[gcc r13-6387] c++: Don't recurse on DECL_INITIAL for DECL_EXPR on non-VAR_DECLs [PR108606]

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:b222e725f53231a0bd9799ca93892a79d592a5f3

commit r13-6387-gb222e725f53231a0bd9799ca93892a79d592a5f3
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Wed Mar 1 10:22:59 2023 +0100

    c++: Don't recurse on DECL_INITIAL for DECL_EXPR on non-VAR_DECLs [PR108606]
    
    The r13-2965-g73d9b0e5947e16 change changed the line touched in this patch
    from
          return RECUR (tmp, want_rval);
    to
          return RECUR (DECL_INITIAL (tmp), want_rval);
    This is on DECL_EXPR handling code, where tmp can be lots of different
    trees and DECL_INITIAL unfortunately also means different things on
    different trees.
    It is the initializer on VAR_DECL, DECL_ARG_TYPE on PARM_DECLs (though
    those are unlikely to have DECL_EXPRs), for FUNCTION_DECLs the body,
    ..., USING_DECL_DECLS on USING_DECLs and DECL_FRIENDLIST on TYPE_DECLs.
    
    The testcase below ICEs because we have a DECL_EXPR for TYPE_DECL
    which has non-NULL DECL_FRIENDLIST and we certainly can't recurse on
    the friend list.
    
    The following patch will RECUR on DECL_INITIAL only for VAR_DECLs and
    for anything else just return true.
    
    2023-03-01  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/108606
            * constexpr.cc (potential_constant_expression_1) <case DECL_EXPR>:
            Only recurse on DECL_INITIAL (tmp) if tmp is a VAR_DECL, otherwise
            just return true.
    
            * g++.dg/cpp1y/pr108606.C: New test.

Diff:
---
 gcc/cp/constexpr.cc                   |  4 +++-
 gcc/testsuite/g++.dg/cpp1y/pr108606.C | 11 +++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index b4d3e95bbd5..89df7d7600c 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -9699,7 +9699,9 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
 		   (tmp, /*constexpr_context_p=*/true, flags))
 	    return false;
 	}
-      return RECUR (DECL_INITIAL (tmp), want_rval);
+      if (VAR_P (tmp))
+	return RECUR (DECL_INITIAL (tmp), want_rval);
+      return true;
 
     case TRY_FINALLY_EXPR:
       return (RECUR (TREE_OPERAND (t, 0), want_rval)
diff --git a/gcc/testsuite/g++.dg/cpp1y/pr108606.C b/gcc/testsuite/g++.dg/cpp1y/pr108606.C
new file mode 100644
index 00000000000..71cfc679317
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/pr108606.C
@@ -0,0 +1,11 @@
+// PR c++/108606
+// { dg-do compile { target c++14 } }
+
+template <typename T>
+void bar (T) {}
+
+void
+foo ()
+{
+  bar ([&] (auto x) { class C { friend void baz (); }; });
+}