# 0064 — non-`$` `T: Type` function param used as a type silently yields `{}`

## Symptom

A function parameter declared `T: Type` (or lowercase `T: type`) — i.e. WITHOUT
the `$` generic-type-parameter sigil — that is then referenced in a type position
(`-> T`, `Closure() -> T`, etc.) silently resolves `T` to a fabricated empty
struct `{}` instead of the caller's argument type. The function "runs" but
produces garbage (the value renders as `T{}`), with no diagnostic.

```sx
idwrap :: (T: Type, f: Closure() -> T) -> T { return f(); }
main :: () -> s32 {
    print("{}\n", idwrap(s32, closure(() -> s32 { return 7; })));  // prints "T{}", want 7
    return 0;
}
```

The correct, working form is the generic `$T: Type` (the `$` introduces the
generic type parameter — see `specs.md` §"`$` generic type parameter
introduction"). With `$T`, the binding is established and the result is `7`.

So this is not a miscompile of a valid program — it's a **missing diagnostic**
for a misuse: a non-generic `Type`-typed value param can't be used as a type, and
should be rejected (or the `$` requirement explained), not silently turned into
an empty struct.

## Reproduction

See above. Compare `idwrap :: ($T: Type, …)` (works, prints 7) vs `idwrap :: (T:
Type, …)` (prints `T{}`).

## Investigation prompt

In [src/ir/lower.zig](../src/ir/lower.zig), `resolveTypeWithBindings` resolves a
`.type_expr` named `T` by checking `type_bindings` (works for `$T`, which
`buildTypeBindings` registers). For a non-`$` `T: Type` param there is no binding,
so resolution falls through to `type_bridge.resolveAstType`, which fabricates an
empty-struct stub for the unknown name `T` — the classic "silent empty struct"
the CLAUDE.md REJECTED-PATTERNS warn about. Fix options: (1) at scan/sema time,
reject referencing a non-`$` `Type`-typed param in a type position with a
diagnostic ("type parameter must be introduced with `$` — write `$T: Type`"); or
(2) make `resolveAstType` return `.unresolved` + a diagnostic for an unknown bare
type name in a generic-eligible position, instead of stubbing `{}`. Deferred —
orthogonal to ERR; the working `$T` idiom exists. Low priority but should not stay
silent.