Compiler bug when trying to return an interface-optional with a fallback default value

[dasidiot]

Describe the bug

/tmp/v_1000/v-option-bug.01HZZ86M38ET1YCNSFHJVPHZQ6.tmp.c:13154: error: cannot convert 'struct src__example__IFoo' to 'struct _option_src__example__IFoo'
builder error: 
==================
C error found. It should never happen, when compiling pure V code.
This is a V compiler bug, please report it using `v bug file.v`,
or goto https://github.com/vlang/v/issues/new/choose .
You can also use #help on Discord: https://discord.gg/vlang .

The function in question tries to return a result of the interface directly from the struct, which itself is an optional. If the struct value is not set it invokes a function to create a default. The produced C code does not seem to realize that the value is no longer an optional.

Reproduction Steps

There is a full reproducer here:
https://github.com/dasidiot/v-option-bug

~/src/v-option-bug$ v -cg run .
/tmp/v_1000/v-option-bug.01HZZ86M38ET1YCNSFHJVPHZQ6.tmp.c:13154: error: cannot convert 'struct src__example__IFoo' to 'struct _option_src__example__IFoo'
builder error: 
==================
C error found. It should never happen, when compiling pure V code.
This is a V compiler bug, please report it using `v bug file.v`,
or goto https://github.com/vlang/v/issues/new/choose .
You can also use #help on Discord: https://discord.gg/vlang .

### Expected Behavior

The program should compile without an error.

### Current Behavior

The C compiler fails to convert from the optional form of the interface to the concrete form of an implementation.

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### V version

Current V version: V 0.4.6 1af7b7c, timestamp: 2024-06-09 17:52:56 +0300

### Environment details (OS name and version, etc.)

getwd: /home/user/src/v
vexe: /home/user/src/v/v
vexe mtime: 2024-06-09 19:42:11

vroot: OK, value: /home/user/src/v
VMODULES: OK, value: /home/user/.vmodules
VTMP: OK, value: /tmp/v_1000

Git version: git version 2.39.2
Git vroot status: weekly.2024.22-47-g1af7b7c3
.git/config present: true

CC version: cc (Debian 12.2.0-14) 12.2.0
thirdparty/tcc status: thirdparty-linux-amd64 40e5cbb5

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