

* Simplify test

* Simplify test

* More general test, each combination of factors will now be tested

* Don't unexpectedly shadow variables

conans/client/graph/compatibility.py

@@ -65,10 +65,10 @@ def cppstd_compat(conanfile):
         new_combinations = []
         for comb in combinations:
             for f in factor:
-                comb = comb.copy()
-                comb.update(f)
-                new_combinations.append(comb)
-        combinations = new_combinations
+                new_comb = comb.copy()
+                new_comb.update(f)
+                new_combinations.append(new_comb)
+        combinations.extend(new_combinations)
 
     ret = []
     for comb in combinations:

test/integration/package_id/compatible_test.py

@@ -400,3 +400,21 @@ def package_id(self):
 
         c.run("install --requires=pdfium/2020.9 -pr=myprofile -s build_type=Debug")
         assert "missing. Using compatible package" in c.out
+
+    def test_compatibility_msvc_and_cppstd(self):
+        """msvc 194 would not find compatible packages built with same version but different cppstd
+        due to an issue in the msvc fallback compatibility rule."""
+        tc = TestClient()
+        profile = textwrap.dedent("""
+                   [settings]
+                   compiler=msvc
+                   compiler.version=194
+                   compiler.runtime=dynamic
+                   """)
+        tc.save({"dep/conanfile.py": GenConanfile("dep", "1.0").with_setting("compiler"),
+                 "conanfile.py": GenConanfile("app", "1.0").with_require("dep/1.0").with_setting("compiler"),
+                 "profile": profile})
+
+        tc.run("create dep -pr=profile -s compiler.cppstd=20")
+        tc.run("create . -pr=profile -s compiler.cppstd=17")
+        tc.assert_listed_binary({"dep/1.0": ("b6d26a6bc439b25b434113982791edf9cab4d004", "Cache")})