82d6b8da0e
A '*[N]T' receiver in an index expression reached LLVM emission with an unresolved element type and tripped the panic sentinel — no read or write spelling worked. ptrToArrayElem on Lowering recognises the shape; the index READ path GEPs the pointee array through the pointer value and loads the element; the write / compound-assign / lvalue / addr-of-element paths and the expression typer resolve the element type through the same helper (their GEP machinery already handled a pointer base). Kept out of getElementType so slice paths don't half-accept a raw pointer base. Regression: examples/0176 (read, write, compound, element ptr + deref).
63 lines
2.8 KiB
Markdown
63 lines
2.8 KiB
Markdown
# 0117 — indexing through a pointer-to-array panics at LLVM emission
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> **RESOLVED** (2026-06-11). `ptrToArrayElem` on Lowering recognises the
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> `*[N]T` receiver; the index READ path GEPs the pointee array through the
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> pointer value and loads the element; the WRITE / compound-assign /
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> lvalue / addr-of-element paths and the expression typer resolve the
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> element type through the same helper (their GEP machinery already
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> handled a pointer base). Postfix deref (`p.*`) was always the deref
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> spelling — the issue's `(*p)[2]` note was a wrong-syntax red herring.
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> Regression test: examples/0176-types-pointer-to-array-index.sx
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> (read, write, compound, element pointer + deref).
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**Symptom.** Indexing through a `*[N]T` pointer is neither lowered nor
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diagnosed: the index expression reaches the LLVM emitter with the
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`.unresolved` type sentinel and trips the panic tripwire.
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- **Observed**: `panic: unresolved type reached LLVM emission`
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(src/backend/llvm/types.zig:175, via `emitIndexGet`).
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- **Expected**: either `p[2]` auto-derefs the pointer-to-array (load the
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pointer, GEP into the array — mirroring the field-access auto-deref on
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struct pointers), or a clean diagnostic if indexing through `*[N]T` is
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out of spec. Never an emit-time panic.
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Pre-existing — reproduces on plain locals with no module-level features
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involved (found while pinning `@K` reads for PLAN-CONST-AGG step 1; the
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same panic fires for `@<var-decl global array>` and `@<array const>`).
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Also note: the explicit-deref spelling `(*p)[2]` does not parse as a
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deref — it lowers as `unknown_expr` ("unresolved 'unknown_expr'"), so
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there is no working spelling for reading an element through a
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pointer-to-array.
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## Reproduction
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```sx
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#import "modules/std.sx";
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main :: () {
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k : [4]s64 = .[11, 22, 33, 44];
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p := @k; // *[4]s64
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print("{}\n", p[2]); // expected 33; panics at emission today
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}
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```
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## Investigation prompt
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`emitIndexGet` receives an `index_get` whose result type is `.unresolved`
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— the lowering of `index_expr` over a receiver of pointer-to-array type
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produces no deref. Suspected area: the index lowering in
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src/ir/lower/expr.zig (`lowerIndexExpr` or equivalent — find the
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`index_expr` arm) and its typing twin in src/ir/expr_typer.zig: both
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handle array / slice / many-pointer receivers but not
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`pointer → array`. The fix likely mirrors the struct-pointer auto-deref:
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when the receiver type is `.pointer` whose pointee is `.array`, load the
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pointer value and index the pointee array (element type = pointee
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element), in BOTH the typer and the lowering. Check the assignment-target
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path too (`p[1] = v` through a pointer-to-array).
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Verification: the repro above prints `33`; add a pin under
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examples/01xx-types-… covering read AND write-through
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(`p[1] = 5; print k[1]` → 5 for a mutable local). Suite + zig build test
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stay green.
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