c++: Fix P0960 in member init list and array [PR92812]

c++: Fix P0960 in member init list and array [PR92812]

This patch nails down the remaining P0960 case in PR92812:

  struct A {
    int ar[2];
    A(): ar(1, 2) {} // doesn't work without this patch
  };

Note that when the target object is not of array type, this already
works:

  struct S { int x, y; };
  struct A {
    S s;
    A(): s(1, 2) { } // OK in C++20
  };

because build_new_method_call_1 takes care of the P0960 magic.

It proved to be quite hairy.  When the ()-list has more than one
element, we can always create a CONSTRUCTOR, because the code was
previously invalid.  But when the ()-list has just one element, it
gets all kinds of difficult.  As usual, we have to handle a("foo")
so as not to wrap the STRING_CST in a CONSTRUCTOR.  Always turning
x(e) into x{e} would run into trouble as in c++/93790.  Another
issue was what to do about x({e}): previously, this would trigger
"list-initializer for non-class type must not be parenthesized".
I figured I'd make this work in C++20, so that given

  struct S { int x, y; };

you can do

   S a[2];
   [...]
   A(): a({1, 2}) // initialize a[0] with {1, 2} and a[1] with {}

It also turned out that, as an extension, we support compound literals:

  F (): m((S[1]) { 1, 2 })

so this has to keep working as before.  Moreover, make sure not to trigger
in compiler-generated code, like =default, where array assignment is allowed.

I've factored out a function that turns a TREE_LIST into a CONSTRUCTOR
to simplify handling of P0960.

paren-init35.C also tests this with vector types.

gcc/cp/ChangeLog:

	PR c++/92812
	* cp-tree.h (do_aggregate_paren_init): Declare.
	* decl.c (do_aggregate_paren_init): New.
	(grok_reference_init): Use it.
	(check_initializer): Likewise.
	* init.c (perform_member_init): Handle initializing an array from
	a ()-list.  Use do_aggregate_paren_init.

gcc/testsuite/ChangeLog:

	PR c++/92812
	* g++.dg/cpp0x/constexpr-array23.C: Adjust dg-error.
	* g++.dg/cpp0x/initlist69.C: Likewise.
	* g++.dg/diagnostic/mem-init1.C: Likewise.
	* g++.dg/init/array28.C: Likewise.
	* g++.dg/cpp2a/paren-init33.C: New test.
	* g++.dg/cpp2a/paren-init34.C: New test.
	* g++.dg/cpp2a/paren-init35.C: New test.
	* g++.old-deja/g++.brendan/crash60.C: Adjust dg-error.
	* g++.old-deja/g++.law/init10.C: Likewise.
	* g++.old-deja/g++.other/array3.C: Likewise.