upgrade extremely useful (and easy to miss) warning to an error

src/poetry/installation/installer.py

@@ -245,12 +245,9 @@ def _do_install(self) -> int:
             locked_repository = self._locker.locked_repository()
 
             if not self._locker.is_fresh():
-                self._io.write_error_line(
-                    "<warning>"
-                    "Warning: poetry.lock is not consistent with pyproject.toml. "
-                    "You may be getting improper dependencies. "
+                raise ValueError(
+                    "poetry.lock is not consistent with pyproject.toml. "
                     "Run `poetry lock [--no-update]` to fix it."
-                    "</warning>"
                 )
 
             locker_extras = {

tests/installation/test_installer.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import json
+import re
 
 from pathlib import Path
 from typing import TYPE_CHECKING
@@ -97,6 +98,7 @@ def __init__(self, lock_path: Path) -> None:
         self._lock = lock_path / "poetry.lock"
         self._written_data = None
         self._locked = False
+        self._fresh = True
         self._lock_data = None
         self._content_hash = self._get_content_hash()
 
@@ -121,8 +123,13 @@ def mock_lock_data(self, data: dict[str, Any]) -> None:
     def is_locked(self) -> bool:
         return self._locked
 
+    def fresh(self, is_fresh: bool = True) -> Locker:
+        self._fresh = is_fresh
+
+        return self
+
     def is_fresh(self) -> bool:
-        return True
+        return self._fresh
 
     def _get_content_hash(self) -> str:
         return "123456789"
@@ -208,6 +215,18 @@ def test_run_no_dependencies(installer: Installer, locker: Locker) -> None:
     assert locker.written_data == expected
 
 
+def test_not_fresh_lock(installer: Installer, locker: Locker) -> None:
+    locker.locked().fresh(False)
+    with pytest.raises(
+        ValueError,
+        match=re.escape(
+            "poetry.lock is not consistent with pyproject.toml."
+            " Run `poetry lock [--no-update]` to fix it."
+        ),
+    ):
+        installer.run()
+
+
 def test_run_with_dependencies(
     installer: Installer, locker: Locker, repo: Repository, package: ProjectPackage
 ) -> None: