TypeSystemAstBuilder writes the C# 14 "operator +=" declaration form, behind a
support flag like the other version-gated operator syntax; a non-public
operator has no legal operator declaration (CS9308) and falls back to a plain
method, except an explicit interface implementation, which is private in
metadata but still written in operator form. An instance operator hides by
signature like an ordinary method, so it can carry "new"; the
[CompilerFeatureRequired] marker the compiler emits is removed like the other
feature markers. Tooltips get the same rendering via a ConversionFlags bit;
widening ConversionFlags.All also turns on the existing checked-operator and
unsigned-right-shift flags, which the tooltip ambience now sets explicitly.
The test fixtures land here, where the pipeline is complete end to end: pretty
and IL round-trips, correctness runs against Roslyn's C# 14 binding (including
the operator-inheritance matrix and a ref-local target), and the ugly
configuration pins the output with the setting off.
Assisted-by: Claude:claude-opus-5:Claude Code
Siegfried Pammer
e4fa167afc