gen-c: the element type of a LOCAL array is passed through unmapped (W583, second position) #3448

What

fn probe(v: i32) -> i32 { var x : [4]GF16; return 0; }

GF16 x[4];
error: use of undeclared identifier 'GF16'

The same element is lowered correctly everywhere else:

This is W583, in a position the repair never reached

The local path gates on is_primitive:

let c_elem = if Self::is_primitive(elem) {
    Self::type_to_c(elem).to_string()
} else {
    elem.to_string()          // t27 text, verbatim
};

Two hundred lines below, on param_type_to_c, sits the note retiring exactly that gate:

W583: this used to be gated on is_primitive, which lists only the integer scalars -- so f32, f64, str, string and gf16 took the pass-through arm and reached C unmapped even after type_to_c learned them. type_to_c already passes genuinely custom types through, so the gate only ever suppressed correct mappings.

Same sentence, same file, other position.

Two more in the same eight lines

• var x : []u8; emits uint8_t x[]; — "definition of variable with array type needs an explicit size or an initializer". Only without an initialiser: T x[] = {…} is legal C and takes its size from the list.
• var x : []const u8; emits const u8* x — "unknown type name 'u8'". The qualifier lives inside the element text, so the element is the literal const u8; param_type_to_c strips it and this path did not.

Measured

Whole corpus, -ferror-limit=0:

Three files improve: math/property_test_template 46→43, nn/hslm 79→76, numeric/gf16 151→150.

A note on every error count I have published this session

Clang's default -ferror-limit=20 was censoring the aggregate. The corpus emits 15133 errors, not the 3849 I have been reporting — the default stops at twenty per file and 141 files reach it.

The per-file splits I reported were sound (they were counted per file), but the totals were floors. Re-measured without the limit, #3447 moved 15188 → 15133 (−55), not the −24 I published.

Found by

tools/backend_parity_table.py extended to a fourth position, local. The first probe there — var x : T; with no initialiser — measured the wrong thing: Zig and Verilog emit nothing for an uninitialised local, so the table was reading their dead-code removal. Initialising from a parameter of the same type fixed it, and the self-check now covers all four positions.