A qualified write to a class-level static var was parsed and visibility-checked
but not lowered — semantic reported it as "not yet supported". It now lowers
to a plain store of the shared global slot, identical to the bare form a static
method writes (StaticVar := V).
Semantic marks the TFieldAssignment with IsClassVarWrite + ClassVarEmitName +
ClassVarLhsType after enforcing visibility. Both backends delegate to their
existing global-store path (EmitAssignment) via a borrowed synthetic
TAssignment — the Expr is shared and nilled before the temp is freed — so the
scalar, class-ARC, and pointer store logic is reused verbatim rather than
duplicated.
Verified on both backends: scalar (int/enum/bool/float), class with ARC
(assign + nil-release). Adds e2e TestRun_StaticVar_QualifiedWrite_Scalar and
_ClassARC (each compiled+run under native and QBE).
Two adjacent limitations remain, both pre-existing and independent of this
change (documented in bugs.txt): interface-typed static vars mishandle the
2-slot fat-pointer store (the bare form is equally affected), and a chained
l-value base through a qualified static var (TFoo.X.field := V) is unresolved
in the assignment-receiver path while the read form works.
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