From 2a5e84e1c4d9c8e4c4236e1eccfa580406a29b6b Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Wed, 3 Feb 2021 16:23:04 +0800
Subject: [PATCH 1/2] Add failing test

---
 tests/functional/test_new_resolver_errors.py | 30 +++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/tests/functional/test_new_resolver_errors.py b/tests/functional/test_new_resolver_errors.py
index e263f4206b8..b4d63a99695 100644
--- a/tests/functional/test_new_resolver_errors.py
+++ b/tests/functional/test_new_resolver_errors.py
@@ -1,4 +1,6 @@
-from tests.lib import create_basic_wheel_for_package
+import sys
+
+from tests.lib import create_basic_wheel_for_package, create_test_package_with_setup
 
 
 def test_new_resolver_conflict_requirements_file(tmpdir, script):
@@ -45,3 +47,29 @@ def test_new_resolver_conflict_constraints_file(tmpdir, script):
 
     message = "The user requested (constraint) pkg!=1.0"
     assert message in result.stdout, str(result)
+
+
+def test_new_resolver_requires_python_error(script):
+    compatible_python = ">={0.major}.{0.minor}".format(sys.version_info)
+    incompatible_python = "<{0.major}.{0.minor}".format(sys.version_info)
+
+    pkga = create_test_package_with_setup(
+        script,
+        name="pkga",
+        version="1.0",
+        python_requires=compatible_python,
+    )
+    pkgb = create_test_package_with_setup(
+        script,
+        name="pkgb",
+        version="1.0",
+        python_requires=incompatible_python,
+    )
+
+    # This always fails because pkgb can never be satisfied.
+    result = script.pip("install", "--no-index", pkga, pkgb, expect_error=True)
+
+    # The error message should mention the Requires-Python: value causing the
+    # conflict, not the compatible one.
+    assert incompatible_python in result.stderr, str(result)
+    assert compatible_python not in result.stderr, str(result)

From d87abdfdfe5175d76019ec2ce52b77656f3a3779 Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Wed, 3 Feb 2021 16:47:22 +0800
Subject: [PATCH 2/2] Fix error reporting on Requires-Python conflicts

---
 news/9541.bugfix.rst                          |  1 +
 .../resolution/resolvelib/factory.py          | 57 ++++++++++++-------
 2 files changed, 37 insertions(+), 21 deletions(-)
 create mode 100644 news/9541.bugfix.rst

diff --git a/news/9541.bugfix.rst b/news/9541.bugfix.rst
new file mode 100644
index 00000000000..88180198c07
--- /dev/null
+++ b/news/9541.bugfix.rst
@@ -0,0 +1 @@
+Fix incorrect reporting on ``Requires-Python`` conflicts.
diff --git a/src/pip/_internal/resolution/resolvelib/factory.py b/src/pip/_internal/resolution/resolvelib/factory.py
index 3181d575336..13ab7e904a0 100644
--- a/src/pip/_internal/resolution/resolvelib/factory.py
+++ b/src/pip/_internal/resolution/resolvelib/factory.py
@@ -1,6 +1,7 @@
 import functools
 import logging
 from typing import (
+    TYPE_CHECKING,
     Dict,
     FrozenSet,
     Iterable,
@@ -60,6 +61,14 @@
     UnsatisfiableRequirement,
 )
 
+if TYPE_CHECKING:
+    from typing import Protocol
+
+    class ConflictCause(Protocol):
+        requirement: RequiresPythonRequirement
+        parent: Candidate
+
+
 logger = logging.getLogger(__name__)
 
 C = TypeVar("C")
@@ -387,21 +396,25 @@ def get_dist_to_uninstall(self, candidate):
             )
         return None
 
-    def _report_requires_python_error(
-        self,
-        requirement,  # type: RequiresPythonRequirement
-        template,  # type: Candidate
-    ):
-        # type: (...) -> UnsupportedPythonVersion
-        message_format = (
-            "Package {package!r} requires a different Python: "
-            "{version} not in {specifier!r}"
-        )
-        message = message_format.format(
-            package=template.name,
-            version=self._python_candidate.version,
-            specifier=str(requirement.specifier),
-        )
+    def _report_requires_python_error(self, causes):
+        # type: (Sequence[ConflictCause]) -> UnsupportedPythonVersion
+        assert causes, "Requires-Python error reported with no cause"
+
+        version = self._python_candidate.version
+
+        if len(causes) == 1:
+            specifier = str(causes[0].requirement.specifier)
+            message = (
+                f"Package {causes[0].parent.name!r} requires a different "
+                f"Python: {version} not in {specifier!r}"
+            )
+            return UnsupportedPythonVersion(message)
+
+        message = f"Packages require a different Python. {version} not in:"
+        for cause in causes:
+            package = cause.parent.format_for_error()
+            specifier = str(cause.requirement.specifier)
+            message += f"\n{specifier!r} (required by {package})"
         return UnsupportedPythonVersion(message)
 
     def _report_single_requirement_conflict(self, req, parent):
@@ -427,12 +440,14 @@ def get_installation_error(
 
         # If one of the things we can't solve is "we need Python X.Y",
         # that is what we report.
-        for cause in e.causes:
-            if isinstance(cause.requirement, RequiresPythonRequirement):
-                return self._report_requires_python_error(
-                    cause.requirement,
-                    cause.parent,
-                )
+        requires_python_causes = [
+            cause
+            for cause in e.causes
+            if isinstance(cause.requirement, RequiresPythonRequirement)
+            and not cause.requirement.is_satisfied_by(self._python_candidate)
+        ]
+        if requires_python_causes:
+            return self._report_requires_python_error(requires_python_causes)
 
         # Otherwise, we have a set of causes which can't all be satisfied
         # at once.