Mismatched types for a conditional containing closures inside method call

[marmac02]

Hello! I have encountered an issue when using a conditional returning closures inside a method call - specifically, the compiler is unable to coerce closure types to Box<dyn Fn(...)->...> for a very specific case. Consider this code.

trait Foo<I : Fn() -> i16>{
    fn foo(&self, param : I) -> ();
}
struct S {}

impl Foo<Box<dyn Fn() -> i16>> for S {
    fn foo(&self, param : Box<dyn Fn() -> i16>) -> () {
        unimplemented!();
    }
}

fn main() {
    let s = S {};
    
    let cond = true;
    s.foo(if cond { //replace cond with cond==true
        Box::new(|| {42})
    }
    else {
        Box::new(|| {42})
    });
}

The above code does not compile, with an error message saying that if and else have incompatible types. When cond is replaced with cond==true, the code will compile.

This issue is very similar to an issue I submitted a while back (#126988). But only now have I found that the compiler behaves differently for different conditionals.

Also, when you enclose the type of param in another type, for example Vec, the code will compile even with just cond if-statement conditional. See adjusted code below that compiles:

trait Foo<I : Fn() -> i16>{
    fn foo(&self, param : Vec<I>) -> ();
}
struct S {}

impl Foo<Box<dyn Fn() -> i16>> for S {
    fn foo(&self, param : Vec<Box<dyn Fn() -> i16>>) -> () {
        unimplemented!();
    }
}

fn main() {
    let s = S {};
    
    let cond = true;
    s.foo(if cond {
        vec![if cond {Box::new(|| {42})} else {Box::new(|| {43})}]
    }
    else {
        vec![Box::new(|| {42})]
    });
}

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rustc 1.75.0 (82e1608df 2023-12-21)
release: 1.75.0
LLVM version: 17.0.6