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

[Jason Merrill]

On 10/19/21 09:24, Jakub Jelinek wrote:
> 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.

OK.

> 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
>