error (instead of warning) on unavailable extra

Author: meejah

src/pip/_internal/exceptions.py

@@ -775,3 +775,26 @@ def __init__(self, *, distribution: "BaseDistribution") -> None:
             ),
             hint_stmt=None,
         )
+
+
+class UnavailableExtra(InstallationError):
+    """A requested extra is not available."""
+
+    def __init__(
+            self,
+            base: str,
+            version: str,
+            extra: str,
+            available_extras: List[str],
+    ):
+        self.base = base
+        self.version = version
+        self.extra = extra
+        self.available_extras = available_extras
+
+    def __str__(self) -> str:
+        nice_available = " ".join(
+            '"{}", '.format(e)
+            for e in self.available_extras
+        )[:-2]
+        return f"{self.base} {self.version} does not provide the extra '{self.extra}'\n{self.base} provides extras: {nice_available}"

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

@@ -11,6 +11,7 @@
     InstallationSubprocessError,
     MetadataInconsistent,
     MetadataInvalid,
+    UnavailableExtra,
 )
 from pip._internal.metadata import BaseDistribution
 from pip._internal.models.link import Link, links_equivalent
@@ -508,11 +509,11 @@ def iter_dependencies(self, with_requires: bool) -> Iterable[Optional[Requiremen
         valid_extras = self.extras.intersection(self.base.dist.iter_provided_extras())
         invalid_extras = self.extras.difference(self.base.dist.iter_provided_extras())
         for extra in sorted(invalid_extras):
-            logger.warning(
-                "%s %s does not provide the extra '%s'",
-                self.base.name,
-                self.version,
-                extra,
+            raise UnavailableExtra(
+                base=self.base.name,
+                version=self.version,
+                extra=extra,
+                available_extras=self.base.dist.iter_provided_extras(),
             )
 
         for r in self.base.dist.iter_dependencies(valid_extras):