Add test: ignore unavailable versions in preexisting output

Rather than fail and exit early,
it's better to discard these items.

The legacy resolver does not pass,
and there is no plan to fix that before
we drop that resolver completely.

tests/test_cli_compile.py

@@ -2030,6 +2030,31 @@ def test_preserve_compiled_prerelease_version(pip_conf, runner):
     assert "small-fake-a==0.3b1" in out.stderr.splitlines()
 
 
+def test_ignore_compiled_unavailable_version(pip_conf, runner, current_resolver):
+    if current_resolver == "legacy":
+        pytest.xfail(
+            "We know this is broken in the legacy resolver, but no fix is planned."
+        )
+
+    with open("requirements.in", "w") as req_in:
+        req_in.write("small-fake-a")
+
+    with open("requirements.txt", "w") as req_txt:
+        req_txt.write("small-fake-a==9999")
+
+    out = runner.invoke(cli, ["--no-annotate", "--no-header"])
+
+    assert out.exit_code == 0, out
+    assert "small-fake-a==" in out.stderr
+    assert "small-fake-a==9999" not in out.stderr.splitlines()
+
+    assert (
+        "Discarding small-fake-a==9999 "
+        "(from -r requirements.txt (line 1)) "
+        "to proceed the resolution"
+    ) in out.stderr
+
+
 def test_prefer_binary_dist(
     pip_conf, make_package, make_sdist, make_wheel, tmpdir, runner
 ):