https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107687
Marek Polacek <mpolacek at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |ASSIGNED
Assignee|unassigned at gcc dot gnu.org |mpolacek at gcc dot
gnu.org
--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
A (very) rudimentary patch that handles the simplest case so far:
>From be2950f8cf3fb5ca0571054b4196de35d9807519 Mon Sep 17 00:00:00 2001
From: Marek Polacek <[email protected]>
Date: Thu, 10 Aug 2023 17:06:11 -0400
Subject: [PATCH] c++: implement P2564, consteval needs to propagate up
[PR107687]
---
gcc/cp/call.cc | 48 ++++++++++++++++++++
gcc/cp/cp-gimplify.cc | 26 +++++++++--
gcc/testsuite/g++.dg/cpp23/consteval-if10.C | 2 +-
gcc/testsuite/g++.dg/cpp2a/consteval-prop1.C | 36 +++++++++++++++
4 files changed, 108 insertions(+), 4 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/consteval-prop1.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 673ec91d60e..34b2fe1884d 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -9740,6 +9740,7 @@ build_trivial_dtor_call (tree instance, bool
no_ptr_deref)
bool
in_immediate_context ()
{
+ // TODO update to say that manifestly constant eval ctx is an IFC
return (cp_unevaluated_operand != 0
|| (current_function_decl != NULL_TREE
&& DECL_IMMEDIATE_FUNCTION_P (current_function_decl))
@@ -9762,6 +9763,34 @@ immediate_invocation_p (tree fn)
&& !in_immediate_context ());
}
+/* Return true if FN is an immediate-escalating function. */
+
+static bool
+immediate_escalating_function_p (tree fn)
+{
+ if (!fn)
+ return false;
+
+ gcc_checking_assert (TREE_CODE (fn) == FUNCTION_DECL);
+
+ /* An immediate-escalating function is
+ -- the call operator of a lambda that is not declared with the consteval
+ specifier */
+ if (LAMBDA_FUNCTION_P (fn) && !DECL_IMMEDIATE_FUNCTION_P (fn))
+ return true;
+ /* -- a defaulted special member function that is not declared with the
+ consteval specifier */
+ special_function_kind sfk = special_memfn_p (fn);
+ if (sfk != sfk_none
+ && DECL_DEFAULTED_FN (fn)
+ && !DECL_IMMEDIATE_FUNCTION_P (fn))
+ return true;
+ /* -- a function that results from the instantiation of a templated entity
+ defined with the constexpr specifier. */
+ // TODO check if the DECL_DEFAULTED_FN part is actually OK here
+ return is_instantiation_of_constexpr (fn);
+}
+
/* Subroutine of the various build_*_call functions. Overload resolution
has chosen a winning candidate CAND; build up a CALL_EXPR accordingly.
ARGS is a TREE_LIST of the unconverted arguments to the call. FLAGS is a
@@ -10484,6 +10513,7 @@ build_over_call (struct z_candidate *cand, int flags,
tsubst_flags_t complain)
tree fndecl = STRIP_TEMPLATE (TREE_OPERAND (fn, 0));
if (immediate_invocation_p (fndecl))
{
+ tree orig_call = call;
tree obj_arg = NULL_TREE;
/* Undo convert_from_reference called by build_cxx_call. */
if (REFERENCE_REF_P (call))
@@ -10508,6 +10538,24 @@ build_over_call (struct z_candidate *cand, int flags,
tsubst_flags_t complain)
obj_arg = TREE_OPERAND (addr, 0);
}
}
+
+ /* [expr.const]p16 "An expression or conversion is
+ immediate-escalating if it is not initially in an immediate
+ function context and it is either
+ -- an immediate invocation that is not a constant expression and
+ is not a subexpression of an immediate invocation."
+
+ If we are in an immediate-escalating function, the
+ immediate-escalating expression or conversion makes it an
+ immediate function. So CALL does not need to produce a constant
+ expression. ??? It's ugly to call cxx_constant_value twice in
+ some cases. */
+ if (immediate_escalating_function_p (current_function_decl)
+ && cxx_constant_value (call, obj_arg, tf_none) ==
error_mark_node)
+ {
+ SET_DECL_IMMEDIATE_FUNCTION_P (current_function_decl);
+ return orig_call;
+ }
call = cxx_constant_value (call, obj_arg, complain);
if (obj_arg && !error_operand_p (call))
call = cp_build_init_expr (obj_arg, call);
diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index 206e791fcfd..5fc0e3cc84b 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -1017,6 +1017,20 @@ maybe_replace_decl (tree *tp, tree decl, tree
replacement)
return true;
}
+/* Maybe say that FN was initially not an immediate function, but was
+ promoted to one because its body contained an immediate-escalating
+ expression or conversion. */
+
+static void
+maybe_explain_promoted_consteval (location_t loc, tree fn)
+{
+ /* A function cannot be marked both constexpr and consteval
+ except when we've promoted the former to the latter. */
+ if (is_instantiation_of_constexpr (fn)
+ && DECL_DECLARED_CONSTEXPR_P (fn))
+ inform (loc, "%qD was promoted to an immediate function", fn);
+}
+
/* Genericization context. */
struct cp_genericize_data
@@ -1050,9 +1064,13 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void
*data_)
&& DECL_IMMEDIATE_FUNCTION_P (PTRMEM_CST_MEMBER (stmt)))
{
if (!data->pset.add (stmt))
- error_at (PTRMEM_CST_LOCATION (stmt),
- "taking address of an immediate function %qD",
- PTRMEM_CST_MEMBER (stmt));
+ {
+ error_at (PTRMEM_CST_LOCATION (stmt),
+ "taking address of an immediate function %qD",
+ PTRMEM_CST_MEMBER (stmt));
+ maybe_explain_promoted_consteval (PTRMEM_CST_LOCATION (stmt),
+ PTRMEM_CST_MEMBER (stmt));
+ }
stmt = *stmt_p = build_zero_cst (TREE_TYPE (stmt));
break;
}
@@ -1065,6 +1083,8 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
error_at (EXPR_LOCATION (stmt),
"taking address of an immediate function %qD",
TREE_OPERAND (stmt, 0));
+ maybe_explain_promoted_consteval (EXPR_LOCATION (stmt),
+ TREE_OPERAND (stmt, 0));
stmt = *stmt_p = build_zero_cst (TREE_TYPE (stmt));
break;
}
diff --git a/gcc/testsuite/g++.dg/cpp23/consteval-if10.C
b/gcc/testsuite/g++.dg/cpp23/consteval-if10.C
index 4c0523fe1d0..f799e81113d 100644
--- a/gcc/testsuite/g++.dg/cpp23/consteval-if10.C
+++ b/gcc/testsuite/g++.dg/cpp23/consteval-if10.C
@@ -10,7 +10,7 @@ bar (int x)
int r = 0;
if consteval // { dg-warning "'if consteval' only available with" ""
{ target c++20_only } }
{
- auto y = [=] { foo (x); }; // { dg-error "'x' is not a constant
expression" }
+ auto y = [=] { foo (x); };
y ();
}
return r;
diff --git a/gcc/testsuite/g++.dg/cpp2a/consteval-prop1.C
b/gcc/testsuite/g++.dg/cpp2a/consteval-prop1.C
new file mode 100644
index 00000000000..9a791b9cf0b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/consteval-prop1.C
@@ -0,0 +1,36 @@
+// P2564R3
+// { dg-do compile { target c++20 } }
+
+consteval int id(int i) { return i; }
+
+template <typename T>
+constexpr int
+f (T t)
+{
+ // OK, f promoted to consteval.
+ return id (t);
+}
+
+constexpr auto a1 = f (3);
+
+// As a consequence of f<int> being promoted to an immediate function, we
+// can't take its address.
+auto p1 = &f<int>; // { dg-error "taking address of an immediate function" }
+
+template <typename T>
+constexpr int
+f2 (T)
+{
+ // This produces a constant; f2 *not* promoted to consteval.
+ return id (42);
+}
+
+// ... so we can take its address.
+auto p2 = &f2<int>;
+
+constexpr int
+f3 (int i)
+{
+ // f3 isn't a function template and those don't get upgraded to consteval.
+ return id (i); // { dg-error "not a constant expression" }
+}
base-commit: ecfd8c7ffecf9e8f851c996ec149fbda7ef202f5
--
2.41.0
