[gcc r11-5178] c++: Fix crash with broken deduction from {} [PR97895]

[gcc r11-5178] c++: Fix crash with broken deduction from {} [PR97895]

[Marek Polacek]

https://gcc.gnu.org/g:25056bdf94d5f3e66bef039702f7fae799ef16b9

commit r11-5178-g25056bdf94d5f3e66bef039702f7fae799ef16b9
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Nov 18 22:49:59 2020 -0500

    c++: Fix crash with broken deduction from {} [PR97895]
    
    Unfortunately, the otherwise beautiful
    
      for (constructor_elt &elt : *CONSTRUCTOR_ELTS (init))
    
    is not immune to an empty constructor, so we have to check
    CONSTRUCTOR_ELTS first.
    
    gcc/cp/ChangeLog:
    
            PR c++/97895
            * pt.c (do_auto_deduction): Don't crash when the constructor has
            zero elements.
    
    gcc/testsuite/ChangeLog:
    
            PR c++/97895
            * g++.dg/cpp0x/auto54.C: New test.

Diff:
---
 gcc/cp/pt.c                         | 11 +++++++----
 gcc/testsuite/g++.dg/cpp0x/auto54.C | 10 ++++++++++
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 1babf833d32..a1b6631d691 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -29250,10 +29250,13 @@ do_auto_deduction (tree type, tree init, tree auto_node,
     return error_mark_node;
 
   if (BRACE_ENCLOSED_INITIALIZER_P (init))
-    /* We don't recurse here because we can't deduce from a nested
-       initializer_list.  */
-    for (constructor_elt &elt : *CONSTRUCTOR_ELTS (init))
-      elt.value = resolve_nondeduced_context (elt.value, complain);
+    {
+      /* We don't recurse here because we can't deduce from a nested
+	 initializer_list.  */
+      if (CONSTRUCTOR_ELTS (init))
+	for (constructor_elt &elt : *CONSTRUCTOR_ELTS (init))
+	  elt.value = resolve_nondeduced_context (elt.value, complain);
+    }
   else
     init = resolve_nondeduced_context (init, complain);
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/auto54.C b/gcc/testsuite/g++.dg/cpp0x/auto54.C
new file mode 100644
index 00000000000..0c1815a99bc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/auto54.C
@@ -0,0 +1,10 @@
+// PR c++/97895
+// { dg-do compile { target c++11 } }
+
+namespace std {
+  template<typename T> struct initializer_list {
+    const T *ptr;
+    decltype(sizeof 0) n;
+  };
+  auto a = {}; // { dg-error "unable to deduce" }
+}