GAMS writer: map non-finite float values to GAMS special values (NA / INF)

[cjck944084735-dot]

Summary

The GAMS writer emits Python's string rendering of non-finite floats (nan, inf, -inf), which are not valid GAMS numeric literals, so the exported .gms file fails in GAMS with errors like the symbol nan is undefined.

Fixes #3911.

Root cause

All numeric output in pyomo/repn/plugins/gams_writer.py goes through ftoa(), which formats with '%.17g'. Non-finite floats therefore render as nan / inf / -inf, e.g. (current main, default warmstart=True):

c2.. GAMS_OBJECTIVE =e= power(x1, 2) + x2 + nan ;
x1.l = nan;
x2.l = inf;

GAMS's special numeric literals are NA, INF/+INF/-INF (and EPS), so every one of those lines is rejected by GAMS. This hits all value-emitting paths of the writer: constraint bounds, expression constants (fixed Vars/Params), warmstart .l values, and fixed-variable .fx values.

The fix

Route every numeric output site in the writer through a small wrapper that post-processes the ftoa() result:

def gams_ftoa(val, parenthesize_negative_values=False):
    a = ftoa(val, parenthesize_negative_values)
    # Python renders non-finite floats as nan/inf, which are not valid GAMS
    # numeric literals: map them to GAMS's special values NA and INF
    if a == 'nan':
        return 'NA'
    if a == '(-inf)':
        return '(-INF)'
    if a.endswith('inf'):
        return a[:-3] + 'INF'
    return a

After the change the same model exports as:

c2.. GAMS_OBJECTIVE =e= power(x1, 2) + x2 + NA ;
x1.l = NA;
x2.l = INF;

Two notes on float semantics that the new tests document:

• -1 * nan is nan (the sign is lost), so - z with z = nan exports as + NA.
• -inf values in monomials are parenthesized by ftoa, hence + (-INF).

Testing

• New test_nonfinite_var_to_string (expression path: fixed nan/inf/-inf Vars) and test_nonfinite_var_values_write_gams (full model write: .l warmstart values and the fixed-expression constant, plus assertions that no bare nan/inf appears anywhere in the file).
• Full existing suite pyomo/repn/tests/gams/ passes (24 passed, 1 skipped) and black --check is clean.
• Verified the reporter's scenario from Handling of float('nan') especially in GAMSWriter #3911 (Var(initialize=float('nan')) + GAMS solve script) no longer emits invalid literals.

Note: gams_writer_v2

pyomo/repn/plugins/gams_writer_v2.py has the same pattern for warmstart values (ostream.write(f"{v}.l = {pyomo_v.value}; "), line ~537). I left it untouched to keep this PR focused on the default gams writer from the issue; happy to apply the equivalent mapping there in this PR or a follow-up, whichever the maintainers prefer.