M2compiler-V3/docs/gaps

Unsupported / non-standard cases

One minimal source per known V3 gap (2026-10-18). Each file's header comment carries its tag, its Modula-2 status, and the gm2 corpus file(s) it stands for. These are not part of the test suite (they are expected to fail); they exist so each case has a runnable example.

Verify one with:

cd compiler && ./build.sh
./M2 ../docs/gaps/gap07_convert.mod     # prints the diagnostic on stdout

Policy: follow Modula-2, not degraded gm2

The corpus is gcc's gm2 testsuite, so "compiles on the corpus" mixes conforming Modula-2 with gm2 leniencies and extensions. This project follows the language:

Standard Modula-2 (PIM and ISO) requires declaration before use; the declaration part is ordered CONST, TYPE, VAR. The only forward reference the language allows is a POINTER's base type (for recursive/self-referential types).

V3 therefore intentionally rejects use-before-declaration; that is conforming behaviour, not a gap. gm2 accepts such programs because it resolves the whole declaration part before checking — a leniency.

The class column says what a change would mean:

• decl-order — not legal Modula-2; gm2 leniency. V3 rejects it correctly. Not a target.
• gm2-ext — not standard; a gm2/PIM extension or leniency. Not a target unless we choose gm2 compatibility.
• iso/system — a standard ISO Modula-2 / SYSTEM facility V3 lacks. A genuine gap.
• v3-deviation — V3 is stricter than the standard (a V3 bug to fix).
• impl-limit — the language allows it; V3 has a fixed implementation limit.

Index

tag	file	class	diagnostic	corpus exemplars
gap01-fwd-type-const-conv	gap01_fwd_type_const_conv.mod	decl-order	undeclared identifier	subrange4, subrange7
gap02-fwd-type-var	gap02_fwd_type_var.mod	decl-order	undeclared identifier	subrange11, arrayptr, arrayptr6, constructor1, constructor3, param3
gap03-fwd-type-heading	gap03_fwd_type_heading.mod	decl-order	undeclared identifier	shift3, shift4
gap04-fwd-const-bound	gap04_fwd_const_bound.mod	decl-order	undeclared identifier	setconst3
gap05-fwd-type-size	gap05_fwd_type_size.mod	decl-order	undeclared identifier	constsize2
gap06-fwd-setconst	gap06_fwd_setconst.mod	decl-order	undeclared identifier	setoverflow
gap08-set-word-cast	gap08_val_cast_set_word.mod	iso/system	not supported yet	constset, constset2, constset3, packed
gap09-scalar-to-bytearray	gap09_scalar_to_bytearray.mod	gm2-ext	invalid call	aochar, arrayofbyte, bytearray
gap10-var-param-type	gap10_var_param_type.mod	gm2-ext	invalid call	testparam2, testparam3
gap11-bare-proc-stmt	gap11_bare_proc_stmt.mod	gm2-ext	invalid call	tinyproc2, tinyproc4
gap12-variant-empty-arms	gap12_variant_empty_arms.mod	gm2-ext	invalid VarLabel	varient2, varient3, varient8, tsize, tsize2
gap13-builtin-proc-name	gap13_builtin_proc_name.mod	v3-deviation	ident expected	fileio.def, fileio.mod
gap14-builtin-pragma	gap14_builtin_pragma.def	gm2-ext	ident expected	builtin.def, builtin.mod
gap15a-qualified-import	gap15a_qualified_import.mod (+ gap15_base.def, gap15_lib.def)	iso/system	undeclared identifier	impf, impk, impl, impq
gap15b-from-import	gap15b_from_import.mod (+ gap15_base.def, gap15_lib.def)	iso/system	undeclared identifier	impi
gap16-wide-set	gap16_wide_set.mod	impl-limit	not supported yet	settest1
gap17-set-of-cardinal	gap17_set_of_cardinal.mod	impl-limit	not supported yet	settest3
gap18-set-case-labels	gap18_set_case_labels.mod	gm2-ext	not supported yet	bitset5
gap19-fwd-array-index	gap19_fwd_array_index.mod	decl-order	not supported yet	record7, complextypes
gap20-fwd-set-of-type	gap20_fwd_set_of_type.mod	decl-order	undeclared identifier	tinyset6, tinyset7
gap21-fwd-record-field	gap21_fwd_record_field.mod	decl-order	undeclared identifier	record14.def, record15.def, record16.def

First diagnostic (message) per file at commit time (line:col varies as comments change):

gap01_fwd_type_const_conv.mod   undeclared identifier   (ind0)
gap02_fwd_type_var.mod          undeclared identifier   (arr)
gap03_fwd_type_heading.mod      undeclared identifier   (large)
gap04_fwd_const_bound.mod       undeclared identifier   (n)
gap05_fwd_type_size.mod         undeclared identifier   (t)
gap06_fwd_setconst.mod          undeclared identifier   (CharSet / tab)
gap08_val_cast_set_word.mod     not supported yet       (VAL of a set)
gap09_scalar_to_bytearray.mod   invalid call            (scalar -> ARRAY OF BYTE)
gap10_var_param_type.mod        invalid call            (VAR actual type)
gap11_bare_proc_stmt.mod        invalid call            (bare proc value)
gap12_variant_empty_arms.mod    invalid VarLabel
gap13_builtin_proc_name.mod     ident expected          (ORDL)
gap14_builtin_pragma.def        ident expected          (__BUILTIN__)
gap15a_qualified_import.mod     undeclared identifier   (Gap15Lib.red)
gap15b_from_import.mod          undeclared identifier   (red / green)
gap16_wide_set.mod              not supported yet       (span > 65536)
gap17_set_of_cardinal.mod       not supported yet       (unbounded base)
gap18_set_case_labels.mod       not supported yet       (set as CASE selector)
gap19_fwd_array_index.mod       not supported yet       (array index declared later)
gap20_fwd_set_of_type.mod       undeclared identifier   (SET OF base declared later)
gap21_fwd_record_field.mod      undeclared identifier   (record field type declared later)

Genuine gaps (worth fixing, standard Modula-2)

• gap07 CONVERT — fixed (2026-10-18): CONVERT(T, x) is accepted as a VAL sibling (procconv2 passes).
• gap08 set↔word CAST/VAL — BITSET has a machine-word representation in Modula-2.
• gap13 builtins as identifiers — ORD, CHR, VAL, ORDL, … are predeclared identifiers, not reserved words, so V3 must accept them as user names. (This is a V3 deviation, not a corpus gap.)
• gap15 enumeration literals with an imported type — ISO import/export semantics.
• Foreign modules FOR "C" / EXPORT UNQUALIFIED — already fixed (§ STATUS).

gap16/gap17 are implementation limits (raising the set span cap).

Not represented by a source

• Size-sensitive V3 miscompile — a latent codegen bug that only manifests at the compiler's own scale (the historical "removing ASTDUMP breaks the fixpoint"); see docs/summary_l3_selfhost_fix.md. No small standalone source triggers it.

Companion files (gap15_base.def, gap15_lib.def) are not gaps themselves; they model the impc → impb re-export chain.