M2compiler-V3/docs/gaps

Unsupported-gap reproducers

One minimal source per known V3 gap (2026-10-18). Each file's header comment carries its tag 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 gap has a runnable, self-contained example.

Verify one with:

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

Classifying the gaps

The corpus is gcc's gm2 testsuite, so "compiles on the corpus" mixes standard Modula-2 with gm2 leniencies and extensions. Standard Modula-2 (PIM and ISO) requires declaration before use; the only forward reference the language allows is a POINTER's base type (for recursive/self-referential types). V3 deliberately enforces that order, so the decl-order cases below are not missing language features — they are gm2 accepting non-conforming programs.

The class column says what a fix would mean:

• decl-order — gm2 resolves the whole declaration part; strict M-2 forbids the use-before-declaration. Not a standard feature.
• iso/system — a standard ISO Modula-2 / SYSTEM facility V3 lacks.
• gm2-ext — a gm2/PIM extension or a leniency (not portable M-2).
• v3-deviation — V3 is stricter than the standard (a V3 bug).
• 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 (procedure-local part)
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
gap07-convert-builtin	gap07_convert.mod	iso/system	undeclared identifier	procconv2
gap08-set-word-cast	gap08_val_cast_set_word.mod	iso/system	not supported yet	constset, constset2, constset3, packed, bitset5
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

Observed first diagnostics (line:col) at commit time:

gap01_fwd_type_const_conv.mod:5:13   undeclared identifier
gap02_fwd_type_var.mod:7:9           undeclared identifier
gap03_fwd_type_heading.mod:5:17      undeclared identifier
gap04_fwd_const_bound.mod:5:19       undeclared identifier
gap05_fwd_type_size.mod:5:17         undeclared identifier
gap06_fwd_setconst.mod:5:12          undeclared identifier
gap07_convert.mod:8:12               undeclared identifier
gap08_val_cast_set_word.mod:7:35     not supported yet
gap09_scalar_to_bytearray.mod:9:13   invalid call
gap10_var_param_type.mod:9:11        invalid call
gap11_bare_proc_stmt.mod:6:7         invalid call
gap12_variant_empty_arms.mod:8:12    invalid VarLabel
gap13_builtin_proc_name.mod:5:11     ident expected
gap14_builtin_pragma.def:5:11        ident expected
gap15a_qualified_import.mod:8:21     undeclared identifier
gap15b_from_import.mod:6:16          undeclared identifier
gap16_wide_set.mod:5:29              not supported yet
gap17_set_of_cardinal.mod:5:19       not supported yet

Why gap01–gap06 are not standard Modula-2

All six name an identifier before its declaration:

CONST low = ind0(-100);        (* ind0's TYPE section comes later *)
TYPE  ind0 = [-100..100];

gm2 accepts this because it resolves the whole declaration part; PIM/ISO Modula-2 require declaration before use (only a POINTER's base type is exempt). V3 rejecting them is standard-conforming. They are listed for corpus-compatibility accounting, not as missing language features.

gap13 is the opposite case: ORDL is not a reserved word in Modula-2 (ORD, CHR, VAL, … are predeclared identifiers, not keywords), so PROCEDURE ORDL … is legal standard M-2 and V3 is wrong to reject it.

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.