Add 'format-missing-comma' extension (closes #484)

[DisciplinedSoftware]

Closes #484.

Many compilers (gfortran, ifort, ifx) accept a missing comma between a character-string edit descriptor and a neighbouring format item:

100 format('a' 1x,'b')   ! literal then descriptor
200 format(15x'a')       ! descriptor then literal (the #484 example)
300 format('a' 'b')      ! adjacent literals

Following the convention described in the developer guide, this adds a format-missing-comma entry to the EXTENSIONS list in utils.py (enabled by default, like the other extensions) and implements the relaxation in Format_Item_C1002:

• the existing match body becomes _standard_match (unchanged logic);
• match now runs the standard pass first and only falls back to _extension_match when the standard pass finds no match and the extension is enabled — so standard-conforming code produces exactly the same tree as before (the standard pass can raise NoMatchError from an eagerly-taken branch, which previously aborted matching entirely; it is now treated as no-match so the fallback can run);
• _extension_match splits the item at the boundary of the first character literal and matches both sides as Format_Items.

The full example from #484 parses with this change (verified verbatim). Also found independently while parsing MODFLOW-2005 (gwf2swr7.f, gwf2swi27.fpp, gwf2lak7.f, gwf2sfr7.f).

Two pre-existing negative tests in test_syntaxerror_c1002 asserted NoMatchError for exactly the constructs this extension legalizes (('hello' 2/), ('hello' 'hello')); they now assert the new behavior with the extension enabled and the old rejection with it disabled.

Note: re-generating a format specification from the parse tree re-introduces the omitted commas (consistent with how Format_Item_C1002 renders its item pairs); this is documented in the extension section of the docs.

• pytest src/fparser passes (2950 passed, 24 xfailed, 1 pre-existing xpassed)
• black clean
• Extension documented in doc/source/fparser2.rst and the Format_Item_C1002 docstring
• CHANGELOG.md updated (the PR-number placeholder will be filled in once this PR's number is known)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a new default-enabled parser extension, format-missing-comma, to accept missing commas between character-string edit descriptors and adjacent format items in FORMAT specifications (as supported by common compilers), addressing issue #484 and updating tests/docs accordingly.

Changes:

• Add format-missing-comma to the default EXTENSIONS list.
• Extend Format_Item_C1002 matching with an extension-aware fallback path to parse missing-comma constructs involving character literals.
• Update/expand tests and user documentation (plus changelog) to reflect the new extension and its behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/fparser/two/utils.py	Registers the new format-missing-comma extension as enabled by default.
src/fparser/two/Fortran2003.py	Implements the extension logic in Format_Item_C1002 and adds helper for splitting character literals.
src/fparser/two/tests/fortran2003/test_format_specification_r1002.py	Updates negative/positive expectations for C1002-related missing-comma cases with the extension enabled/disabled.
src/fparser/two/tests/fortran2003/test_format_item_c1002.py	Adds direct tests covering the new missing-comma parsing behavior and disabled-extension behavior.
doc/source/fparser2.rst	Documents the new extension and notes re-generation behavior (commas reintroduced).
CHANGELOG.md	Records the new extension addition.

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[DisciplinedSoftware]

This is intentional: a char-string-edit-desc may not have a kind parameter — F2008 (J3/10-007r1) 10.3.2, C1013 (R1021): "A kind parameter shall not be specified for the char-literal-constant"; likewise F2003 (WG5/N1601) 10.2.1, C1012 (R1019). Compilers enforce this (e.g. gfortran rejects FORMAT(4_'a')), so since this extension exists to mirror what compilers accept for missing commas, recognising kind-prefixed literals here would accept code that is invalid both per the standard and per the compilers. Happy to add kind-param handling if the maintainers prefer consistency with fparser's (lax) Char_Literal_Constant instead.