sx/issues/0041-pointer-type-not-parsed-as-expression.md

issue-0041 — Pointer types don't parse as expressions / type-argument positions

Symptom

A pointer type like *u8 or *void does not parse in positions where a type expression is expected as a value, e.g.:

• As an argument to a $T: Type builtin: size_of(*u8), align_of(*u8).
• On the RHS of a type alias: Ptr :: *u8;.

In each case the parser emits error: unexpected token in expression at the column of the *.

Pointer types DO parse correctly in dedicated type-annotation positions: function parameters ((p: *u8)), struct fields (field: *u8;), variable annotations (p: *u8 = ...;). So the bug is a parsing inconsistency between "type-annotation context" and "expression context where a type is expected".

This is pre-existing — it affects size_of (already shipping) and was just made more visible by adding align_of in Phase 0.6 of the MEM plan. Not a regression introduced by 0.6, but a real limitation worth pinning down because:

• Phase 1+ of the MEM plan will need size_of(*T) / align_of(*T) in user-facing allocator helpers if we want to stay terse — e.g. serializing a pointer-typed field in field_value_int patterns.
• It's a discoverability cliff. New users WILL write size_of(*u8), see "unexpected token", and have to learn the workaround.

Reproduction

#import "modules/std.sx";

main :: () -> s32 {
    n := size_of(*u8);   // error: unexpected token in expression
    print("{}\n", n);
    0;
}

Also fails on the alias form:

#import "modules/std.sx";

Ptr :: *u8;              // error: unexpected token in expression

main :: () -> s32 { 0; }

Both sx run and sx build reject identically.

Confirmed working workarounds

A pointer type DOES resolve when bound through a *void-style variable type and then cast, or routed via a helper:

// Workaround A: anonymous struct holding the pointer field, then
// pull alignment from the wrapping struct (clumsy).
Wrap :: struct { p: *u8; }
n := align_of(Wrap);          // 8 — correct for pointer alignment.

// Workaround B: explicit *void
n := size_of(*void);          // ALSO fails — same parse error.

Workaround B is NOT functional — it has the same parse error. Only the wrap-in-struct or type-alias-via-typedef trick is currently viable for code that needs pointer size/alignment.

There is no clean way today to write size_of(*u8). The whole class of "ptr type as type-expression value" is unsupported.

Investigation prompt

Pointer types parse via a dedicated parseTypeExpr (or similar) path that the parser invokes in type-annotation positions (param lists, field declarations, variable annotations). The expression grammar used in argument positions (e.g. inside size_of(...)) dispatches through parseExpr instead, which treats * as "either prefix unary deref or infix multiplication" — neither matches the desired "type literal" interpretation.

The fix likely belongs in the call-argument parser path: when the callee is a builtin that takes $T: Type, OR more broadly whenever the parser sees a * at the start of an expression followed by an identifier that resolves to a type, it should dispatch to parseTypeExpr instead of parsePrefixUnary.

Implementation sketch:

• Check src/parser.zig for the expression entry point that handles * prefix. Today it likely returns a unary_op { op = deref, operand = … } AST node.
• Look at how lower.zig's resolveTypeArg consumes the AST node for size_of(s32) — what AST shape does it expect for a type literal? Probably an identifier whose name resolves to a type.
• The fix should extend resolveTypeArg to also accept a unary_op { op = deref, ... } and treat it as "pointer to resolved type" — equivalent to Ptr$T in spec terms.
• For the type-alias case (Ptr :: *u8;), the RHS of a :: const decl is parsed as an expression. The parser needs to recognize that the LHS-determined shape (type-level alias) should bias the RHS parser toward parseTypeExpr. Or: extend the constant-fold path to interpret unary_op { deref, T } as a type literal when used as a type.

Verification:

1. Add examples/issue-0041.sx with the repro above and tests/expected/issue-0041.txt capturing the expected output (size_of(*u8) → 8).
2. Confirm bash tests/run_examples.sh still passes everything else (151 tests currently).
3. Run tools/verify-step.sh to confirm chess on three platforms.
4. Also bake into examples/50-smoke.sx near the existing align_of lines — add align_of(*u8), size_of(*u8), align_of(*void) and regen.

Hazard: any change to expression parsing affects a huge surface. Watch for these contexts to make sure they still work post-fix:

• a * b (multiplication)
• *p (prefix deref read)
• *p = … (prefix deref write)
• func(a, *b) (deref as argument) A surgical "is the next token a built-in type identifier" lookahead at the * site is probably less invasive than a wholesale type-expression-in-expression-position rewrite.

Plan-level impact

None for Phase 0.6 — align_of shipped and works for every shape that size_of works for (primitives, structs, type aliases through non-pointer types). The 50-smoke test addition uses only non-pointer types, so it's stable.

Phase 1+ should bake an align_of(*u8) test once the parser fix lands, since the allocator API will want to round-trip pointer alignments at some call sites.