diff --git a/lib/Sema/CSDiagnostics.cpp b/lib/Sema/CSDiagnostics.cpp
index 2dc883d2b1a..f55616749d2 100644
--- a/lib/Sema/CSDiagnostics.cpp
+++ b/lib/Sema/CSDiagnostics.cpp
@@ -15,11 +15,13 @@
 //===----------------------------------------------------------------------===//
 
 #include "CSDiagnostics.h"
-#include "ConstraintSystem.h"
 #include "CSDiag.h"
+#include "ConstraintSystem.h"
 #include "MiscDiagnostics.h"
+#include "swift/AST/Decl.h"
 #include "swift/AST/Expr.h"
 #include "swift/AST/GenericSignature.h"
+#include "swift/AST/ParameterList.h"
 #include "swift/AST/Types.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/SmallString.h"
@@ -611,3 +613,56 @@ bool RValueTreatedAsLValueFailure::diagnoseAsError() {
                             subElementDiagID, rvalueDiagID);
   return true;
 }
+
+bool TrailingClosureAmbiguityFailure::diagnoseAsNote() {
+  const auto *expr = getParentExpr();
+  auto *callExpr = dyn_cast<CallExpr>(expr);
+  if (!callExpr)
+    return false;
+  if (!callExpr->hasTrailingClosure())
+    return false;
+  if (callExpr->getFn() != getAnchor())
+    return false;
+
+  llvm::SmallMapVector<Identifier, const ValueDecl *, 8> choicesByLabel;
+  for (const auto &choice : Choices) {
+    auto *callee = dyn_cast<AbstractFunctionDecl>(choice.getDecl());
+    if (!callee)
+      return false;
+
+    const ParameterList *paramList = callee->getParameters();
+    const ParamDecl *param = paramList->getArray().back();
+
+    // Sanity-check that the trailing closure corresponds to this parameter.
+    if (!param->hasValidSignature() ||
+        !param->getInterfaceType()->is<AnyFunctionType>())
+      return false;
+
+    Identifier trailingClosureLabel = param->getArgumentName();
+    auto &choiceForLabel = choicesByLabel[trailingClosureLabel];
+
+    // FIXME: Cargo-culted from diagnoseAmbiguity: apparently the same decl can
+    // appear more than once?
+    if (choiceForLabel == callee)
+      continue;
+
+    // If just providing the trailing closure label won't solve the ambiguity,
+    // don't bother offering the fix-it.
+    if (choiceForLabel != nullptr)
+      return false;
+
+    choiceForLabel = callee;
+  }
+
+  // If we got here, then all of the choices have unique labels. Offer them in
+  // order.
+  for (const auto &choicePair : choicesByLabel) {
+    auto diag = emitDiagnostic(
+        expr->getLoc(), diag::ambiguous_because_of_trailing_closure,
+        choicePair.first.empty(), choicePair.second->getFullName());
+    swift::fixItEncloseTrailingClosure(getTypeChecker(), diag, callExpr,
+                                       choicePair.first);
+  }
+
+  return true;
+}
diff --git a/lib/Sema/CSDiagnostics.h b/lib/Sema/CSDiagnostics.h
index fadc55a5919..2dbd74c8b0a 100644
--- a/lib/Sema/CSDiagnostics.h
+++ b/lib/Sema/CSDiagnostics.h
@@ -467,6 +467,21 @@ public:
   bool diagnoseAsError() override;
 };
 
+class TrailingClosureAmbiguityFailure final : public FailureDiagnostic {
+  ArrayRef<OverloadChoice> Choices;
+
+public:
+  TrailingClosureAmbiguityFailure(Expr *root, ConstraintSystem &cs,
+                                  Expr *anchor,
+                                  ArrayRef<OverloadChoice> choices)
+      : FailureDiagnostic(root, cs, cs.getConstraintLocator(anchor)),
+        Choices(choices) {}
+
+  bool diagnoseAsError() override { return false; }
+
+  bool diagnoseAsNote() override;
+};
+
 } // end namespace constraints
 } // end namespace swift
 
diff --git a/lib/Sema/ConstraintSystem.cpp b/lib/Sema/ConstraintSystem.cpp
index 90a92c57a6a..be96f4994d2 100644
--- a/lib/Sema/ConstraintSystem.cpp
+++ b/lib/Sema/ConstraintSystem.cpp
@@ -17,7 +17,7 @@
 //===----------------------------------------------------------------------===//
 #include "ConstraintSystem.h"
 #include "ConstraintGraph.h"
-#include "MiscDiagnostics.h"
+#include "CSDiagnostics.h"
 #include "TypeCheckType.h"
 #include "swift/AST/GenericEnvironment.h"
 #include "swift/AST/ParameterList.h"
@@ -2283,61 +2283,6 @@ static DeclName getOverloadChoiceName(ArrayRef<OverloadChoice> choices) {
   return name;
 }
 
-/// Returns true if any diagnostics were emitted.
-static bool
-tryDiagnoseTrailingClosureAmbiguity(TypeChecker &tc, const Expr *expr,
-                                    const Expr *anchor,
-                                    ArrayRef<OverloadChoice> choices) {
-  auto *callExpr = dyn_cast<CallExpr>(expr);
-  if (!callExpr)
-    return false;
-  if (!callExpr->hasTrailingClosure())
-    return false;
-  if (callExpr->getFn() != anchor)
-    return false;
-
-  llvm::SmallMapVector<Identifier, const ValueDecl *, 8> choicesByLabel;
-  for (const OverloadChoice &choice : choices) {
-    auto *callee = dyn_cast<AbstractFunctionDecl>(choice.getDecl());
-    if (!callee)
-      return false;
-
-    const ParameterList *paramList = callee->getParameters();
-    const ParamDecl *param = paramList->getArray().back();
-
-    // Sanity-check that the trailing closure corresponds to this parameter.
-    if (!param->hasValidSignature() ||
-        !param->getInterfaceType()->is<AnyFunctionType>())
-      return false;
-
-    Identifier trailingClosureLabel = param->getArgumentName();
-    auto &choiceForLabel = choicesByLabel[trailingClosureLabel];
-
-    // FIXME: Cargo-culted from diagnoseAmbiguity: apparently the same decl can
-    // appear more than once?
-    if (choiceForLabel == callee)
-      continue;
-
-    // If just providing the trailing closure label won't solve the ambiguity,
-    // don't bother offering the fix-it.
-    if (choiceForLabel != nullptr)
-      return false;
-
-    choiceForLabel = callee;
-  }
-
-  // If we got here, then all of the choices have unique labels. Offer them in
-  // order.
-  for (const auto &choicePair : choicesByLabel) {
-    auto diag =
-        tc.diagnose(expr->getLoc(), diag::ambiguous_because_of_trailing_closure,
-                    choicePair.first.empty(), choicePair.second->getFullName());
-    swift::fixItEncloseTrailingClosure(tc, diag, callExpr, choicePair.first);
-  }
-
-  return true;
-}
-
 bool ConstraintSystem::diagnoseAmbiguity(Expr *expr,
                                          ArrayRef<Solution> solutions) {
   // Produce a diff of the solutions.
@@ -2415,7 +2360,9 @@ bool ConstraintSystem::diagnoseAmbiguity(Expr *expr,
                                   : diag::ambiguous_decl_ref,
                 name);
 
-    if (tryDiagnoseTrailingClosureAmbiguity(tc, expr, anchor, overload.choices))
+    TrailingClosureAmbiguityFailure failure(expr, *this, anchor,
+                                            overload.choices);
+    if (failure.diagnoseAsNote())
       return true;
 
     // Emit candidates.  Use a SmallPtrSet to make sure only emit a particular