Index: lib/AST/ASTDiagnostic.cpp
===================================================================
--- lib/AST/ASTDiagnostic.cpp
+++ lib/AST/ASTDiagnostic.cpp
@@ -959,10 +959,12 @@
                           ToIter.isEnd() && ToExpr);
           if (DefaultNTTPD->getType()->isIntegralOrEnumerationType()) {
             if (FromExpr)
-              FromInt = GetInt(FromIter, FromExpr);
+              FromInt = GetInt(FromIter, FromExpr, HasFromInt);
             if (ToExpr)
-              ToInt = GetInt(ToIter, ToExpr);
-            Tree.SetNode(FromInt, ToInt, FromExpr, ToExpr);
+              ToInt = GetInt(ToIter, ToExpr, HasToInt);
+          }
+          if (HasFromInt && HasToInt) {
+            Tree.SetNode(FromInt, ToInt, HasFromInt, HasToInt);
             Tree.SetSame(IsSameConvertedInt(ParamWidth, FromInt, ToInt));
             Tree.SetKind(DiffTree::Integer);
           } else {
@@ -970,14 +972,10 @@
             Tree.SetKind(DiffTree::Expression);
           }
         } else if (HasFromInt || HasToInt) {
-          if (!HasFromInt && FromExpr) {
-            FromInt = GetInt(FromIter, FromExpr);
-            HasFromInt = true;
-          }
-          if (!HasToInt && ToExpr) {
-            ToInt = GetInt(ToIter, ToExpr);
-            HasToInt = true;
-          }
+          if (!HasFromInt && FromExpr)
+            FromInt = GetInt(FromIter, FromExpr, HasFromInt);
+          if (!HasToInt && ToExpr)
+            ToInt = GetInt(ToIter, ToExpr, HasToInt);
           Tree.SetNode(FromInt, ToInt, HasFromInt, HasToInt);
           Tree.SetSame(IsSameConvertedInt(ParamWidth, FromInt, ToInt));
           Tree.SetDefault(FromIter.isEnd() && HasFromInt,
@@ -1121,9 +1119,10 @@
 
   /// GetInt - Retrieves the template integer argument, including evaluating
   /// default arguments.
-  llvm::APInt GetInt(const TSTiterator &Iter, Expr *ArgExpr) {
+  llvm::APInt GetInt(const TSTiterator &Iter, Expr *ArgExpr, bool &HasInt) {
     // Default, value-depenedent expressions require fetching
     // from the desugared TemplateArgument
+    HasInt = true;
     if (Iter.isEnd() && ArgExpr->isValueDependent())
       switch (Iter.getDesugar().getKind()) {
         case TemplateArgument::Integral:
@@ -1134,7 +1133,11 @@
         default:
           llvm_unreachable("Unexpected template argument kind");
       }
-    return ArgExpr->EvaluateKnownConstInt(Context);
+    if (ArgExpr->isEvaluatable(Context))
+      return ArgExpr->EvaluateKnownConstInt(Context);
+
+    HasInt = false;
+    return llvm::APInt();
   }
 
   /// GetValueDecl - Retrieves the template Decl argument, including
Index: test/Misc/diag-template-diffing.cpp
===================================================================
--- test/Misc/diag-template-diffing.cpp
+++ test/Misc/diag-template-diffing.cpp
@@ -1068,6 +1068,36 @@
 }
 }
 
+namespace PR15677 {
+template <bool>
+struct A{};
+
+template <typename T>
+using B = A<T::value>;
+
+template <typename T>
+using B = A<!T::value>;
+// CHECK-ELIDE-NOTREE: type alias template redefinition with different types ('A<!T::value>' vs 'A<T::value>')
+
+template <int>
+struct C{};
+
+template <typename T>
+using D = C<T::value>;
+
+template <typename T>
+using D = C<T::value + 1>;
+// CHECK-ELIDE-NOTREE: type alias template redefinition with different types ('C<T::value + 1>' vs 'C<T::value>')
+
+template <typename T>
+using E = C<T::value>;
+
+template <typename T>
+using E = C<42>;
+// CHECK-ELIDE-NOTREE: type alias template redefinition with different types ('C<42>' vs 'C<T::value>')
+}
+}
+
 // CHECK-ELIDE-NOTREE: {{[0-9]*}} errors generated.
 // CHECK-NOELIDE-NOTREE: {{[0-9]*}} errors generated.
 // CHECK-ELIDE-TREE: {{[0-9]*}} errors generated.