[PATCH] c++, v2: Don't defer local statics initialized with constant expressions [PR108702]

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Mar 02, 2023 at 11:48:04AM -0500, Jason Merrill wrote:
> > The stmtexpr19.C testcase used to be rejected as it has a static
> > variable in statement expression in constexpr context, but as that
> > static variable is initialized by constant expression, when P2647R1
> > was implemented we agreed to make it valid.
> > 
> > Now, as reported, the testcase compiles fine, but doesn't actually link
> > because the static variable isn't defined anywhere, and with -flto ICEs
> > because of this problem.  This is because we never
> > varpool_node::finalize_decl those vars, the constant expression in which
> > the DECL_EXPR is present for the static VAR_DECL is folded (constant
> > evaluated) into just the address of the VAR_DECL.
> 
> Would it make sense to define it when we see the DECL_EXPR in constant
> evaluation?

So like this?
Passes GXX_TESTSUITE_STDS=98,11,14,17,20,2b make check-g++ so far.

2023-03-03  Jakub Jelinek  <jakub@redhat.com>

	PR c++/108702
	* constexpr.cc: Include toplev.h.
	(cxx_eval_constant_expression) <case DECL_EXPR>: When seeing a local
	static initialized by constant expression outside of a constexpr
	function which has been deferred by make_rtl_for_nonlocal_decl,
	call rest_of_decl_compilation on it.

	* g++.dg/ext/stmtexpr19.C: Use dg-do link rather than dg-do compile.

--- gcc/cp/constexpr.cc.jj	2023-03-03 00:34:44.113679918 +0100
+++ gcc/cp/constexpr.cc	2023-03-03 13:26:57.602871900 +0100
@@ -40,6 +40,7 @@ along with GCC; see the file COPYING3.
 #include "attribs.h"
 #include "fold-const.h"
 #include "intl.h"
+#include "toplev.h"
 
 static bool verify_constant (tree, bool, bool *, bool *);
 #define VERIFY_CONSTANT(X)						\
@@ -7127,6 +7128,24 @@ cxx_eval_constant_expression (const cons
 	    break;
 	  }
 
+	/* make_rtl_for_nonlocal_decl could have deferred emission of
+	   a local static var, but if it appears in a statement expression
+	   which is constant expression evaluated to e.g. just the address
+	   of the variable, its DECL_EXPR will never be seen during
+	   gimple lowering's record_vars_into as the statement expression
+	   will not be in the IL at all.  */
+	if (VAR_P (r)
+	    && TREE_STATIC (r)
+	    && !DECL_REALLY_EXTERN (r)
+	    && DECL_FUNCTION_SCOPE_P (r)
+	    && !var_in_maybe_constexpr_fn (r)
+	    && decl_constant_var_p (r))
+	  {
+	    varpool_node *node = varpool_node::get (r);
+	    if (node == NULL || !node->definition)
+	      rest_of_decl_compilation (r, 0, at_eof);
+	  }
+
 	if (AGGREGATE_TYPE_P (TREE_TYPE (r))
 	    || VECTOR_TYPE_P (TREE_TYPE (r)))
 	  {
--- gcc/testsuite/g++.dg/ext/stmtexpr19.C.jj	2023-02-09 15:52:29.623359240 +0100
+++ gcc/testsuite/g++.dg/ext/stmtexpr19.C	2023-03-03 12:24:20.217186735 +0100
@@ -1,6 +1,6 @@
 // PR c++/81073
 // { dg-options "" }
-// { dg-do compile { target c++11 } }
+// { dg-do link { target c++11 } }
 
 struct test { const int *addr; };
 


	Jakub