[gcc r14-9658] c++: broken direct-init with trailing array member [PR114439]

[Marek Polacek via Gcc-cvs]

https://gcc.gnu.org/g:de0886d48032332d10e4acb5d15c8789b281b6fe

commit r14-9658-gde0886d48032332d10e4acb5d15c8789b281b6fe
Author: Marek Polacek <pola...@redhat.com>
Date:   Mon Mar 25 15:32:20 2024 -0400

    c++: broken direct-init with trailing array member [PR114439]
    
    can_init_array_with_p is wrongly saying that the init for 's' here:
    
      struct S {
        int *list = arr;
        int arr[];
      };
    
      struct A {
        A() {}
        S s[2]{};
      };
    
    is invalid.  But as process_init_constructor_array says, for "non-constant
    initialization of trailing elements with no explicit initializers" we use
    a VEC_INIT_EXPR wrapped in a TARGET_EXPR, built in process_init_constructor.
    
    Unfortunately we didn't have a test for this scenario so I didn't
    realize can_init_array_with_p must handle it.
    
            PR c++/114439
    
    gcc/cp/ChangeLog:
    
            * init.cc (can_init_array_with_p): Return true for a VEC_INIT_EXPR
            wrapped in a TARGET_EXPR.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/init/array65.C: New test.

Diff:
---
 gcc/cp/init.cc                      |  6 +++++-
 gcc/testsuite/g++.dg/init/array65.C | 38 +++++++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
index dbd37d47cbf..a93ce00800c 100644
--- a/gcc/cp/init.cc
+++ b/gcc/cp/init.cc
@@ -950,12 +950,16 @@ can_init_array_with_p (tree type, tree init)
      mem-initializers of a constructor.  */
   if (DECL_DEFAULTED_FN (current_function_decl))
     return true;
-  /* As an extension, we allow copying from a compound literal.  */
   if (TREE_CODE (init) == TARGET_EXPR)
     {
       init = TARGET_EXPR_INITIAL (init);
+      /* As an extension, we allow copying from a compound literal.  */
       if (TREE_CODE (init) == CONSTRUCTOR)
        return CONSTRUCTOR_C99_COMPOUND_LITERAL (init);
+      /* VEC_INIT_EXPR is used for non-constant initialization of trailing
+        elements with no explicit initializers.  */
+      else if (TREE_CODE (init) == VEC_INIT_EXPR)
+       return true;
     }
 
   return false;
diff --git a/gcc/testsuite/g++.dg/init/array65.C 
b/gcc/testsuite/g++.dg/init/array65.C
new file mode 100644
index 00000000000..0b144f45a9d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/init/array65.C
@@ -0,0 +1,38 @@
+// PR c++/114439
+// { dg-do compile { target c++11 } }
+
+struct S {
+  int *list = arr;
+  __extension__ int arr[];
+};
+
+struct R {
+  int *list = arr;
+  int arr[2];
+};
+
+struct A {
+  A() {}
+  S s[2]{};
+};
+
+struct A2 {
+  A2() {}
+  S s[2]{ {}, {} };
+};
+
+struct B {
+  B() {}
+  R r[2]{};
+};
+
+struct B2 {
+  B2() {}
+  R r[2]{ {}, {} };
+};
+
+struct S1 { S1(); };
+struct S2 {
+  S2() {}
+  S1 a[1] {};
+};