diff --git a/lib/AST/RequirementMachine/RequirementLowering.cpp b/lib/AST/RequirementMachine/RequirementLowering.cpp
index 33705845b9b..fe7d52b0448 100644
--- a/lib/AST/RequirementMachine/RequirementLowering.cpp
+++ b/lib/AST/RequirementMachine/RequirementLowering.cpp
@@ -136,7 +136,23 @@ static void desugarConformanceRequirement(Type subjectType, Type constraintType,
   // Fast path.
   if (constraintType->is<ProtocolType>()) {
     if (!subjectType->isTypeParameter()) {
-      // FIXME: Check conformance, diagnose redundancy or conflict upstream
+      // Check if the subject type actually conforms.
+      auto *protoDecl = constraintType->castTo<ProtocolType>()->getDecl();
+      auto *module = protoDecl->getParentModule();
+      auto conformance = module->lookupConformance(subjectType, protoDecl);
+      if (conformance.isInvalid()) {
+        // FIXME: Diagnose a conflict.
+        return;
+      }
+
+      // FIXME: Diagnose a redundancy.
+      assert(conformance.isConcrete());
+      auto *concrete = conformance.getConcrete();
+
+      // Introduce conditional requirements if the subject type is concrete.
+      for (auto req : concrete->getConditionalRequirements()) {
+        desugarRequirement(req, result);
+      }
       return;
     }
 
diff --git a/test/Generics/conditional_requirement_inference_2.swift b/test/Generics/conditional_requirement_inference_2.swift
index b63d59e94aa..b6fbf35a26e 100644
--- a/test/Generics/conditional_requirement_inference_2.swift
+++ b/test/Generics/conditional_requirement_inference_2.swift
@@ -15,6 +15,18 @@ protocol Sequence {
 extension Array : Sequence {}
 
 struct EquatableSequenceBox<T : Sequence> where T.Element : Equatable {
-  // CHECK: Generic signature: <T, U where T == Array<Array<U>>, U : Equatable>
+  // CHECK-LABEL: EquatableSequenceBox.withArrayArray@
+  // CHECK-NEXT: Generic signature: <T, U where T == Array<Array<U>>, U : Equatable>
   func withArrayArray<U>(_: U) where T == Array<Array<U>> {}
-}
\ No newline at end of file
+}
+
+// Make sure requirement desugaring handles conditional conformances.
+protocol Hashable : Equatable {}
+
+struct Set<Element : Hashable> {}
+
+extension Array : Hashable where Element : Hashable {}
+
+// CHECK-LABEL: doStuff@
+// CHECK-NEXT: Generic signature: <U where U : Hashable>
+func doStuff<U>(_: Set<Array<U>>) {}