c++: treat NON_DEPENDENT_EXPR as not potentially constant [PR104507]

Here we're crashing from potential_constant_expression because it tries
to perform trial evaluation of the first operand '(bool)__r' of the
conjunction (which is overall wrapped in a NON_DEPENDENT_EXPR), but
cxx_eval_constant_expression ICEs on unsupported trees (of which CAST_EXPR
is one).  The sequence of events is:

  1. build_non_dependent_expr for the array subscript yields
     NON_DEPENDENT_EXPR<<<(bool)__r && __s>>> ? 1 : 2
  2. cp_build_array_ref calls fold_non_dependent_expr on this subscript
     (after this point, processing_template_decl is cleared)
  3. during which, the COND_EXPR case of tsubst_copy_and_build calls
     fold_non_dependent_expr on the first operand
  4. during which, we crash from p_c_e_1 because it attempts trial
     evaluation of the CAST_EXPR '(bool)__r'.

Note that even if this crash didn't happen, fold_non_dependent_expr
from cp_build_array_ref would still ultimately be one big no-op here
since neither constexpr evaluation nor tsubst handle NON_DEPENDENT_EXPR.

In light of this and of the observation that we should never see
NON_DEPENDENT_EXPR in a context where a constant expression is needed
(it's used primarily in the build_x_* family of functions), it seems
futile for p_c_e_1 to ever return true for NON_DEPENDENT_EXPR.  And the
otherwise inconsistent handling of NON_DEPENDENT_EXPR between p_c_e_1,
cxx_evaluate_constexpr_expression and tsubst apparently leads to weird
bugs such as this one.

	PR c++/104507

gcc/cp/ChangeLog:

	* constexpr.cc (potential_constant_expression_1)
	<case NON_DEPENDENT_EXPR>: Return false instead of recursing.
	Assert tf_error isn't set.

gcc/testsuite/ChangeLog:

	* g++.dg/template/non-dependent21.C: New test.

2 files changed, 17 insertions, 1 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 7274c3b..4716694 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -9065,6 +9065,14 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
     case BIND_EXPR:
       return RECUR (BIND_EXPR_BODY (t), want_rval);
 
+    case NON_DEPENDENT_EXPR:
+      /* Treat NON_DEPENDENT_EXPR as non-constant: it's not handled by
+	 constexpr evaluation or tsubst, so fold_non_dependent_expr can't
+	 do anything useful with it.  And we shouldn't see it in a context
+	 where a constant expression is strictly required, hence the assert.  */
+      gcc_checking_assert (!(flags & tf_error));
+      return false;
+
     case CLEANUP_POINT_EXPR:
     case MUST_NOT_THROW_EXPR:
     case TRY_CATCH_EXPR:
@@ -9072,7 +9080,6 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
     case EH_SPEC_BLOCK:
     case EXPR_STMT:
     case PAREN_EXPR:
-    case NON_DEPENDENT_EXPR:
       /* For convenience.  */
     case LOOP_EXPR:
     case EXIT_EXPR:
diff --git a/gcc/testsuite/g++.dg/template/non-dependent21.C b/gcc/testsuite/g++.dg/template/non-dependent21.C
new file mode 100644
index 0000000..89900837
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/non-dependent21.C
@@ -0,0 +1,9 @@
+// PR c++/104507
+
+extern const char *_k_errmsg[];
+
+template<class>
+const char* DoFoo(int __r, int __s) {
+  const char* n = _k_errmsg[(bool)__r && __s ? 1 : 2];
+  return n;
+}