`$new(T.pointee_type)` allocates wrong type when generic function is instantiated for multiple pointer types

[enghitalo]

Describe the bug

When a generic function uses $new(T.pointee_type) and is instantiated with two or more different pointer types in the same program, every instantiation generates C code that allocates the same (last-seen) pointee type rather than the per-instantiation type. The result is silently wrong code: the returned pointer's static V type is correct but the heap object behind it is of a different type, leading to memory corruption, wrong values, or runtime errors.

Introduced by 8b5f74e.

Reproduction Steps

module main

struct Foo {
    name string
}

fn make_pointer[T]() T {
    $if T.indirections == 1 {
        mut p := $new(T.pointee_type)
        return p
    } $else {
        return $zero(T)
    }
}

fn main() {
    int_ptr := make_pointer[&int]()
    foo_ptr := make_pointer[&Foo]()
    dump(*int_ptr)
    dump(*foo_ptr)
}

Expected Behavior

[file.v:18] *int_ptr: 0
[file.v:19] *foo_ptr: Foo{
    name: ''
}

Current Behavior

[file.v:18] *int_ptr: 8636372
[file.v:19] *foo_ptr: Foo{
    name: ''
}

*int_ptr reads garbage. Inspecting the generated C with v -keepc shows the make_pointer[&int] instantiation allocates a Foo, not an int:

VV_LOC int* main__make_pointer_T___ptr__int(void) {
    main__Foo* p = HEAP(main__Foo, ((main__Foo){.name = (string){.str=(byteptr)\"\", .is_lit=1},}));
    return p;
}

Both _T___ptr__int and _T___ptr__main__Foo instantiations emit HEAP(main__Foo, ...). The "last seen" pointee type wins for all instantiations.

This blocks refactoring vlib/x/json2/decode.v to use $new(T.pointee_type) instead of the manual create_decoded_ptr helper.

Possible Solution

Resolve T.pointee_type per-instantiation in the C generator instead of caching/sharing across instantiations.

V version

V 0.5.1 6dd9033.eb1d47b

Environment details

linux, Ubuntu 24.04 LTS, gcc 14.2.0, tcc 0.9.28rc

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