[naga] automatic conversion in return statements

[k2d222]

related #4400

Description
on 24.0.0 naga does not seem to support automatic type conversion in return statements. Is it a bug?

this fails validation

fn foo(x: u32) -> u32 {
    return 0;    // this is ERR, but return 0u; is OK
}

error: Function [0] 'foo' is invalid
  ┌─ wgsl:1:1
  │  
1 │ ╭ fn foo(x: u32) -> u32 {
2 │ │     return 0;
  │ │            ^ naga::Expression [1]
  │ ╰─────────────^ naga::Function [0]
  │  
  = The `return` value Some([1]) does not match the function return value

Repro steps

link to WESL playground. hit compile.

Expected vs observed behavior

The WGSL spec is not completely clear about when automatic conversion is allowed to happen (it is clarified on a case-by-case basis, e.g. for call statements in the overload resolution section, but the return statement spec just says The type of the return value must match the return type of the function.)

I suspect that the spec intent for return expressions is the same as for declarations with an abstract initializer and explicit type, i.e. the expression is automatically converted to the return type if a conversion exists. this is also what tint/dawn does.

[ErichDonGubler]

Tagging in @jimblandy and @jamienicol.

[jamienicol]

This should have been fixed by #7035. @k2d222 can you still reproduce on trunk?