c++: Fix up handling of for/while loops with declarations in condition [PR86769]

As the following testcase show (note, just for-{3,4,6,7,8}.C, constexpr-86769.C
and stmtexpr27.C FAIL without the patch, the rest is just that I couldn't
find coverage for some details and so added tests we don't regress or for5.C
is from Marek's attempt in the PR), we weren't correctly handling for/while
loops with declarations as conditions.

The C++ FE has the simplify_loop_decl_cond function which transforms
such loops as mentioned in the comment:
            while (A x = 42) { }
            for (; A x = 42;) { }
   becomes
            while (true) { A x = 42; if (!x) break; }
            for (;;) { A x = 42; if (!x) break; }
For for loops this is not enough, as the x declaration should be
still in scope when expr (if any) is executed, and injecting the
expr expression into the body in the FE needs to have the continue
label in between, something normally added by the c-family
genericization.  One of my thoughts was to just add there an artificial
label plus the expr expression in the FE and tell c-family about that
label, so that it doesn't create it but uses what has been emitted.

Unfortunately break/continue are resolved to labels only at c-family
genericization time and by moving the condition (and its preparation
statements such as the DECL_EXPR) into the body (and perhaps by also
moving there the (increment) expr as well) we resolve incorrectly any
break/continue statement appearing in cond (or newly perhaps also expr)
expression(s).  While in standard C++ one can't have something like that
there, with statement expressions they are possible there, and we actually
have testsuite coverage that when they appear outside of the body of the
loop they bind to an outer loop rather than the inner one.  When the FE
moves everything into the body, c-family can't distinguish any more between
the user body vs. the condition/preparation statements vs. expr expression.

So, the following patch instead keeps them separate and does the merging
only at the c-family loop genericization time.  For that the patch
introduces two new operands of FOR_STMT and WHILE_STMT, *_COND_PREP
which is forced to be a BIND_EXPR which contains the preparation statements
like DECL_EXPR, and the initialization of that variable, so basically what
{FOR,WHILE}_BODY has when we encounter the function dealing with this,
except one optional CLEANUP_STMT at the end which holds cleanups for the
variable if it needs to be destructed.  This CLEANUP_STMT is removed and
the actual cleanup saved into another new operand, *_COND_CLEANUP.

The c-family loop genericization handles such loops roughly the way
https://eel.is/c++draft/stmt.for and https://eel.is/c++draft/stmt.while
specifies, so the body is (if *_COND_CLEANUP is non-NULL)
{ A x = 42; try { if (!x) break; body; cont_label: expr; } finally { cleanup; } }
and otherwise
{ A x = 42; if (!x) break; body; cont_label: expr; }
i.e. the *_COND, *_BODY, optional continue label, FOR_EXPR  are appended
into the body of the *_COND_PREP BIND_EXPR.

And when doing constexpr evaluation of such FOR/WHILE loops, we treat
it similarly, first evaluate *_COND_PREP except the
      for (tree decl = BIND_EXPR_VARS (t); decl; decl = DECL_CHAIN (decl))
        destroy_value_checked (ctx, decl, non_constant_p);
part of BIND_EXPR handling for it, then evaluate *_COND (and decide based
on whether it was false or true like before), then *_BODY, then FOR_EXPR,
then *_COND_CLEANUP (similarly to the way how CLEANUP_STMT handling handles
that) and finally do those destroy_value_checked.

Note, the constexpr-86769.C testcase FAILs with both clang++ and MSVC (note,
the rest of tests PASS with clang++) but I believe it must be just a bug
in those compilers, new int is done in all the constructors and delete is
done in the destructor, so when clang++ reports one of the new int weren't
deallocated during constexpr evaluation I don't see how that would be
possible.  When the same test has all the constexpr stuff, all the new int
are properly deleted at runtime when compiled by both compilers and valgrind
is happy about it, no leaks.

2025-02-07  Jakub Jelinek  <jakub@redhat.com>
	    Jason Merrill  <jason@redhat.com>

	PR c++/86769
gcc/c-family/
	* c-common.def (FOR_STMT): Add 2 operands and document them.
	(WHILE_STMT): Likewise.
	* c-common.h (WHILE_COND_PREP, WHILE_COND_CLEANUP): Define.
	(FOR_COND_PREP, FOR_COND_CLEANUP): Define.
	* c-gimplify.cc (genericize_c_loop): Add COND_PREP and COND_CLEANUP
	arguments, handle them if they are non-NULL.
	(genericize_for_stmt, genericize_while_stmt, genericize_do_stmt):
	Adjust callers.
gcc/c/
	* c-parser.cc (c_parser_while_statement): Add 2 further NULL_TREE
	operands to build_stmt.
	(c_parser_for_statement): Likewise.
gcc/cp/
	* semantics.cc (set_one_cleanup_loc): New function.
	(set_cleanup_locs): Use it.
	(simplify_loop_decl_cond): Remove.
	(adjust_loop_decl_cond): New function.
	(begin_while_stmt): Add 2 further NULL_TREE operands to build_stmt.
	(finish_while_stmt_cond): Call adjust_loop_decl_cond instead of
	simplify_loop_decl_cond.
	(finish_while_stmt): Call do_poplevel also on WHILE_COND_PREP if
	non-NULL and also use pop_stmt_list rather than do_poplevel for
	WHILE_BODY in that case.  Call set_one_cleanup_loc.
	(begin_for_stmt): Add 2 further NULL_TREE operands to build_stmt.
	(finish_for_cond): Call adjust_loop_decl_cond instead of
	simplify_loop_decl_cond.
	(finish_for_stmt): Call do_poplevel also on FOR_COND_PREP if non-NULL
	and also use pop_stmt_list rather than do_poplevel for FOR_BODY in
	that case.  Call set_one_cleanup_loc.
	* constexpr.cc (cxx_eval_loop_expr): Handle
	{WHILE,FOR}_COND_{PREP,CLEANUP}.
	(check_for_return_continue): Handle {WHILE,FOR}_COND_PREP.
	(potential_constant_expression_1): RECUR on
	{WHILE,FOR}_COND_{PREP,CLEANUP}.
gcc/testsuite/
	* g++.dg/diagnostic/redeclaration-7.C: New test.
	* g++.dg/expr/for3.C: New test.
	* g++.dg/expr/for4.C: New test.
	* g++.dg/expr/for5.C: New test.
	* g++.dg/expr/for6.C: New test.
	* g++.dg/expr/for7.C: New test.
	* g++.dg/expr/for8.C: New test.
	* g++.dg/ext/stmtexpr27.C: New test.
	* g++.dg/cpp2a/constexpr-86769.C: New test.
	* g++.dg/cpp26/name-independent-decl7.C: New test.
	* g++.dg/cpp26/name-independent-decl8.C: New test.