Fix another decltype crash.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@75175 91177308-0d34-0410-b5e6-96231b3b80d8

lib/AST/Type.cpp

@@ -125,7 +125,7 @@ QualType Type::getDesugaredType(bool ForDisplay) const {
   if (const TypeOfType *TOT = dyn_cast<TypeOfType>(this))
     return TOT->getUnderlyingType().getDesugaredType();
   if (const DecltypeType *DTT = dyn_cast<DecltypeType>(this))
-    return DTT->getUnderlyingExpr()->getType().getDesugaredType();
+    return DTT->getCanonicalTypeInternal();
   if (const TemplateSpecializationType *Spec 
         = dyn_cast<TemplateSpecializationType>(this)) {
     if (ForDisplay)

test/SemaCXX/decltype-crash.cpp

@@ -0,0 +1,5 @@
+int& a();
+
+void f() {
+  decltype(a()) c;
+}