Assertion on empty union switch

[Skytrias]

Context

• Operating System: Linux Mint 20 - Kernel 5.4.0-73-generic
• Please paste odin version output: odin version dev-2021-08:7b2f6aaa

Expected Behavior

Compilation should be disallowed when switching on an empty union.

Current Behavior

Compiles, but asserts at runtime / startup.

Failure Information (for bugs)

src/llvm_backend_general.cpp(1108): Assertion Failure: type_size_of(ut) > 0

Steps to Reproduce

Test :: struct {
    a: TestState,
}

TestState :: union {

}

call_a :: proc(test: ^Test) {
    #partial switch thing in &test.a { // <--- this is really it

    }
}

call_b :: proc(test: Test) {
    #partial switch thing in test.a { // <--- this is really it

    }
}

main :: proc() {
    // both break
    call_a({});
    call_b({});
}

[ghost]

@Skytrias does this really break at runtime? I'm not even able to compile it. The compiler fails on that assertion.

No binary is produced when I run it over here.

System info

OS Name: Microsoft Windows 10 Enterprise
OS Version: 10.0.18363 N/A Build 18363
OS Manufacturer: Microsoft Corporation
Odin version: dev-2021-09:461ca408

Console output:

p:\>\Odin\odin.exe build repro.odin -debug -opt:0
p:\Odin\src\llvm_backend_general.cpp(1106): Assertion Failure: `type_size_of(ut) > 0`

---

In any case, maybe empty unions should be disallowed entirely? I'm not sure I see the case for allowing it at all.
This should probably be a semantic analysis check to fail even before code gen starts.

@gingerBill is allowing empty unions something intended? 🤔

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