Fix wrong cppstd detection for newer apple-clang compilers (#14837)

* Fix wrong cppstd detection for newer apple-clang compilers

* Update test to cover real detection

* Separate detect_cppstd from default_cppstd

conan/internal/api/detect_api.py

@@ -228,6 +228,16 @@ def _mcst_lcc_cppstd_default(version):
     return default
 
 
+def detect_cppstd(compiler, compiler_version):
+    cppstd = default_cppstd(compiler, compiler_version)
+    if compiler == "apple-clang" and compiler_version >= "11":
+        # Conan does not detect the default cppstd for apple-clang,
+        # because it's still 98 for the compiler (eben though xcode uses newer in projects)
+        # and having it be so old would be annoying for users
+        cppstd = "gnu17"
+    return cppstd
+
+
 def detect_compiler():
     """
     find the default compiler on the build machine

conans/client/conf/detect.py

@@ -1,6 +1,6 @@
 from conan.api.output import ConanOutput
 from conan.internal.api.detect_api import detect_os, detect_arch, default_msvc_runtime, \
-    detect_libcxx, default_cppstd, detect_compiler, default_compiler_version
+    detect_libcxx, detect_cppstd, detect_compiler, default_compiler_version
 
 
 def detect_defaults_settings():
@@ -31,7 +31,7 @@ def detect_defaults_settings():
     libcxx = detect_libcxx(compiler, version)
     if libcxx:
         result.append(("compiler.libcxx", libcxx))
-    cppstd = default_cppstd(compiler, version)
+    cppstd = detect_cppstd(compiler, version)
     if cppstd:
         result.append(("compiler.cppstd", cppstd))
     result.append(("build_type", "Release"))

conans/test/unittests/tools/build/test_cppstd.py

@@ -1,7 +1,9 @@
 import pytest
 
+from conan.internal.api.detect_api import detect_cppstd
 from conan.tools.build import supported_cppstd, check_min_cppstd, valid_min_cppstd, default_cppstd
 from conans.errors import ConanException, ConanInvalidConfiguration
+from conans.model.version import Version
 from conans.test.utils.mocks import MockSettings, ConanFileMock
 
 
@@ -25,6 +27,23 @@ def test_supported_cppstd_clang(compiler, compiler_version, values):
     assert sot == values
 
 
+@pytest.mark.parametrize("compiler,compiler_version,result", [
+    ("gcc", "5", 'gnu98'),
+    ("gcc", "8", "gnu14"),
+    ("gcc", "12", "gnu17"),
+    ("msvc", "190", "14"),
+    ("apple-clang", "10.0", "gnu98"),
+    # We diverge from the default cppstd for apple-clang >= 11
+    ("apple-clang", "12.0", "gnu17"),
+    ("clang", "4", "gnu98"),
+    ("clang", "6", "gnu14"),
+    ("clang", "17", "gnu17")
+])
+def test_detected_cppstd(compiler, compiler_version, result):
+    sot = detect_cppstd(compiler, Version(compiler_version))
+    assert sot == result
+
+
 def test_supported_cppstd_with_specific_values():
     settings = MockSettings({})
     conanfile = ConanFileMock(settings)
@@ -104,6 +123,7 @@ def test_supported_cppstd_mcst(compiler, compiler_version, values):
     sot = supported_cppstd(conanfile)
     assert sot == values
 
+
 @pytest.mark.parametrize("compiler,compiler_version,values", [
     ("qcc", "4.4", ['98', 'gnu98']),
     ("qcc", "5.4", ['98', 'gnu98', '11', 'gnu11', "14", "gnu14", "17", "gnu17"]),
@@ -116,20 +136,6 @@ def test_supported_cppstd_qcc(compiler, compiler_version, values):
     assert sot == values
 
 
-@pytest.mark.parametrize("compiler,compiler_version,result", [
-    ("gcc", "5", 'gnu98'),
-    ("gcc", "8", "gnu14"),
-    ("msvc", "190", "14"),
-    ("apple-clang", "12.0", "gnu98"),
-    ("clang", "6", "gnu14"),
-])
-def test_default_cppstd_gcc(compiler, compiler_version, result):
-    settings = MockSettings({"compiler": compiler, "compiler.version": compiler_version})
-    conanfile = ConanFileMock(settings)
-    sot = default_cppstd(conanfile)
-    assert sot == result
-
-
 def test_check_cppstd_type():
     """ cppstd must be a number
     """

conans/test/unittests/util/detect_test.py

@@ -3,6 +3,7 @@
 
 from parameterized import parameterized
 
+from conan.internal.api.detect_api import detect_cppstd
 from conans.client.conf.detect import detect_defaults_settings
 from conans.model.version import Version
 from conans.test.utils.mocks import RedirectedTestOutput