Skip conditional build req without matching marker

news/10883.bugfix.rst

@@ -0,0 +1,2 @@
+When checking for conflicts in the build environment, correctly skip requirements
+containing markers that do not match the current environment.

src/pip/_internal/build_env.py

@@ -175,6 +175,8 @@ def check_requirements(
             )
             for req_str in reqs:
                 req = Requirement(req_str)
+                if req.marker is not None and not req.marker.evaluate():
+                    continue  # FIXME: Consider extras?
                 dist = env.get_distribution(req.name)
                 if not dist:
                     missing.add(req_str)

tests/functional/test_build_env.py

@@ -165,6 +165,25 @@ def test_build_env_requirements_check(script: PipTestEnvironment) -> None:
         """,
     )
 
+    run_with_build_env(
+        script,
+        """
+        build_env.install_requirements(
+            finder,
+            ["bar==3.0"],
+            "normal",
+            kind="installing bar in normal",
+        )
+        r = build_env.check_requirements(
+            [
+                "bar==2.0; python_version < '3.0'",
+                "bar==3.0; python_version >= '3.0'",
+            ],
+        )
+        assert r == (set(), set()), repr(r)
+        """,
+    )
+
 
 def test_build_env_overlay_prefix_has_priority(script: PipTestEnvironment) -> None:
     create_basic_wheel_for_package(script, "pkg", "2.0")