Index: lib/AST/ASTDiagnostic.cpp
===================================================================
--- lib/AST/ASTDiagnostic.cpp
+++ lib/AST/ASTDiagnostic.cpp
@@ -65,11 +65,20 @@
       continue;
     }
 
-    // Don't desugar template specializations, unless it's an alias template.
+    // Don't desugar template specializations, unless it's an alias template
+    // or decltype appears anywhere in the type.
     if (const TemplateSpecializationType *TST
-          = dyn_cast<TemplateSpecializationType>(Ty))
-      if (!TST->isTypeAlias())
+        = dyn_cast<TemplateSpecializationType>(Ty)) {
+      for (auto Arg : *TST) {
+          if (Arg.getKind() == TemplateArgument::Type)
+              Desugar(Context, Arg.getAsType(), ShouldAKA);
+          if (ShouldAKA)
+              break;
+      }
+
+      if (!ShouldAKA && !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 +45,12 @@
 void test() {
  helper(&ns::str::method); // expected-error{{no matching function for call to 'helper'}}
 }
+
+// PR10405
+template <typename T>
+class Temp {};
+
+int t1 = Temp<decltype(1 + 2)>(); // expected-error{{no viable conversion from 'Temp<decltype(1 + 2)>' (aka 'Temp<int>') to 'int'}}
+int t2 = Temp<Temp<decltype(1 + 2)>>(); // expected-error{{no viable conversion from 'Temp<Temp<decltype(1 + 2)> >' (aka 'Temp<Temp<int> >') to 'int'}}
+int t3 = Temp<__typeof(1 + 2)>(); // expected-error{{no viable conversion from 'Temp<typeof (1 + 2)>' (aka 'Temp<int>') to 'int'}}
+int t4 = Temp<Temp<__typeof(1 + 2)>>(); // expected-error{{no viable conversion from 'Temp<Temp<typeof (1 + 2)> >' (aka 'Temp<Temp<int> >') to 'int'}}