Re: [PATCH] c++: Reject identifier label in constexpr [PR97846]

[Marek Polacek via Gcc-patches]

On Fri, Nov 20, 2020 at 05:18:55PM -0500, Jason Merrill via Gcc-patches wrote:
> On 11/16/20 9:58 PM, Marek Polacek wrote:
> > [dcl.constexpr]/3 says that the function-body of a constexpr function
> > shall not contain an identifier label, but we aren't enforcing that.
> > 
> > This patch implements that.  Of course, we can't reject artificial
> > labels.
> > 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > 
> > gcc/cp/ChangeLog:
> > 
> > PR c++/97846
> > * constexpr.c (potential_constant_expression_1): Reject
> > LABEL_EXPRs that use non-artifical LABEL_DECLs.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > PR c++/97846
> > * g++.dg/cpp1y/constexpr-label.C: New test.
> > ---
> >   gcc/cp/constexpr.c   | 9 -
> >   gcc/testsuite/g++.dg/cpp1y/constexpr-label.C | 9 +
> >   2 files changed, 17 insertions(+), 1 deletion(-)
> >   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-label.C
> > 
> > diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
> > index e6ab5eecd68..e4fbce14065 100644
> > --- a/gcc/cp/constexpr.c
> > +++ b/gcc/cp/constexpr.c
> > @@ -7484,7 +7484,6 @@ potential_constant_expression_1 (tree t, bool 
> > want_rval, bool strict, bool now,
> >   case OVERLOAD:
> >   case TEMPLATE_ID_EXPR:
> >   case LABEL_DECL:
> > -case LABEL_EXPR:
> >   case CASE_LABEL_EXPR:
> >   case PREDICT_EXPR:
> >   case CONST_DECL:
> > @@ -8393,6 +8392,14 @@ potential_constant_expression_1 (tree t, bool 
> > want_rval, bool strict, bool now,
> > return false;
> > }
> > +case LABEL_EXPR:
> > +  t = LABEL_EXPR_LABEL (t);
> > +  if (DECL_ARTIFICIAL (t) && DECL_IGNORED_P (t))
> 
> Is it useful to check DECL_IGNORED_P?  I'd think we want to allow any
> artificial labels, regardless of whether we're emitting debug info for them.

Not really; I only checked it because create_artificial_label sets it.  I'll
commit the patch without that check.

> OK either way.

Thanks,

Marek

Re: [PATCH] c++: Reject identifier label in constexpr [PR97846]

[Jason Merrill via Gcc-patches]

On 11/16/20 9:58 PM, Marek Polacek wrote:

[dcl.constexpr]/3 says that the function-body of a constexpr function
shall not contain an identifier label, but we aren't enforcing that.

This patch implements that.  Of course, we can't reject artificial
labels.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

gcc/cp/ChangeLog:

PR c++/97846
* constexpr.c (potential_constant_expression_1): Reject
LABEL_EXPRs that use non-artifical LABEL_DECLs.

gcc/testsuite/ChangeLog:

PR c++/97846
* g++.dg/cpp1y/constexpr-label.C: New test.
---
  gcc/cp/constexpr.c   | 9 -
  gcc/testsuite/g++.dg/cpp1y/constexpr-label.C | 9 +
  2 files changed, 17 insertions(+), 1 deletion(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-label.C

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index e6ab5eecd68..e4fbce14065 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -7484,7 +7484,6 @@ potential_constant_expression_1 (tree t, bool want_rval, 
bool strict, bool now,
  case OVERLOAD:
  case TEMPLATE_ID_EXPR:
  case LABEL_DECL:
-case LABEL_EXPR:
  case CASE_LABEL_EXPR:
  case PREDICT_EXPR:
  case CONST_DECL:
@@ -8393,6 +8392,14 @@ potential_constant_expression_1 (tree t, bool want_rval, 
bool strict, bool now,
return false;
}
  
+case LABEL_EXPR:

+  t = LABEL_EXPR_LABEL (t);
+  if (DECL_ARTIFICIAL (t) && DECL_IGNORED_P (t))


Is it useful to check DECL_IGNORED_P?  I'd think we want to allow any 
artificial labels, regardless of whether we're emitting debug info for 
them.  OK either way.



+   return true;
+  else if (flags & tf_error)
+   error_at (loc, "label definition is not a constant expression");
+  return false;
+
  case ANNOTATE_EXPR:
return RECUR (TREE_OPERAND (t, 0), rval);
  
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-label.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-label.C

new file mode 100644
index 000..a2d113c186f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-label.C
@@ -0,0 +1,9 @@
+// PR c++/97846
+// { dg-do compile { target c++14 } }
+
+constexpr int
+f ()
+{
+x: // { dg-error "label definition is not a constant expression" }
+  return 42;
+}

base-commit: 814e016318646d06b1662219cc716d502b76d8ce