fix #593: allow calver-by-date on top of legacy version

src/setuptools_scm/version.py

@@ -323,21 +323,27 @@ def guess_next_date_ver(version, node_date=None, date_fmt=None, version_cls=None
     """
     match = date_ver_match(version)
     if match is None:
-        raise ValueError(
-            "{version} does not correspond to a valid versioning date, "
-            "please correct or use a custom version scheme".format(version=version)
+        warnings.warn(
+            f"{version} does not correspond to a valid versioning date, "
+            "assuming legacy version"
         )
+        if date_fmt is None:
+            date_fmt = "%y.%m.%d"
+
     # deduct date format if not provided
     if date_fmt is None:
         date_fmt = "%Y.%m.%d" if len(match.group("year")) == 4 else "%y.%m.%d"
     head_date = node_date or datetime.date.today()
     # compute patch
-    tag_date = datetime.datetime.strptime(match.group("date"), date_fmt).date()
+    if match is None:
+        tag_date = datetime.date.today()
+    else:
+        tag_date = datetime.datetime.strptime(match.group("date"), date_fmt).date()
     if tag_date == head_date:
-        patch = match.group("patch") or "0"
+        patch = "0" if match is None else (match.group("patch") or "0")
         patch = int(patch) + 1
     else:
-        if tag_date > head_date:
+        if tag_date > head_date and match is not None:
             # warn on future times
             warnings.warn(
                 "your previous tag  ({}) is ahead your node date ({})".format(

testing/test_version.py

@@ -208,7 +208,7 @@ def date_to_str(date_=None, days_offset=0, fmt="{dt:%y}.{dt.month}.{dt.day}"):
             meta(date_to_str() + ".1", config=c), date_to_str() + ".1", id="exact patch"
         ),
         pytest.param(
-            meta(date_to_str(fmt="20.01.02"), config=c),
+            meta("20.01.02", config=c),
             "20.1.2",
             id="leading 0s",
         ),
@@ -256,6 +256,19 @@ def date_to_str(date_=None, days_offset=0, fmt="{dt:%y}.{dt.month}.{dt.day}"):
             date_to_str(date.today() - timedelta(days=2)) + ".3.dev2",
             id="node date distance",
         ),
+        pytest.param(
+            meta(
+                "1.2.0",
+                config=c,
+                distance=2,
+                node_date=date.today() - timedelta(days=2),
+            ),
+            date_to_str(days_offset=2) + ".0.dev2",
+            marks=pytest.mark.filterwarnings(
+                "ignore:.*not correspond to a valid versioning date.*:UserWarning"
+            ),
+            id="using on old version tag",
+        ),
     ],
 )
 def test_calver_by_date(version, expected_next):