Incremental compilation fails for valid source when correctly widening the return error set

[Zirunis]

Zig Version

0.16.0-dev.1246+4b593a6c2

Steps to Reproduce and Observed Behavior

Wrong code:

const std = @import("std");

pub fn main() void {  //no error set declared here!
    try std.testing.expect(2 + 2 == 5);
}

zig build --watch -fincremental correctly points out the error:

src\main.zig:4:5: error: expected type 'void', found '@typeInfo(@typeInfo(@TypeOf(testing.expect)).@"fn".return_type.?).error_union.error_set'
    try std.testing.expect(2 + 2 == 5);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src\main.zig:3:15: note: function cannot return an error
pub fn main() void {
              ^~~~

Fix the code:

const std = @import("std");

pub fn main() !void {  //now correct
    try std.testing.expect(2 + 2 == 5);
}

zig build --watch -fincremental incorrectly throws an error despite the source being valid:

error: Found return instr that returns non-void in Function of void return type!
         ret i16 %1, !dbg !50393
        voidFound return instr that returns non-void in Function of void return type!
         ret i16 0, !dbg !50393
        voidLLVM ERROR: Broken module found, compilation aborted!

error: process exited with error code 9

Expected Behavior

Incremental compilation should lead to an equivalent result as running zig build again. In this case that means building successfully.

Related: #25874