[gcc r11-4231] c++: Handle RANGE_EXPR index in init_subob_ctx [PR97328]

[Patrick Palka]

https://gcc.gnu.org/g:3d423c6f6a69d87ad52ba3af75f3debd8a8b8810

commit r11-4231-g3d423c6f6a69d87ad52ba3af75f3debd8a8b8810
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Oct 22 07:40:40 2020 -0400

    c++: Handle RANGE_EXPR index in init_subob_ctx [PR97328]
    
    In the testcase below, we're ICEing during constexpr evaluation of the
    CONSTRUCTOR {.data={{}, [1 ... 7]={}}} of type 'vector'.  The interesting
    thing about this CONSTRUCTOR is that it has a RANGE_EXPR index for an
    element initializer which doesn't satisfy reduced_constant_expression_p
    (because the field 't' is uninitialized).
    
    This is a problem because init_subob_ctx currently punts on setting up a
    sub-aggregate initialization context when given a RANGE_EXPR index, so
    we later trip over the asserts in verify_ctor_sanity when recursing into
    cxx_eval_bare_aggregate on this element initializer.
    
    Fix this by making init_subob_ctx set up an appropriate initialization
    context when supplied a RANGE_EXPR index.
    
    gcc/cp/ChangeLog:
    
            PR c++/97328
            * constexpr.c (init_subob_ctx): Don't punt on RANGE_EXPR
            indexes, instead build a sub-aggregate initialization context
            with no subobject.
    
    gcc/testsuite/ChangeLog:
    
            PR c++/97328
            * g++.dg/cpp2a/constexpr-init19.C: New test.
            * g++.dg/cpp2a/constexpr-init20.C: New test.

Diff:
---
 gcc/cp/constexpr.c                            | 11 +++++++++--
 gcc/testsuite/g++.dg/cpp2a/constexpr-init19.C | 15 +++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/constexpr-init20.C | 15 +++++++++++++++
 3 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index c566376e4ee..4707d29f3bc 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3954,7 +3954,8 @@ init_subob_ctx (const constexpr_ctx *ctx, constexpr_ctx &new_ctx,
   new_ctx = *ctx;
 
   if (index && TREE_CODE (index) != INTEGER_CST
-      && TREE_CODE (index) != FIELD_DECL)
+      && TREE_CODE (index) != FIELD_DECL
+      && TREE_CODE (index) != RANGE_EXPR)
     /* This won't have an element in the new CONSTRUCTOR.  */
     return;
 
@@ -3967,7 +3968,13 @@ init_subob_ctx (const constexpr_ctx *ctx, constexpr_ctx &new_ctx,
      update object to refer to the subobject and ctor to refer to
      the (newly created) sub-initializer.  */
   if (ctx->object)
-    new_ctx.object = build_ctor_subob_ref (index, type, ctx->object);
+    {
+      if (index == NULL_TREE || TREE_CODE (index) == RANGE_EXPR)
+	/* There's no well-defined subobject for this index.  */
+	new_ctx.object = NULL_TREE;
+      else
+	new_ctx.object = build_ctor_subob_ref (index, type, ctx->object);
+    }
   tree elt = build_constructor (type, NULL);
   CONSTRUCTOR_NO_CLEARING (elt) = true;
   new_ctx.ctor = elt;
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-init19.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-init19.C
new file mode 100644
index 00000000000..d354c5ad609
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-init19.C
@@ -0,0 +1,15 @@
+// PR c++/97328
+// { dg-do compile { target c++20 } }
+
+struct vector {
+  struct storage {
+    int t;
+    constexpr storage() {}
+  } data[8];
+};
+
+constexpr auto foo() {
+  vector i;
+  return i;
+}
+auto val = foo();
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-init20.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-init20.C
new file mode 100644
index 00000000000..1a6ed8d86dd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-init20.C
@@ -0,0 +1,15 @@
+// PR c++/97328
+// { dg-do compile { target c++20 } }
+
+struct vector {
+  union storage {
+    int t;
+    constexpr storage() {}
+  } data[8];
+};
+
+constexpr auto foo() {
+  vector i;
+  return i;
+}
+auto val = foo();