[PATCH] c++: Reject addresses of immediate functions in constexpr vars inside of immediate functions or consteval if [PR102753]

[Jakub Jelinek <jakub@redhat.com>]

On Tue, Oct 19, 2021 at 02:00:21PM +0200, Jakub Jelinek via Gcc-patches wrote:
> And another thing isn't in a patch, but I'm wondering whether we don't
> handle it incorrectly.  constexpr.c has:
>   /* Check that immediate invocation does not return an expression referencing
>      any immediate function decls.  They need to be allowed while parsing
>      immediate functions, but can't leak outside of them.  */
>   if (is_consteval
>       && t != r
>       && (current_function_decl == NULL_TREE
> 	  || !DECL_IMMEDIATE_FUNCTION_P (current_function_decl)))
> as condition for the discovery of embedded immediate FUNCTION_DECLs
> (or now PTRMEM_CSTs).  If I remove the && (current... ..._decl))
> then g++.dg/cpp2a/consteval7.C's
> struct S { int b; int (*c) (); };
> consteval S baz () { return { 5, foo }; }
> consteval int qux () { S s = baz (); return s.b + s.c (); }
> consteval int quux () { constexpr S s = baz (); return s.b + s.c (); }
> quux line fails, but based on
> http://eel.is/c++draft/expr.const#11
> I wonder if it shouldn't fail (clang++ -std=c++20 rejects it),
> and be only accepted without the constexpr keyword before S s.

Here is an incremental patch that implements that.

2021-10-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/102753
	* constexpr.c (cxx_eval_outermost_constant_expr): Perform
	find_immediate_fndecl discovery if is_consteval or
	in_immediate_context () rather than if is_consteval, t != r
	and not in immediate function's body.

	* g++.dg/cpp2a/consteval7.C: Expect diagnostics on quux.
	* g++.dg/cpp2a/consteval24.C: New test.
	* g++.dg/cpp23/consteval-if12.C: New test.

--- gcc/cp/constexpr.c.jj	2021-10-19 12:22:35.583964001 +0200
+++ gcc/cp/constexpr.c	2021-10-19 13:58:22.545182032 +0200
@@ -7472,12 +7472,8 @@ cxx_eval_outermost_constant_expr (tree t
     }
 
   /* Check that immediate invocation does not return an expression referencing
-     any immediate function decls.  They need to be allowed while parsing
-     immediate functions, but can't leak outside of them.  */
-  if (is_consteval
-      && t != r
-      && (current_function_decl == NULL_TREE
-	  || !DECL_IMMEDIATE_FUNCTION_P (current_function_decl)))
+     any immediate function decls.  */
+  if (is_consteval || in_immediate_context ())
     if (tree immediate_fndecl
 	= cp_walk_tree_without_duplicates (&r, find_immediate_fndecl,
 					   NULL))
--- gcc/testsuite/g++.dg/cpp2a/consteval7.C.jj	2020-01-12 11:54:37.140402440 +0100
+++ gcc/testsuite/g++.dg/cpp2a/consteval7.C	2021-10-19 13:59:54.033897061 +0200
@@ -7,7 +7,7 @@ constexpr auto a = bar ();	// { dg-error
 struct S { int b; int (*c) (); };
 consteval S baz () { return { 5, foo }; }
 consteval int qux () { S s = baz (); return s.b + s.c (); }
-consteval int quux () { constexpr S s = baz (); return s.b + s.c (); }
+consteval int quux () { constexpr S s = baz (); return s.b + s.c (); }	// { dg-error "immediate evaluation returns address of immediate function 'consteval int foo\\(\\)'" }
 constexpr auto d = baz ();	// { dg-error "immediate evaluation returns address of immediate function 'consteval int foo\\(\\)'" }
 constexpr auto e = qux ();
 constexpr auto f = quux ();
--- gcc/testsuite/g++.dg/cpp2a/consteval24.C.jj	2021-10-19 14:32:51.858019368 +0200
+++ gcc/testsuite/g++.dg/cpp2a/consteval24.C	2021-10-19 14:49:11.618177303 +0200
@@ -0,0 +1,30 @@
+// PR c++/102753
+// { dg-do compile { target c++20 } }
+
+struct S {
+  constexpr S () : s (0) {}
+  consteval int foo () { return 1; }
+  virtual consteval int bar () { return 2; }
+  int s;
+};
+
+consteval int foo () { return 42; }
+consteval auto baz () { return foo; }
+consteval auto qux () { return &S::foo; }
+consteval auto corge () { return &S::bar; }
+
+consteval int
+bar ()
+{
+  S s;
+  constexpr auto fn1 = foo;		// { dg-error "immediate evaluation returns address of immediate function" }
+  constexpr auto fn2 = &foo;		// { dg-error "immediate evaluation returns address of immediate function" }
+  constexpr auto fn3 = &S::foo;		// { dg-error "immediate evaluation returns address of immediate function" }
+  constexpr auto fn4 = &S::bar;		// { dg-error "immediate evaluation returns address of immediate function" }
+  constexpr auto fn5 = baz ();		// { dg-error "immediate evaluation returns address of immediate function" }
+  constexpr auto fn6 = qux ();		// { dg-error "immediate evaluation returns address of immediate function" }
+  constexpr auto fn7 = corge ();	// { dg-error "immediate evaluation returns address of immediate function" }
+  return fn1 () + fn2 () + (s.*fn3) () + (s.*fn4) () + fn5 () + (s.*fn6) () + (s.*fn7) ();
+}
+
+auto a = bar ();
--- gcc/testsuite/g++.dg/cpp23/consteval-if12.C.jj	2021-10-19 14:54:05.123023731 +0200
+++ gcc/testsuite/g++.dg/cpp23/consteval-if12.C	2021-10-19 14:55:29.026844039 +0200
@@ -0,0 +1,34 @@
+// PR c++/102753
+// { dg-do compile { target c++20 } }
+// { dg-options "" }
+
+struct S {
+  constexpr S () : s (0) {}
+  consteval int foo () { return 1; }
+  virtual consteval int bar () { return 2; }
+  int s;
+};
+
+consteval int foo () { return 42; }
+consteval auto baz () { return foo; }
+consteval auto qux () { return &S::foo; }
+consteval auto corge () { return &S::bar; }
+
+constexpr int
+bar ()
+{
+  S s;
+  if consteval {			// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    constexpr auto fn1 = foo;		// { dg-error "immediate evaluation returns address of immediate function" }
+    constexpr auto fn2 = &foo;		// { dg-error "immediate evaluation returns address of immediate function" }
+    constexpr auto fn3 = &S::foo;	// { dg-error "immediate evaluation returns address of immediate function" }
+    constexpr auto fn4 = &S::bar;	// { dg-error "immediate evaluation returns address of immediate function" }
+    constexpr auto fn5 = baz ();	// { dg-error "immediate evaluation returns address of immediate function" }
+    constexpr auto fn6 = qux ();	// { dg-error "immediate evaluation returns address of immediate function" }
+    constexpr auto fn7 = corge ();	// { dg-error "immediate evaluation returns address of immediate function" }
+    return fn1 () + fn2 () + (s.*fn3) () + (s.*fn4) () + fn5 () + (s.*fn6) () + (s.*fn7) ();
+  }
+  return 0;
+}
+
+auto a = bar ();


	Jakub