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

[Marek Polacek via Gcc-patches]

[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))
+       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 00000000000..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
-- 
2.28.0