[TypeChecker] SE-0347: Allow same-type requirements connecting dependent members

Since the types don't flow from member to the base, there is no risk
that dependent member type is going to affect other generic parameters
through this kind of requirement.

Resolves: SR-16069

Author: xedin

lib/Sema/TypeCheckConstraints.cpp

@@ -621,6 +621,12 @@ Type TypeChecker::typeCheckParameterDefault(Expr *&defaultValue,
             !containsTypes(rhsTy, genericParameters))
           continue;
 
+        // If both sides are dependent members, that's okay because types
+        // don't flow from member to the base e.g. `T.Element == U.Element`.
+        if (lhsTy->is<DependentMemberType>() &&
+            rhsTy->is<DependentMemberType>())
+          continue;
+
         // Allow a subset of generic same-type requirements that only mention
         // "in scope" generic parameters e.g. `T.X == Int` or `T == U.Z`
         if (!containsGenericParamsExcluding(lhsTy, genericParameters) &&

test/Constraints/type_inference_from_default_exprs.swift

@@ -175,3 +175,27 @@ func test_magic_defaults() {
   let _ = with_magic()
   let _: String = generic_with_magic()
 }
+
+// SR-16069
+func test_allow_same_type_between_dependent_types() {
+  struct Default : P {
+    typealias X = Int
+  }
+
+  struct Other : P {
+    typealias X = Int
+  }
+
+  struct S<T: P> {
+    func test<U: P>(_: U = Default()) where U.X == T.X { // expected-note {{where 'T.X' = 'String', 'U.X' = 'Default.X' (aka 'Int')}}
+    }
+  }
+
+  func test_ok<T: P>(s: S<T>) where T.X == Int {
+    s.test() // Ok: U == Default
+  }
+
+  func test_bad<T: P>(s: S<T>) where T.X == String {
+    s.test() // expected-error {{instance method 'test' requires the types 'String' and 'Default.X' (aka 'Int') be equivalent}}
+  }
+}