add test for #116599

Fixes #116599

Author: matthiaskrgr

tests/ui/nll/unexpected-inference-var-ice-116599.rs

@@ -0,0 +1,44 @@
+// ICE unexpected inference var
+// issue: rust-lang/rust#116599
+//@ check-pass
+
+pub trait EvaluateConstMethods {
+    type Trait: TraitWithConstMethods;
+
+    /// **This block breaks**
+    const DATA_3: Data3 = {
+        <<<Self::Trait as TraitWithConstMethods>::Method2 as ConstFn<_, _>>::Body<
+            <<Self::Trait as TraitWithConstMethods>::Method1 as ConstFn<_, _>>::Body<ContainsData1>,
+        > as Contains<_>>::ITEM
+    };
+}
+
+pub trait TraitWithConstMethods {
+    /// "const trait method" of signature `fn(Data1) -> Data2`
+    type Method1: ConstFn<Data1, Data2>;
+
+    /// "const trait method" of signature `fn(Data2) -> Data3`
+    type Method2: ConstFn<Data2, Data3>;
+}
+
+/// A trait which tries to implement const methods in traits
+pub trait ConstFn<Arg, Ret> {
+    type Body<T: Contains<Arg>>: Contains<Ret>;
+}
+
+/// A ZST which represents / "contains" a const value which can be pass to a [`ConstFn`]
+pub trait Contains<T> {
+    const ITEM: T;
+}
+
+pub struct ContainsData1;
+impl Contains<Data1> for ContainsData1 {
+    const ITEM: Data1 = Data1 {};
+}
+
+// Arbitrary data
+pub struct Data1 {}
+pub struct Data2 {}
+pub struct Data3 {}
+
+pub fn main() {}