Add regression test to close #51217

Author: AnthonyLatsis

test/Generics/unbound.swift

@@ -106,3 +106,34 @@ func nested(_: OuterStruct.InnerStruct<Int>) {}
 
 func nested(_: OuterStruct<Int>.InnerStruct) {}
 // expected-error@-1 {{reference to generic type 'OuterStruct<Int>.InnerStruct' requires arguments in <...>}}
+
+
+func assertExactType<T>(of _: T, is _: T.Type) {}
+
+// https://github.com/apple/swift/issues/51217
+protocol P {
+  associatedtype A
+  associatedtype B
+}
+do {
+  struct Concrete: P {
+    typealias A = Int
+    typealias B = Bool
+  }
+  struct Generic<A, B>: P {}
+
+  struct BinderGenericParams1<T1: P, T2: P>
+  where T1.A == T2.A, T1.B == T2.B {
+    static func bind(_: T1, _: T2) -> T2 {}
+  }
+  struct BinderGenericParams2 {
+    static func bind<T1: P, T2: P>(_: T1, _: T2) -> T2
+    where T1.A == T2.A, T1.B == T2.B {}
+  }
+
+  let x1 = BinderGenericParams1.bind(Concrete(), Generic())
+  let x2 = BinderGenericParams2.bind(Concrete(), Generic())
+
+  assertExactType(of: x1, is: Generic<Int, Bool>.self)
+  assertExactType(of: x2, is: Generic<Int, Bool>.self)
+}