c++: scoped variable template-id of reference type [PR97340]

lookup_and_finish_template_variable calls convert_from_reference, which
means for a variable template-id of reference type the function wraps
the corresponding VAR_DECL in an INDIRECT_REF.  But the downstream logic
of two callers, tsubst_qualified_id and finish_class_member_access_expr,
expect a DECL_P result and this unexpected INDIRECT_REF leads to an ICE
resolving such a (dependently scoped) template-id as in the first testcase.
(Note these two callers eventually call convert_from_reference on the
result anyway, so calling it earlier seems redundant in this case.)

This patch fixes this by pulling out the convert_from_reference call
from lookup_and_finish_template_variable and into the callers that
actually need it, which turns out to only be tsubst_copy_and_build
(if we got rid of the call there we'd mishandle the second testcase).

	PR c++/97340

gcc/cp/ChangeLog:

	* pt.cc (lookup_and_finish_template_variable): Don't call
	convert_from_reference.
	(tsubst_copy_and_build) <case TEMPLATE_ID_EXPR>: Call
	convert_from_reference on the result of
	lookup_and_finish_template_variable.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/var-templ80.C: New test.
	* g++.dg/cpp1y/var-templ81.C: New test.

3 files changed, 38 insertions, 1 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 696233e..7fb3e75 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -10394,7 +10394,7 @@ lookup_and_finish_template_variable (tree templ, tree targs,
   complain &= ~tf_partial;
   var = finish_template_variable (var, complain);
   mark_used (var);
-  return convert_from_reference (var);
+  return var;
 }
 
 /* If the set of template parameters PARMS contains a template parameter
@@ -20462,6 +20462,7 @@ tsubst_copy_and_build (tree t,
 	  {
 	    tree r = lookup_and_finish_template_variable (templ, targs,
 							  complain);
+	    r = convert_from_reference (r);
 	    r = maybe_wrap_with_location (r, EXPR_LOCATION (t));
 	    RETURN (r);
 	  }
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ80.C b/gcc/testsuite/g++.dg/cpp1y/var-templ80.C
new file mode 100644
index 0000000..4439bee
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ80.C
@@ -0,0 +1,22 @@
+// PR c++/97340
+// { dg-do compile { target c++14 } }
+
+template<class>
+struct A {
+  template<class>
+  static constexpr const int& var = 0;
+};
+
+template<class T>
+struct B {
+  static constexpr int x1 = A<T>::template var<int>;
+  static constexpr int y1 = A<T>{}.template var<int>;
+
+  static constexpr int x2 = A<int>::template var<T>;
+  static constexpr int y2 = A<int>{}.template var<T>;
+
+  static constexpr int x3 = A<int>::template var<int>;
+  static constexpr int y3 = A<int>{}.template var<int>;
+};
+
+template struct B<int>;
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ81.C b/gcc/testsuite/g++.dg/cpp1y/var-templ81.C
new file mode 100644
index 0000000..3efe779
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ81.C
@@ -0,0 +1,14 @@
+// Verify we don't ICE on an invalid dereference of a variable
+// template-id of reference type.
+// { dg-do compile { target c++14 } }
+
+template<class>
+static constexpr const int& var = 0;
+
+template<class T>
+struct B {
+  static constexpr int x = *var<T>; // { dg-error "argument of unary" }
+  static constexpr const int& y = *var<T>; // { dg-error "argument of unary" }
+};
+
+template struct B<int>;