don't assume standard version matches file format

asdf/_asdf.py

@@ -734,6 +734,10 @@ def _parse_header_line(cls, line):
             msg = f"Unparsable version in ASDF file: {parts[1]}"
             raise ValueError(msg) from err
 
+        if version != versioning._FILE_FORMAT_VERSION:
+            msg = f"Unsupported ASDF file format version {version}"
+            raise ValueError(msg)
+
         return version
 
     @classmethod
@@ -809,13 +813,15 @@ def _open_asdf(
                 msg = "Does not appear to be a ASDF file."
                 raise ValueError(msg) from e
             self._file_format_version = cls._parse_header_line(header_line)
-            self.version = self.file_format_version
 
             self._comments = cls._read_comment_section(fd)
 
             version = cls._find_asdf_version_in_comments(self._comments)
             if version is not None:
                 self.version = version
+            else:
+                # If no ASDF_STANDARD comment is found...
+                self.version = versioning.AsdfVersion("1.0.0")
 
             # Now that version is set for good, we can add any additional
             # extensions, which may have narrow ASDF Standard version

asdf/_tests/test_file_format.py

@@ -166,7 +166,7 @@ def test_invalid_version(tmp_path):
 foo : bar
 ..."""
     buff = io.BytesIO(content)
-    with pytest.raises(ValueError, match=r"ASDF Standard version .* is not supported by asdf==.*"), asdf.open(buff):
+    with pytest.raises(ValueError, match=r"Unsupported ASDF file format version*"), asdf.open(buff):
         pass
 
 

asdf/_tests/test_yaml.py

@@ -199,7 +199,8 @@ def check_asdf(asdf):
 
 
 def test_explicit_tags():
-    yaml = f"""#ASDF {asdf.versioning.default_version}
+    yaml = """#ASDF 1.0.0
+#ASDF_STANDARD 1.5.0
 %YAML 1.1
 --- !<tag:stsci.edu:asdf/core/asdf-1.1.0>
 foo: !<tag:stsci.edu:asdf/core/ndarray-1.0.0> [1, 2, 3]