Hi!
We emit weird diagnostics on the following testcase in C++11.
If it is not a template, maybe_save_constexpr_fundef calls first
if (!is_valid_constexpr_fn (fun, complain))
return;
(which doesn't fail) and then
tree massaged = massage_constexpr_body (fun, DECL_SAVED_TREE (fun));
if (massaged == NULL_TREE || massaged == error_mark_node)
{
if (!DECL_CONSTRUCTOR_P (fun) && complain)
error ("body of %<constexpr%> function %qD not a return-statement",
fun);
return;
}
which diagnoses it and if even that would succeed, go on with
bool potential = potential_rvalue_constant_expression (massaged);
if (!potential && complain)
require_potential_rvalue_constant_expression_fncheck (massaged);
In templates, maybe_save_constexpr_fundef returns early:
if (processing_template_decl
|| cp_function_chain->invalid_constexpr
|| (DECL_CLONED_FUNCTION_P (fun) && !DECL_DELETING_DESTRUCTOR_P (fun)))
return;
and then it is called again during instantiation.
But in that case DECL_GENERATED_P (fun) is true and so we silently return
on errors without diagnosing them:
bool complain = !DECL_GENERATED_P (fun) && !implicit;
Now, when we actually try to constexpr evaluate those (if at all), we
emit an error and then
'constexpr ...' is not usable as a 'constexpr' function because:
message and then explain_invalid_constexpr_fn tries to diagnose
the errors by calling is_valid_constexpr_fn (fun, true) and
require_potential_rvalue_constant_expression (massaged). So it diagnoses
those 2 cases, but misses the one where massaged was NULL or error_mark_node
for a non-constructor, so after the because: there is no reason emitted.
The following patch diagnoses even that.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2026-02-03 Jakub Jelinek <[email protected]>
PR c++/123889
* constexpr.cc (explain_invalid_constexpr_fn): Diagnose
NULL or error_mark_node massaged on non-constructor.
* g++.dg/cpp0x/constexpr-123889.C: New test.
--- gcc/cp/constexpr.cc.jj 2026-01-31 10:16:31.950164802 +0100
+++ gcc/cp/constexpr.cc 2026-02-02 14:35:43.506094992 +0100
@@ -1116,6 +1116,10 @@ explain_invalid_constexpr_fn (tree fun)
else
body = DECL_SAVED_TREE (fun);
tree massaged = massage_constexpr_body (fun, body);
+ if ((massaged == NULL_TREE || massaged == error_mark_node)
+ && !DECL_CONSTRUCTOR_P (fun))
+ error ("body of %<constexpr%> function %qD not a return-statement",
+ fun);
require_potential_rvalue_constant_expression (massaged);
if (DECL_CONSTRUCTOR_P (fun))
{
--- gcc/testsuite/g++.dg/cpp0x/constexpr-123889.C.jj 2026-02-02
14:42:30.345152859 +0100
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-123889.C 2026-02-02
14:44:10.958435899 +0100
@@ -0,0 +1,14 @@
+// PR c++/123889
+// { dg-do compile { target c++11 } }
+
+template <class T>
+constexpr int func (T) {
+// { dg-message "'constexpr int func\\\(T\\\) \\\[with T = int\\\]' is not
usable as a 'constexpr' function because:" "" { target c++11_only } .-1 }
+// { dg-error "body of 'constexpr' function 'constexpr int func\\\(T\\\)
\\\[with T = int\\\]' not a return-statement" "" { target c++11_only } .-2 }
+ return 1;
+ return 2;
+}
+
+static_assert (func (1) == 1, "");
+// { dg-error "non-constant condition for static assertion" "" { target
c++11_only } .-1 }
+// { dg-error "'constexpr int func\\\(T\\\) \\\[with T = int\\\]' called in a
constant expression" "" { target c++11_only } .-2 }
Jakub
