Index: lib/AST/ASTDiagnostic.cpp
===================================================================
--- lib/AST/ASTDiagnostic.cpp
+++ lib/AST/ASTDiagnostic.cpp
@@ -65,11 +65,21 @@
       continue;
     }
 
-    // Don't desugar template specializations, unless it's an alias template.
+    // Don't desugar template specializations, unless it's an alias template
+    // or one of the template arguments is a decltype.
     if (const TemplateSpecializationType *TST
-          = dyn_cast<TemplateSpecializationType>(Ty))
-      if (!TST->isTypeAlias())
+        = dyn_cast<TemplateSpecializationType>(Ty)) {
+      bool AnyDecltypeArgs = false;
+      for (auto Arg : *TST)
+        if (Arg.getKind() == TemplateArgument::Type &&
+            Arg.getAsType()->getAs<DecltypeType>()) {
+            AnyDecltypeArgs = true;
+          break;
+        }
+
+      if (!AnyDecltypeArgs && !TST->isTypeAlias())
         break;
+    }
 
     // Don't desugar magic Objective-C types.
     if (QualType(Ty,0) == Context.getObjCIdType() ||
Index: test/Misc/diag-aka-types.cpp
===================================================================
--- test/Misc/diag-aka-types.cpp
+++ test/Misc/diag-aka-types.cpp
@@ -45,3 +45,9 @@
 void test() {
  helper(&ns::str::method); // expected-error{{no matching function for call to 'helper'}}
 }
+
+// PR10405
+template <typename T>
+class Temp {};
+
+int t = Temp<decltype(1 + 2)>(); // expected-error{{no viable conversion from 'Temp<decltype(1 + 2)>' (aka 'Temp<int>') to 'int'}}