c++: constantness of non-dependent NTTP argument [PR107437]

Here we're rejecting the use of the lambda capture of 't' (of empty
type) as a template argument ultimately because convert_nontype_argument
checks constantness using is_constant_expression, which returns false
for lambda captures since want_rval=false.  But in this case I believe
an lvalue-to-rvalue conversion of the argument is implied, so we should
be using is_rvalue_constant_expression instead (which would return true
here).

However, it doesn't seem necessary to consider constantness at all
when deciding whether to instantiate a non-dependent argument in
convert_nontype_argument.  So this patch gets rid of the problematic
constantness test altogether, which incidentally also fixes the similar
dg-ice'd testcase from PR87765.  This is in line with a similar
change we made to finish_decltype_type in r12-7564-gec0f53a3a542e7.

	PR c++/107437
	PR c++/87765

gcc/cp/ChangeLog:

	* pt.cc (convert_nontype_argument): Relax is_nondep_const_expr
	test to !inst_dep_expr_p.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/lambda-generic-107437.C: New test.
	* g++.dg/cpp1z/constexpr-lambda26.C: Remove dg-ice.

3 files changed, 22 insertions, 2 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index bc566ab..2516cca 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -7318,7 +7318,7 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
       && has_value_dependent_address (expr))
     /* If we want the address and it's value-dependent, don't fold.  */;
   else if (processing_template_decl
-	   && is_nondependent_constant_expression (expr))
+	   && !instantiation_dependent_expression_p (expr))
     non_dep = true;
   if (error_operand_p (expr))
     return error_mark_node;
diff --git a/gcc/testsuite/g++.dg/cpp1y/lambda-generic-107437.C b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-107437.C
new file mode 100644
index 0000000..f9b4e01
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-107437.C
@@ -0,0 +1,21 @@
+// PR c++/107437
+// { dg-do compile { target c++14 } }
+
+struct integral_constant {
+  constexpr operator int() const { return 42; }
+};
+
+template<int N>
+struct A {
+  static constexpr int value = N;
+};
+
+template<class T>
+void f(T t) {
+  [=](auto) {
+    A<t> a; // { dg-bogus "constant" }
+    return a.value;
+  }(0);
+}
+
+template void f(integral_constant);
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-lambda26.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-lambda26.C
index 0cdb400..e66cd1d 100644
--- a/gcc/testsuite/g++.dg/cpp1z/constexpr-lambda26.C
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-lambda26.C
@@ -1,7 +1,6 @@
 // PR c++/87765
 // { dg-do compile { target c++17 } }
 // { dg-additional-options "-fchecking" }
-// { dg-ice "cxx_eval_constant_expression" }
 
 template <int N>
 using foo = int;