c++: unevaluated array new-expr size constantness [PR108219]

Here we're mishandling the unevaluated array new-expressions due to a
supposed non-constant array size ever since r12-5253-g4df7f8c79835d569
made us no longer perform constant evaluation of non-manifestly-constant
expressions within unevaluated contexts.  This shouldn't make a difference
here since the array sizes are constant literals, except they're expressed
as NON_LVALUE_EXPR location wrappers around INTEGER_CST, wrappers which
used to get stripped as part of constant evaluation and now no longer do.
Moreover it means build_vec_init can't constant fold the MINUS_EXPR
'maxindex' passed from build_new_1 when in an unevaluated context (since
it tries reducing it via maybe_constant_value called with mce_unknown).

This patch fixes these issues by making maybe_constant_value (and
fold_non_dependent_expr) try folding an unevaluated non-manifestly-constant
operand via fold(), as long as it simplifies to a simple constant, rather
than doing no simplification at all.  This covers e.g. simple arithmetic
and casts including stripping of location wrappers around INTEGER_CST.

In passing, this patch also fixes maybe_constant_value to avoid constant
evaluating an unevaluated operand when called with mce_false, by adjusting
the early exit test appropriately.

Co-authored-by: Jason Merrill <jason@redhat.com>

	PR c++/108219
	PR c++/108218

gcc/cp/ChangeLog:

	* constexpr.cc (fold_to_constant): Define.
	(maybe_constant_value): Move up early exit test for unevaluated
	operands.  Try reducing an unevaluated operand to a constant via
	fold_to_constant.
	(fold_non_dependent_expr_template): Add early exit test for
	CONSTANT_CLASS_P nodes.  Try reducing an unevaluated operand
	to a constant via fold_to_constant.
	* cp-tree.h (fold_to_constant): Declare.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/new6.C: New test.
	* g++.dg/cpp2a/concepts-new1.C: New test.

gcc/cp/constexpr.cc

@@ -8498,6 +8498,19 @@ fold_simple (tree t)
   return t;
 }
 
+/* Try folding the expression T to a simple constant.
+   Returns that constant, otherwise returns T.  */
+
+tree
+fold_to_constant (tree t)
+{
+  tree r = fold (t);
+  if (CONSTANT_CLASS_P (r) && !TREE_OVERFLOW (r))
+    return r;
+  else
+    return t;
+}
+
 /* If T is a constant expression, returns its reduced value.
    Otherwise, if T does not have TREE_CONSTANT set, returns T.
    Otherwise, returns a version of T without TREE_CONSTANT.
@@ -8523,6 +8536,11 @@ maybe_constant_value (tree t, tree decl /* = NULL_TREE */,
     /* No caching or evaluation needed.  */
     return t;
 
+  /* Don't constant evaluate an unevaluated non-manifestly-constant operand,
+     but at least try folding it to a simple constant.  */
+  if (cp_unevaluated_operand && manifestly_const_eval != mce_true)
+    return fold_to_constant (t);
+
   if (manifestly_const_eval != mce_unknown)
     return cxx_eval_outermost_constant_expr (t, true, true,
 					     manifestly_const_eval, false, decl);
@@ -8544,10 +8562,6 @@ maybe_constant_value (tree t, tree decl /* = NULL_TREE */,
       return r;
     }
 
-  /* Don't evaluate an unevaluated operand.  */
-  if (cp_unevaluated_operand)
-    return t;
-
   uid_sensitive_constexpr_evaluation_checker c;
   r = cxx_eval_outermost_constant_expr (t, true, true,
 					manifestly_const_eval, false, decl);
@@ -8611,10 +8625,15 @@ fold_non_dependent_expr_template (tree t, tsubst_flags_t complain,
 	    }
 	  return t;
 	}
-
-      if (cp_unevaluated_operand && !manifestly_const_eval)
+      else if (CONSTANT_CLASS_P (t))
+	/* No evaluation needed.  */
 	return t;
 
+      /* Don't constant evaluate an unevaluated non-manifestly-constant operand,
+	 but at least try folding it to a simple constant.  */
+      if (cp_unevaluated_operand && !manifestly_const_eval)
+	return fold_to_constant (t);
+
       tree r = cxx_eval_outermost_constant_expr (t, true, true,
 						 mce_value (manifestly_const_eval),
 						 false, object);

gcc/cp/cp-tree.h

@@ -8516,6 +8516,7 @@ extern tree fold_non_dependent_init		(tree,
 						 tsubst_flags_t = tf_warning_or_error,
 						 bool = false, tree = NULL_TREE);
 extern tree fold_simple				(tree);
+extern tree fold_to_constant			(tree);
 extern bool reduced_constant_expression_p       (tree);
 extern bool is_instantiation_of_constexpr       (tree);
 extern bool var_in_constexpr_fn                 (tree);

gcc/testsuite/g++.dg/cpp0x/new6.C

@@ -0,0 +1,13 @@
+// PR c++/108218
+// { dg-do compile { target c++11 } }
+
+template<class T>
+void f() {
+  decltype(new int[-1]) p; // { dg-error "negative" }
+  decltype(new int[0-1]) q; // { dg-error "negative" }
+  decltype(new int[1*-1]) r; // { dg-error "negative" }
+}
+
+decltype(new int[-1]) p; // { dg-error "negative" }
+decltype(new int[0-1]) q; // { dg-error "negative" }
+decltype(new int[1*-1]) r; // { dg-error "negative" }

gcc/testsuite/g++.dg/cpp2a/concepts-new1.C

@@ -0,0 +1,13 @@
+// PR c++/108219
+// { dg-do compile { target c++20 } }
+
+template<class T>
+concept C = requires { new T[1]{{ 42 }}; };
+
+template<class T>
+concept D = requires { new T[2][1]{{{ 42 }}, {{ 42 }}}; };
+
+struct A { A(int); };
+
+static_assert(C<A>);
+static_assert(D<A>);