Drop sdist re-installation behaviour

This was deprecated in favour of explicitly passing `--force-reinstall`.

news/8711.removal.rst

@@ -0,0 +1 @@
+Stop reinstalling source distributions when a package, with matching name *and* version, is already installed.

src/pip/_internal/exceptions.py

@@ -61,6 +61,23 @@ def __str__(self) -> str:
         )
 
 
+class SamePackageAlreadyInstalled(InstallationError):
+    """Installing an sdist, when a matching name-version is already installed."""
+
+    def __init__(self, *, refused_to_install: str) -> None:
+        super().__init__()
+
+        self.refused_to_install = refused_to_install
+
+    def __str__(self) -> str:
+        return (
+            f"Refusing to install {self.refused_to_install}\n"
+            "There is already an installed package with the same name and version. "
+            "To install the source distribution over an existing matching "
+            "package, use the --force-reinstall flag."
+        )
+
+
 class UserInstallationInvalid(InstallationError):
     """A --user install is requested on an environment without user site."""
 

src/pip/_internal/resolution/resolvelib/resolver.py

@@ -9,6 +9,7 @@
 from pip._vendor.resolvelib.structs import DirectedGraph
 
 from pip._internal.cache import WheelCache
+from pip._internal.exceptions import SamePackageAlreadyInstalled
 from pip._internal.index.package_finder import PackageFinder
 from pip._internal.operations.prepare import RequirementPreparer
 from pip._internal.req.req_install import InstallRequirement
@@ -19,7 +20,6 @@
     PipDebuggingReporter,
     PipReporter,
 )
-from pip._internal.utils.deprecation import deprecated
 from pip._internal.utils.filetypes import is_archive_file
 
 from .base import Candidate, Requirement
@@ -141,21 +141,12 @@ def resolve(
                     and candidate.source_link.ext != ".zip"
                 )
                 if looks_like_sdist:
-                    # is a local sdist -- show a deprecation warning!
-                    reason = (
-                        "Source distribution is being reinstalled despite an "
-                        "installed package having the same name and version as "
-                        "the installed package."
-                    )
-                    replacement = "use --force-reinstall"
-                    deprecated(
-                        reason=reason,
-                        replacement=replacement,
-                        gone_in="21.3",
-                        issue=8711,
+                    # is a local sdist -- disallow.
+                    raise SamePackageAlreadyInstalled(
+                        refused_to_install=candidate.source_link.file_path
                     )
 
-                # is a local sdist or path -- reinstall
+                # is a local path -- reinstall.
                 ireq.should_reinstall = True
             else:
                 continue

tests/functional/test_new_resolver.py

@@ -1206,7 +1206,7 @@ def test_new_resolver_check_wheel_version_normalized(
     script.assert_installed(simple="0.1.0+local.1")
 
 
-def test_new_resolver_does_reinstall_local_sdists(script):
+def test_new_resolver_refuses_to_reinstall_local_sdists(script):
     archive_path = create_basic_sdist_for_package(
         script,
         "pkg",
@@ -1225,10 +1225,11 @@ def test_new_resolver_does_reinstall_local_sdists(script):
         "--no-cache-dir",
         "--no-index",
         archive_path,
-        expect_stderr=True,
+        expect_error=True,
     )
-    assert "Installing collected packages: pkg" in result.stdout, str(result)
-    assert "DEPRECATION" in result.stderr, str(result)
+    assert "Installing collected packages: pkg" not in result.stdout, str(result)
+    assert archive_path in result.stderr
+    assert "--force-reinstall" in result.stderr
     script.assert_installed(pkg="1.0")