2 files changed, 12 insertions, 2 deletions
diff --git a/lib/AST/ASTDiagnostic.cpp b/lib/AST/ASTDiagnostic.cpp
index d67423ed3d..e21caa9af2 100644
--- a/lib/AST/ASTDiagnostic.cpp
+++ b/lib/AST/ASTDiagnostic.cpp
@@ -1093,7 +1093,7 @@ class TemplateDiff {
     Expr::EvalResult FromResult, ToResult;
     if (!FromExpr->EvaluateAsRValue(FromResult, Context) ||
         !ToExpr->EvaluateAsRValue(ToResult, Context))
-      assert(0 && "Template arguments must be known at compile time.");
+      return false;
 
     APValue &FromVal = FromResult.Val;
     APValue &ToVal = ToResult.Val;
diff --git a/test/Misc/diag-template-diffing-cxx98.cpp b/test/Misc/diag-template-diffing-cxx98.cpp
index cd40ccc374..f374fbc417 100644
--- a/test/Misc/diag-template-diffing-cxx98.cpp
+++ b/test/Misc/diag-template-diffing-cxx98.cpp
@@ -4,4 +4,14 @@ namespace PR14342 {
   template<typename T, char a> struct X {};
   X<int, 1> x = X<long, 257>();
   // CHECK: error: no viable conversion from 'X<long, [...]>' to 'X<int, [...]>'
-}
-\ No newline at end of file
+}
+
+namespace PR15513 {
+  template <int x, int y = x+1>
+  class A {};
+
+  void foo(A<0> &M) {
+    // CHECK: no viable conversion from 'A<[...], (default) x + 1>' to 'A<[...], 0>'
+    A<0, 0> N = M;
+  }
+}