c++: Handle CONSTRUCTORs without indexes in find_array_ctor_elt [PR93549]

c++: Handle CONSTRUCTORs without indexes in find_array_ctor_elt [PR93549]

My change
* typeck2.c (store_init_value): Don't call cp_fully_fold_init on
initializers of automatic non-constexpr variables in constexpr
functions.
-  value = cp_fully_fold_init (value);
+  /* Don't fold initializers of automatic variables in constexpr functions,
+     that might fold away something that needs to be diagnosed at constexpr
+     evaluation time.  */
+  if (!current_function_decl
+      || !DECL_DECLARED_CONSTEXPR_P (current_function_decl)
+      || TREE_STATIC (decl))
+    value = cp_fully_fold_init (value);
from the constexpr new change apparently broke the following testcase.
When handling COND_EXPR, we build_vector_from_val, however as the argument we
pass to it is not an INTEGER_CST/REAL_CST, but that wrapped in a
NON_LVALUE_EXPR location wrapper, we end up with a CONSTRUCTOR and as it is
middle-end that builds it, it doesn't bother with indexes.  The
cp_fully_fold_init call used to fold it into VECTOR_CST in the past, but as
we intentionally don't invoke it anymore as it might fold away something
that needs to be diagnosed during constexpr evaluation, we end up evaluating
ARRAY_REF into the index-less CONSTRUCTOR.  The following patch fixes the
ICE by teaching find_array_ctor_elt to handle CONSTRUCTORs without indexes
(that itself could be still very efficient) and CONSTRUCTORs with some
indexes present and others missing (the rules are that if the index on the
first element is missing, then it is the array's lowest index (in C/C++ 0)
and if other indexes are missing, they are the index of the previous element
+ 1).

Here is a new version, which assumes CONSTRUCTORs with all or none indexes
and for CONSTRUCTORs without indexes performs the verification for
flag_checking directly in find_array_ctor_elt.  For CONSTRUCTORs with
indexes, it doesn't do the verification of all elts, because some CONSTRUCTORs
can be large, and it "verifies" only what it really needs - if all elts
touched during the binary search have indexes, that is actually all we care
about because we are sure we found the right elt.  It is just if we see a
missing index we need assurance that all are missing to be able to directly
access it.

The assumption then simplifies the patch, for no index CONSTRUCTORs we can
use direct access like for CONSTRUCTORs where last elt index is equal to the
elt position.  If we append right after the last elt, we just should clear
the index so that we don't violate the assumption, and if we need a gap
between the elts and the elt to be added, we need to add indexes.

2020-02-08  Jakub Jelinek  <jakub@redhat.com>

	PR c++/93549
	* constexpr.c (find_array_ctor_elt): If last element has no index,
	for flag_checking verify all elts have no index.  If i is within the
	elts, return it directly, if it is right after the last elt, append
	if NULL index, otherwise force indexes on all elts.
	(cxx_eval_store_expression): Allow cep->index to be NULL.

	* g++.dg/ext/constexpr-pr93549.C: New test.