#4197 Unions containing flexible-array structures bypass containment diagnostics

[Christopher Bazley]

1: Sample code that reproduces the problem.

struct S { int size; char data[]; };
union U { struct S value; int other; };
struct T { union U value; };

2: Exact command used to run SDCC on this sample code.

sdcc -mz80 --std=c2y --nostdinc -S bug.c -o bug.asm

3: SDCC version tested.

SDCC 4.6.4 #17064 (Linux), tested 21 October 2026.

4: Observed and expected behaviour.

SDCC exits successfully and emits no diagnostic.

WG14 N3886,
6.7.3.2 paragraph 3, specifies the constraint violated by the sample.

A union may contain a structure with a flexible array member. That union, including any union recursively containing it, must not itself be a structure member or an array element. Both prohibited contexts are silently accepted, including an extra union nesting level. A union declaration alone is valid; fixed-size arrays make both containing contexts valid.

5: Additional checks.

The sample is silently accepted on Z80, MCS51 default, STM8 default, PDK14 and MOS65C02 in C2Y mode. These are translation and diagnostic checks; no runtime result is claimed.

Compiler Explorer: SDCC 4.6.0, SDCC trunk and GCC 16.2. All three panes use the sample above. The saved source, compiler selections and options were round-trip verified on 21 October 2026. Both SDCC panes use -mz80 --std=c2y; GCC uses -std=c2y -pedantic-errors -fsyntax-only. The public trunk build is r17013, distinct from the local r17064 retest.

CE compiler	Observed result
SDCC 4.6.0	Accepted silently
SDCC trunk r17013	Accepted silently
GCC 16.2	Rejected with diagnostics

Case	Expected	Current SDCC results
flex-fixed-control.c	Valid	Accepted silently on all five targets
flex-nested-union-array.c	Diagnostic required	Accepted silently on all five targets
flex-nested-union-member.c	Diagnostic required	Accepted silently on all five targets
flex-union-array.c	Diagnostic required	Accepted silently on all five targets
flex-union-control.c	Valid	Accepted silently on all five targets
flex-union-member.c	Diagnostic required	Accepted silently on all five targets

Related report: [bugs:#4094]. The earlier report tests an array whose element is the flexible-array structure itself; this report tests containment through a union, including recursive unions.