Rollup merge of #79514 - Julian-Wollersberger:order-dependent-bounds,…

… r=Mark-Simulacrum

Add test for issue #54121: order dependent trait bounds

This adds a test for #54121, which has already been fixed by #73905. Now that issue can be closed.

I tested the test [on the playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=6cb061d3b81518f268649551eb67769f) where it indeed fails on stable 1.48, but compiles successfully on beta and nightly.

fixes #54121

src/test/ui/associated-type-bounds/order-dependent-bounds-issue-54121.rs

@@ -0,0 +1,47 @@
+// check-pass
+
+// From https://github.com/rust-lang/rust/issues/54121/
+//
+// Whether the code compiled depended on the order of the trait bounds in
+// `type T: Tr<u8, u8> + Tr<u16, u16>`
+// But both should compile as order shouldn't matter.
+
+trait Tr<A, B> {
+    fn exec(a: A, b: B);
+}
+
+trait P {
+    // This compiled successfully
+    type T: Tr<u16, u16> + Tr<u8, u8>;
+}
+
+trait Q {
+    // This didn't compile
+    type T: Tr<u8, u8> + Tr<u16, u16>;
+}
+
+#[allow(dead_code)]
+fn f<S: P>() {
+    <S as P>::T::exec(0u8, 0u8)
+}
+
+#[allow(dead_code)]
+fn g<S: Q>() {
+    // A mismatched types error was emitted on this line.
+    <S as Q>::T::exec(0u8, 0u8)
+}
+
+// Another reproduction of the same issue
+trait Trait {
+    type Type: Into<Self::Type1> + Into<Self::Type2> + Copy;
+    type Type1;
+    type Type2;
+}
+
+#[allow(dead_code)]
+fn foo<T: Trait>(x: T::Type) {
+    let _1: T::Type1 = x.into();
+    let _2: T::Type2 = x.into();
+}
+
+fn main() { }