Double same-bound where clause causes failed type inference

[collinoc]

I came across this scenario where having two functions with the same where-bounds causes type inference failure:

pub fn foo<S>(_: S) where String: From<S> {}
pub fn bar<S>(s: S) where String: From<S> {
    // Mismatched types error!
    foo(String::from(s)); 
}

I would expect the type inference to hold up and allow for calling foo with a String constructed in bar. Instead it fails with a mismatched types error (E0308) saying expected type parameter `S`, found `String`

Notably, ANY String or &str constructed in bar and passed to foo will fail type inference. For example:

pub fn baz<S>(_: S) where String: From<S> {
    // Mismatched types error!
    foo(""); 
}

pub fn bam<S>(_: S) where String: From<S> {
    // Mismatched types error!
    let s: String = "".to_owned();
    foo(s); 
}

Some examples of similar scenarios that work, but show that there shouldn't be anything wrong with the actual operation (particularly bap):

pub fn bam<S>(s: S) where String: From<S> {
    // OK!
    foo(s); 
}
pub fn bap<S>(s: S) where String: From<S> {
    // OK!
    foo::<String>(String::from(s)); 
}
pub fn bao() {
    // OK!
    let s: String = "".to_owned();
    foo(s); 
}

// Strangely, inverting the From bound to an Into bound fixes things
pub fn foo2<S>(_: S) where S: Into<String> {}
pub fn bar2<S>(s: S) where String: From<S> {
    // OK!
    foo2(String::from(s)); 
}

Meta

This occurs on stable (1.82.0), beta (1.83.0-beta.4 // 2024-11-02 67512de), and nightly (1.83.0-nightly // 2024-11-03 b8c8287) in debug and release modes.

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[hanna-kruppe]

I think this is #24066 yet again.

[jieyouxu]

Labelling as needs-investigation for someone more knowledgeable to double-check if it's the same root cause.
@rustbot label +E-needs-investigation -needs-triage