On Mon, May 18, 2026 at 09:34:23AM -0400, Patrick Palka wrote:
> On Fri, 15 May 2026, Marek Polacek wrote:
>
> > On Thu, May 14, 2026 at 09:50:52PM -0400, Patrick Palka wrote:
> > > On Thu, 14 May 2026, Marek Polacek wrote:
> > >
> > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/16.2?
> > > >
> > > > -- >8 --
> > > > Thanks to DR 696 (r253266), this works:
> > > >
> > > > int g;
> > > > void fn ()
> > > > {
> > > > int &c = g;
> > > > auto l = [] { c++; };
> > > > l();
> > > > }
> > > >
> > > > because `c` in the lambda body is not an odr-use because we can
> > > > evaluate it to a constant and so there's no capture. But when
> > > > fn is a template, we reject the code and crash. This patch fixes
> > > > both.
> > > >
> > > > Outside a template, the call to maybe_constant_value in mark_use
> > > > evaluates `c` to `(int&) &g` but in a template, it remains `c`.
> > > > Then we emit an error, and crash on the error_mark_node from
> > > > process_outer_var_ref. One of the reasons is
> > > > else if (TYPE_REF_P (TREE_TYPE (expression)))
> > > > /* FIXME cp_finish_decl doesn't fold reference initializers. */
> > > > return true;
> > > > in value_dependent_expression_p but even if that changed, we still
> > > > wouldn't get the referent because decl_really_constant_value wouldn't
> > > > give it to us; the DECL_INITIAL is not a TREE_CONSTANT yet.
> > > >
> > > > So I stopped trying to make this work in a template, and instead
> > > > I'm delaying the processing to instantiating when we know that
> > > > maybe_constant_value wouldn't even try to evaluate.
> > >
> > > It seems with this patch we still incorrectly reject the case where the
> > > outer
> > > local variable is initially type-dependent (not a regression):
> > >
> > > int g;
> > >
> > > template<class T>
> > > void
> > > fn1 ()
> > > {
> > > T c = g;
> > > auto l = [] { c++; }; // bogus error: 'c' is not captured
> > > l();
> > > }
> > >
> > > void
> > > bar ()
> > > {
> > > fn1<int&> ();
> > > }
> > >
> > > via process_outer_var_ref, this time called from finish_id_expression_1.
> > > Maybe we can make both testcases work if we instead give
> > > process_outer_var_ref
> > > an early exit for when the outer variable is from a template?
> >
> > Thanks for that testcase; I hadn't considered it.
> >
> > What do y'all think about this approach, then? I think we want the new
> > check only after checking !odr_use && decl_constant_var_p.
> >
> > dg.exp passed thus far.
> >
> > -- >8 --
> > Thanks to DR 696 (r253266), this works:
> >
> > int g;
> > void fn ()
> > {
> > int &c = g;
> > auto l = [] { c++; };
> > l();
> > }
> >
> > because `c` in the lambda body is not an odr-use because we can
> > evaluate it to a constant and so there's no capture. But when
> > fn is a template, we reject the code and crash. This patch fixes
> > both.
> >
> > Outside a template, the call to maybe_constant_value in mark_use
> > evaluates `c` to `(int&) &g` but in a template, it remains `c`.
> > Then we emit an error, and crash on the error_mark_node from
> > process_outer_var_ref. One of the reasons is
> > else if (TYPE_REF_P (TREE_TYPE (expression)))
> > /* FIXME cp_finish_decl doesn't fold reference initializers. */
> > return true;
> > in value_dependent_expression_p but even if that changed, we still
> > wouldn't get the referent because decl_really_constant_value wouldn't
> > give it to us; the DECL_INITIAL is not a TREE_CONSTANT yet.
> >
> > So I stopped trying to make this work in a template, and instead
> > I'm deferring the error in process_outer_var_ref to instantiation
> > when it's instantiation-dependent.
> >
> > PR c++/123536
> >
> > gcc/cp/ChangeLog:
> >
> > * semantics.cc (process_outer_var_ref): Return decl when it is
> > instantiation-dependent.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp0x/lambda/lambda-const12.C: New test.
> > * g++.dg/cpp0x/lambda/lambda-const13.C: New test.
> > ---
> > gcc/cp/semantics.cc | 5 ++
> > .../g++.dg/cpp0x/lambda/lambda-const12.C | 48 +++++++++++++++++++
> > .../g++.dg/cpp0x/lambda/lambda-const13.C | 19 ++++++++
> > 3 files changed, 72 insertions(+)
> > create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const12.C
> > create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const13.C
> >
> > diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
> > index 6564d9e37a6..201519ef411 100644
> > --- a/gcc/cp/semantics.cc
> > +++ b/gcc/cp/semantics.cc
> > @@ -4699,6 +4699,11 @@ process_outer_var_ref (tree decl, tsubst_flags_t
> > complain, bool odr_use)
> > return var;
> > else if (lambda_expr)
> > {
> > + /* Don't complain when DECL is dependent, because it can turn out to
> > + be constant (and therefore needing no capture) when instantiating. */
> > + if (instantiation_dependent_expression_p (decl))
> > + return decl;
> > +
>
> Presumably we also want to accept the nested class version of
> lambda-const13.C:
>
> int g;
>
> template<class T>
> void
> fn1 ()
> {
> T c = g;
> struct A { static void f() { c++; } };
> A::f();
> }
>
> void
> bar ()
> {
> fn1<int&> ();
> }
>
>
> So the early exit should be independent of whether we're inside a
> lambda.
Oop, yes. I moved the check up and also added VAR_P because
I *think* for PARM_DECLs we don't need/want that i_d_e_p check.
Without it pr57416.C regresses its diagnostic a bit.
> Does instantiation_dependent_expression_p work for:
>
> template<class T>
> void
> fn2 ()
> {
> int& c = T::static_data_member; // not marked constant at parse time
> auto l = [] { c++; };
> l();
> }
It does seem to work. I discovered a crash on invalid when trying
out this test, thus the mark_use hunk. Tested in lambda-const14.C.
> Maybe the early exit test should just be whether the context of the
> outer variable is dependent.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/16.2?
-- >8 --
Thanks to DR 696 (r253266), this works:
int g;
void fn ()
{
int &c = g;
auto l = [] { c++; };
l();
}
because `c` in the lambda body is not an odr-use because we can
evaluate it to a constant and so there's no capture. But when
fn is a template, we reject the code and crash. This patch fixes
both.
Outside a template, the call to maybe_constant_value in mark_use
evaluates `c` to `(int&) &g` but in a template, it remains `c`.
Then we emit an error, and crash on the error_mark_node from
process_outer_var_ref. One of the reasons is
else if (TYPE_REF_P (TREE_TYPE (expression)))
/* FIXME cp_finish_decl doesn't fold reference initializers. */
return true;
in value_dependent_expression_p but even if that changed, we still
wouldn't get the referent because decl_really_constant_value wouldn't
give it to us; the DECL_INITIAL is not a TREE_CONSTANT yet.
So I stopped trying to make this work in a template, and instead
I'm deferring the error in process_outer_var_ref to instantiation
when it's instantiation-dependent. The VAR_P check there is not
to regress the diagnostic in pr57416.C.
The mark_use hunk is to fix a crash on invalid (lambda-const14.C).
PR c++/123536
gcc/cp/ChangeLog:
* expr.cc (mark_use): Return if mark_rvalue_use returns
error_mark_node.
* semantics.cc (process_outer_var_ref): Return decl when it is
instantiation-dependent.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/lambda/lambda-const12.C: New test.
* g++.dg/cpp0x/lambda/lambda-const13.C: New test.
* g++.dg/cpp0x/lambda/lambda-const14.C: New test.
* g++.dg/template/local11.C: New test.
---
gcc/cp/expr.cc | 2 +
gcc/cp/semantics.cc | 6 +-
.../g++.dg/cpp0x/lambda/lambda-const12.C | 62 +++++++++++++++++++
.../g++.dg/cpp0x/lambda/lambda-const13.C | 19 ++++++
.../g++.dg/cpp0x/lambda/lambda-const14.C | 16 +++++
gcc/testsuite/g++.dg/template/local11.C | 18 ++++++
6 files changed, 122 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const12.C
create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const13.C
create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const14.C
create mode 100644 gcc/testsuite/g++.dg/template/local11.C
diff --git a/gcc/cp/expr.cc b/gcc/cp/expr.cc
index 4d017d530ef..516cea084a6 100644
--- a/gcc/cp/expr.cc
+++ b/gcc/cp/expr.cc
@@ -182,6 +182,8 @@ mark_use (tree expr, bool rvalue_p, bool read_p,
}
}
tree r = mark_rvalue_use (ref, loc, reject_builtin);
+ if (r == error_mark_node)
+ return error_mark_node;
if (r != ref)
{
if (!rvalue_p)
diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 6564d9e37a6..5e64a220e4f 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -4697,6 +4697,10 @@ process_outer_var_ref (tree decl, tsubst_flags_t
complain, bool odr_use)
constant without odr-use. So don't complain yet. */
else if (!odr_use && decl_constant_var_p (var))
return var;
+ /* Don't complain when DECL is dependent, because it can turn out to
+ be constant (and therefore needing no capture) when instantiating. */
+ else if (VAR_P (decl) && instantiation_dependent_expression_p (decl))
+ return decl;
else if (lambda_expr)
{
if (complain & tf_error)
@@ -4715,7 +4719,7 @@ process_outer_var_ref (tree decl, tsubst_flags_t
complain, bool odr_use)
}
return error_mark_node;
}
- else if (processing_contract_condition && (TREE_CODE (decl) == PARM_DECL))
+ else if (processing_contract_condition && TREE_CODE (decl) == PARM_DECL)
/* Use of a parameter in a contract condition is fine. */
return decl;
else
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const12.C
b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const12.C
new file mode 100644
index 00000000000..6dc69f64405
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const12.C
@@ -0,0 +1,62 @@
+// PR c++/123536
+// { dg-do compile { target c++11 } }
+
+int g;
+
+struct S {
+ static int static_data_member;
+};
+
+template<int = 1>
+void
+fn1 ()
+{
+ int &c = g;
+ auto l = [] { c++; };
+ l();
+}
+
+template<int N = 1>
+void
+fn2 ()
+{
+ const int &c = N;
+ auto l = [] { int i = c; (void) i; }; // { dg-error ".c. is not captured" }
+ l();
+}
+
+void
+fn3 ()
+{
+ int &c = g;
+ auto l = [] { c++; };
+ l();
+}
+
+void
+fn4 ()
+{
+ int n = 42;
+ const int &c = n;
+ auto l = [] { int i = c; (void) i; }; // { dg-error ".c. is not captured" }
+ l();
+}
+
+template<class T>
+void
+fn5 ()
+{
+ int& c = T::static_data_member;
+ auto l = [] { c++; };
+ l();
+}
+
+void
+bar ()
+{
+ fn1 ();
+ fn2 ();
+ fn3 ();
+ fn4 ();
+ fn5<S>();
+}
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const13.C
b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const13.C
new file mode 100644
index 00000000000..a6044c52e7e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const13.C
@@ -0,0 +1,19 @@
+// PR c++/123536
+// { dg-do compile { target c++11 } }
+
+int g;
+
+template<class T>
+void
+fn1 ()
+{
+ T c = g;
+ auto l = [] { c++; };
+ l();
+}
+
+void
+bar ()
+{
+ fn1<int&> ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const14.C
b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const14.C
new file mode 100644
index 00000000000..2299556a4e6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const14.C
@@ -0,0 +1,16 @@
+// PR c++/123536
+// { dg-do compile { target c++11 } }
+
+template<class T>
+void
+f ()
+{
+ int& c = T::x; // { dg-error ".x. is not a member of .int." }
+ auto l = [] { c++; }; // { dg-error ".c. is not captured" }
+}
+
+void
+g ()
+{
+ f<int>();
+}
diff --git a/gcc/testsuite/g++.dg/template/local11.C
b/gcc/testsuite/g++.dg/template/local11.C
new file mode 100644
index 00000000000..f75b748caa8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/local11.C
@@ -0,0 +1,18 @@
+// PR c++/123536
+
+int g;
+
+template<class T>
+void
+fn1 ()
+{
+ T c = g;
+ struct A { static void f() { c++; } };
+ A::f();
+}
+
+void
+bar ()
+{
+ fn1<int&> ();
+}
base-commit: 1af06a4bfbc04309f944ce50d2ce7a88f5dc150f
--
2.54.0
