Fix crash and rejects-valid when a later template parameter or default

template argument contains a backreference to a dependently-typed
earlier parameter.

In a case like:
  template<typename T, T A, decltype(A) = A> struct X {};
  template<typename U> auto Y = X<U, 0>();
we previously treated both references to `A` in the third parameter as
being of type `int` when checking the template-id in `Y`. That`s wrong;
the type of `A` in these contexts is the dependent type `U`.

When we encounter a non-type template argument that we can't convert to
the parameter type because of type-dependence, we now insert a dependent
conversion node so that the SubstNonTypeTemplateParmExpr for the
template argument will have the parameter's type rather than whatever
type the argument had.

llvm-svn: 363972

1 files changed, 3 insertions, 1 deletions

diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index d7a05e731ea..3941643893a 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -491,6 +491,7 @@ ExprResult Sema::ImpCastExprToType(Expr *E, QualType Ty,
     default:
       llvm_unreachable("can't implicitly cast lvalue to rvalue with this cast "
                        "kind");
+    case CK_Dependent:
     case CK_LValueToRValue:
     case CK_ArrayToPointerDecay:
     case CK_FunctionToPointerDecay:
@@ -499,7 +500,8 @@ ExprResult Sema::ImpCastExprToType(Expr *E, QualType Ty,
       break;
     }
   }
-  assert((VK == VK_RValue || !E->isRValue()) && "can't cast rvalue to lvalue");
+  assert((VK == VK_RValue || Kind == CK_Dependent || !E->isRValue()) &&
+         "can't cast rvalue to lvalue");
 #endif
 
   diagnoseNullableToNonnullConversion(Ty, E->getType(), E->getBeginLoc());