[PATCH] c++: Handle RANGE_EXPR indexes in init_subob_ctx [PR97328]

[Patrick Palka]

In the testcase below, we're ICEing during constexpr evaluation of the
CONSTRUCTOR {.data={{}, [1 ... 7]={}}} of type 'vector'.  The apparently
unique thing about this CONSTRUCTOR is that it has a RANGE_EXPR index
whose corresponding sub-aggregate initializer doesn't satisfy
reduced_constant_expression_p (because its field 't' is uninitialized).

This is a problem because init_subob_ctx currently punts if the
constructor index is a RANGE_EXPR, so when cxx_eval_bare_aggregate
recurses into this sub-aggregate initializer we trip over the
same_type_p assert in verify_ctor_sanity.

Fix this by making init_subob_ctx set up an appropriate sub-aggregate
initialization context even when the index is a RANGE_EXPR.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk and the 10 branch?

gcc/cp/ChangeLog:

	PR c++/97328
	* constexpr.c (init_subob_ctx): Don't punt if the index is a
	RANGE_EXPR, 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.
---
 gcc/cp/constexpr.c                            | 13 +++++++------
 gcc/testsuite/g++.dg/cpp2a/constexpr-init19.C | 15 +++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/constexpr-init20.C | 15 +++++++++++++++
 3 files changed, 37 insertions(+), 6 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-init19.C
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-init20.C

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index a118f8a810b..e50a2a220cb 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3953,11 +3953,6 @@ 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)
-    /* This won't have an element in the new CONSTRUCTOR.  */
-    return;
-
   tree type = initialized_type (value);
   if (!AGGREGATE_TYPE_P (type) && !VECTOR_TYPE_P (type))
     /* A non-aggregate member doesn't get its own CONSTRUCTOR.  */
@@ -3967,7 +3962,13 @@ 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();
-- 
2.29.0.rc0