[gcc r15-10980] c++: Fix up sb as condition instantiation error-recovery [PR120039]

[Jakub Jelinek]

https://gcc.gnu.org/g:e66c207e111e77c2c67d564ef2a7fdf95c958e8a

commit r15-10980-ge66c207e111e77c2c67d564ef2a7fdf95c958e8a
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Mon Mar 16 08:06:52 2026 +0100

    c++: Fix up sb as condition instantiation error-recovery [PR120039]
    
    The following two testcases ICE during instantation.
    Normally for the artificial var used for structured binding as condition
    (which has NULL DECL_NAME) there is a DECL_EXPR which registers local
    specialization and so tsubst_expr works fine.
    But if there are errors while parsing the initializer, the VAR_DECL
    has still NULL_TREE DECL_NAME, but error_mark_node TREE_TYPE and
    when tsubst_expr is called on it, it falls back to calling lookup_name
    (NULL_TREE) and ICEs on that.
    
    The following patch fixes it by not calling lookup_name if DECL_NAME is
    NULL.
    
    2026-03-16  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/120039
            PR c++/122559
            * pt.cc (tsubst_expr) <case VAR_DECL>: Don't call lookup_name on
            DECL_NAME (t) if it is NULL_TREE.
    
            * g++.dg/cpp26/decomp28.C: New test.
            * g++.dg/cpp26/decomp29.C: New test.
    
    Reviewed-by: Jason Merrill <jason@redhat.com>
    (cherry picked from commit caae42b31118f754fbd1f9566373f589d7e1568f)

Diff:
---
 gcc/cp/pt.cc                          |  3 ++-
 gcc/testsuite/g++.dg/cpp26/decomp28.C | 23 +++++++++++++++++++++++
 gcc/testsuite/g++.dg/cpp26/decomp29.C | 19 +++++++++++++++++++
 3 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index dbb7e170e50b..7b8c887ac38a 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -22046,7 +22046,8 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	  else if (r == NULL_TREE)
 	    {
 	      /* First try name lookup to find the instantiation.  */
-	      r = lookup_name (DECL_NAME (t));
+	      if (DECL_NAME (t))
+		r = lookup_name (DECL_NAME (t));
 	      if (r)
 		{
 		  if (!VAR_P (r))
diff --git a/gcc/testsuite/g++.dg/cpp26/decomp28.C b/gcc/testsuite/g++.dg/cpp26/decomp28.C
new file mode 100644
index 000000000000..015d3bf60374
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/decomp28.C
@@ -0,0 +1,23 @@
+// PR c++/120039
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+struct S {
+  int a; long long b; short c;
+  explicit operator bool () const noexcept { return true; }
+};
+
+template <int N>
+void
+foo ()
+{
+  S s = S { 1, 2, 3 };
+  if (auto [sx, sy, sz] : s)	// { dg-warning "structured bindings in conditions only available with" "" { target c++23_down } }
+    ;				// { dg-error "expected initializer before ':' token" "" { target *-*-* } .-1 }
+}				// { dg-error "expected '\\\)' before ':' token" "" { target *-*-* } .-2 }
+
+int
+main ()
+{
+  foo <0> ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp26/decomp29.C b/gcc/testsuite/g++.dg/cpp26/decomp29.C
new file mode 100644
index 000000000000..aafb08b043b1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/decomp29.C
@@ -0,0 +1,19 @@
+// PR c++/122559
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+enum class A : bool { B, C };
+
+template <typename T>
+A type (T &&x)
+{
+  if (auto [value = x ()])	// { dg-warning "structured bindings in conditions only available with" "" { target c++23_down } }
+    return A::C;		// { dg-error "expected '\\\]' before '=' token" "" { target *-*-* } .-1 }
+  return A::B;			// { dg-error "expected initializer before '\\\)' token" "" { target *-*-* } .-2 }
+}
+
+int
+main ()
+{
+  auto _ = type (A::B);
+}