Re: C++ PATCH to fix static init with () in a template (PR c++/84582)

[Marek Polacek <polacek at redhat dot com>]

On Thu, Mar 01, 2018 at 01:56:50PM -0500, Jason Merrill wrote:
> On Thu, Mar 1, 2018 at 8:17 AM, Marek Polacek <polacek@redhat.com> wrote:
> > On Wed, Feb 28, 2018 at 04:50:39PM -0500, Jason Merrill wrote:
> >> On Wed, Feb 28, 2018 at 4:19 PM, Marek Polacek <polacek@redhat.com> wrote:
> >> > On Wed, Feb 28, 2018 at 10:51:17AM -0500, Jason Merrill wrote:
> >> >> On Wed, Feb 28, 2018 at 9:32 AM, Marek Polacek <polacek@redhat.com> wrote:
> >> >> > On Tue, Feb 27, 2018 at 04:16:31PM -0500, Jason Merrill wrote:
> >> >> >> On 02/27/2018 02:13 PM, Marek Polacek wrote:
> >> >> >> > My recent change introducing cxx_constant_init caused this code
> >> >> >> >
> >> >> >> > template <class> class A {
> >> >> >> >    static const long b = 0;
> >> >> >> >    static const unsigned c = (b);
> >> >> >> > };
> >> >> >> >
> >> >> >> > to be rejected.  The reason is that force_paren_expr turns "b" into "*(const
> >> >> >> > long int &) &b", where the former is not value-dependent but the latter is
> >> >> >> > value-dependent.  So when we get to maybe_constant_init_1:
> >> >> >> > 5147   if (!is_nondependent_static_init_expression (t))
> >> >> >> > 5148     /* Don't try to evaluate it.  */;
> >> >> >> > it's not evaluated and we get the non-constant initialization error.
> >> >> >> > (Before we'd always evaluated the expression.)
> >> >> >> >
> >> >> >> > Bootstrapped/regtested on x86_64-linux, ok for trunk?
> >> >> >> >
> >> >> >> > 2018-02-27  Marek Polacek  <polacek@redhat.com>
> >> >> >> >
> >> >> >> >     PR c++/84582
> >> >> >> >     * semantics.c (force_paren_expr): Avoid creating a static cast
> >> >> >> >     when processing a template.
> >> >> >> >
> >> >> >> >     * g++.dg/cpp1z/static1.C: New test.
> >> >> >> >     * g++.dg/template/static37.C: New test.
> >> >> >> >
> >> >> >> > diff --git gcc/cp/semantics.c gcc/cp/semantics.c
> >> >> >> > index 35569d0cb0d..b48de2df4e2 100644
> >> >> >> > --- gcc/cp/semantics.c
> >> >> >> > +++ gcc/cp/semantics.c
> >> >> >> > @@ -1697,7 +1697,7 @@ force_paren_expr (tree expr)
> >> >> >> >       expr = build1 (PAREN_EXPR, TREE_TYPE (expr), expr);
> >> >> >> >     else if (VAR_P (expr) && DECL_HARD_REGISTER (expr))
> >> >> >> >       /* We can't bind a hard register variable to a reference.  */;
> >> >> >> > -  else
> >> >> >> > +  else if (!processing_template_decl)
> >> >> >>
> >> >> >> Hmm, this means that we forget about the parentheses in a template.  I'm
> >> >> >> surprised that this didn't break anything in the testsuite.  In particular,
> >> >> >> auto-fn15.C.  I've attached an addition to auto-fn15.C to catch this issue.
> >> >> >
> >> >> > Thanks, you're right.  I'll use it.
> >> >> >
> >> >> >> Can we use PAREN_EXPR instead of the static_cast in a template?
> >> >> >
> >> >> > I don't think so, it would fix the issue you pointed out in auto-fn15.C but
> >> >> > it wouldn't fix the original test.  The problem with using PAREN_EXPR in a
> >> >> > template is that instantiate_non_dependent_expr will turn in into the
> >> >> > static cast anyway; tsubst_copy_and_build has
> >> >> >     case PAREN_EXPR:
> >> >> >       RETURN (finish_parenthesized_expr (RECUR (TREE_OPERAND (t, 0))));
> >> >> > so it calls force_paren_expr and this time we're not in a template.  And
> >> >> > then when calling cxx_constant_init we have the same issue.
> >> >>
> >> >> Then maybe we need something like fold_non_dependent_expr, which
> >> >> checks for dependency before substitution and then immediately
> >> >> evaluates the result.
> >> >
> >> > I hope you meant something like this.  Further testing also revealed that
> >> > maybe_undo_parenthesized_ref should be able to unwrap PAREN_EXPR (so that
> >> > (fn1)(); in paren2.C is handled correctly), and that lvalue_kind should look
> >> > into PAREN_EXPR so as to give the correct answer regarding lvalueness: we
> >> > should accept
> >> >
> >> > template<typename T>
> >> > void foo (int i)
> >> > {
> >> >   ++(i);
> >> > }
> >> >
> >> > Apologies if I'm on the wrong track.
> >> >
> >> > Bootstrapped/regtested on x86_64-linux, ok for trunk?
> >> >
> >> > 2018-02-28  Marek Polacek  <polacek@redhat.com>
> >> >             Jason Merrill  <jason@redhat.com>
> >> >
> >> >         PR c++/84582
> >> >         * semantics.c (force_paren_expr): Avoid creating the static cast
> >> >         when in a template.  Create a PAREN_EXPR when in a template.
> >> >         (maybe_undo_parenthesized_ref): Unwrap PAREN_EXPR.
> >> >         * typeck2.c (store_init_value): Call fold_non_dependent_expr instead
> >> >         of instantiate_non_dependent_expr.
> >> >         * tree.c (lvalue_kind): Handle PAREN_EXPR like NON_DEPENDENT_EXPR.
> >> >
> >> >         * g++.dg/cpp1y/auto-fn15.C: Extend testing.
> >> >         * g++.dg/cpp1z/static1.C: New test.
> >> >         * g++.dg/template/static37.C: New test.
> >> >
> >> > diff --git gcc/cp/semantics.c gcc/cp/semantics.c
> >> > index 35569d0cb0d..722e3718a14 100644
> >> > --- gcc/cp/semantics.c
> >> > +++ gcc/cp/semantics.c
> >> > @@ -1697,7 +1697,7 @@ force_paren_expr (tree expr)
> >> >      expr = build1 (PAREN_EXPR, TREE_TYPE (expr), expr);
> >> >    else if (VAR_P (expr) && DECL_HARD_REGISTER (expr))
> >> >      /* We can't bind a hard register variable to a reference.  */;
> >> > -  else
> >> > +  else if (!processing_template_decl)
> >> >      {
> >> >        cp_lvalue_kind kind = lvalue_kind (expr);
> >> >        if ((kind & ~clk_class) != clk_none)
> >> > @@ -1713,6 +1713,8 @@ force_paren_expr (tree expr)
> >> >             REF_PARENTHESIZED_P (expr) = true;
> >> >         }
> >> >      }
> >> > +  else
> >> > +    expr = build1 (PAREN_EXPR, TREE_TYPE (expr), expr);
> >>
> >> There's already a branch for building PAREN_EXPR, let's just replace
> >> its condition.
> >
> > Sure.
> >
> >> > -      value = instantiate_non_dependent_expr (value);
> >> > +      value = fold_non_dependent_expr (value);
> >>
> >> I was thinking that we want a parallel fold_non_dependent_init (that
> >> hopefully shares most of the implementation).  Then we shouldn't need
> >> the call to maybe_constant_init anymore.
> >
> > If you mean fold_non_dependent_init that would be like fold_non_dependent_expr
> > but with maybe_constant_init and not maybe_constant_value
> 
> And is_nondependent_static_init_expression, and different arguments to
> cxx_eval_outermost_constant_expression, yes.

Ah.  Maybe it'll be useful sometime in the future.

> > then that would break e.g.
> >
> > const double d = 9.0;   // missing constexpr
> > constexpr double j = d; // should give error
> >
> > because maybe_constant_value checks is_nondependent_constant_expression, and
> > "d" in the example above is not a constant expression, so we don't evaluate,
> > and "d" stays "d", so require_constant_expression gives the error.  On the
> > other hand, maybe_constant_init checks is_nondependent_static_init_expression,
> > and "d" is that, so we evaluate "d" to "9.0".  Then require_constant_expression
> > doesn't complain.
> 
> Ah, I see.  You're right, let's stick with fold_non_dependent_expr.

Thanks, so this is the final patch then:

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2018-03-01  Marek Polacek  <polacek@redhat.com>
	    Jason Merrill  <jason@redhat.com>

	PR c++/84582
	* semantics.c (force_paren_expr): Create a PAREN_EXPR when in
	a template.
	(maybe_undo_parenthesized_ref): Unwrap PAREN_EXPR.
	* typeck2.c (store_init_value): Call fold_non_dependent_expr instead
	of instantiate_non_dependent_expr.
	* tree.c (lvalue_kind): Handle PAREN_EXPR like NON_DEPENDENT_EXPR.

	* g++.dg/cpp1y/auto-fn15.C: Extend testing.
	* g++.dg/cpp1z/static1.C: New test.
	* g++.dg/template/static37.C: New test.

diff --git gcc/cp/semantics.c gcc/cp/semantics.c
index 87c5c669a55..1ac1d23e761 100644
--- gcc/cp/semantics.c
+++ gcc/cp/semantics.c
@@ -1693,7 +1693,8 @@ force_paren_expr (tree expr)
   if (TREE_CODE (expr) == COMPONENT_REF
       || TREE_CODE (expr) == SCOPE_REF)
     REF_PARENTHESIZED_P (expr) = true;
-  else if (type_dependent_expression_p (expr))
+  else if (type_dependent_expression_p (expr)
+	   || processing_template_decl)
     expr = build1 (PAREN_EXPR, TREE_TYPE (expr), expr);
   else if (VAR_P (expr) && DECL_HARD_REGISTER (expr))
     /* We can't bind a hard register variable to a reference.  */;
@@ -1724,9 +1725,10 @@ force_paren_expr (tree expr)
 tree
 maybe_undo_parenthesized_ref (tree t)
 {
-  if (cxx_dialect >= cxx14
-      && INDIRECT_REF_P (t)
-      && REF_PARENTHESIZED_P (t))
+  if (cxx_dialect < cxx14)
+    return t;
+
+  if (INDIRECT_REF_P (t) && REF_PARENTHESIZED_P (t))
     {
       t = TREE_OPERAND (t, 0);
       while (TREE_CODE (t) == NON_LVALUE_EXPR
@@ -1737,6 +1739,8 @@ maybe_undo_parenthesized_ref (tree t)
 		  || TREE_CODE (t) == STATIC_CAST_EXPR);
       t = TREE_OPERAND (t, 0);
     }
+  else if (TREE_CODE (t) == PAREN_EXPR)
+    t = TREE_OPERAND (t, 0);
 
   return t;
 }
diff --git gcc/cp/tree.c gcc/cp/tree.c
index 9b9e36a1173..19f1c0629c9 100644
--- gcc/cp/tree.c
+++ gcc/cp/tree.c
@@ -239,6 +239,7 @@ lvalue_kind (const_tree ref)
       return lvalue_kind (BASELINK_FUNCTIONS (CONST_CAST_TREE (ref)));
 
     case NON_DEPENDENT_EXPR:
+    case PAREN_EXPR:
       return lvalue_kind (TREE_OPERAND (ref, 0));
 
     case VIEW_CONVERT_EXPR:
diff --git gcc/cp/typeck2.c gcc/cp/typeck2.c
index 153b46cca77..583c65d4d0a 100644
--- gcc/cp/typeck2.c
+++ gcc/cp/typeck2.c
@@ -822,7 +822,7 @@ store_init_value (tree decl, tree init, vec<tree, va_gc>** cleanups, int flags)
   if (decl_maybe_constant_var_p (decl) || TREE_STATIC (decl))
     {
       bool const_init;
-      value = instantiate_non_dependent_expr (value);
+      value = fold_non_dependent_expr (value);
       if (DECL_DECLARED_CONSTEXPR_P (decl)
 	  || (DECL_IN_AGGR_P (decl) && !DECL_VAR_DECLARED_INLINE_P (decl)))
 	{
diff --git gcc/testsuite/g++.dg/cpp1y/auto-fn15.C gcc/testsuite/g++.dg/cpp1y/auto-fn15.C
index ba9f3579f62..0db428f7270 100644
--- gcc/testsuite/g++.dg/cpp1y/auto-fn15.C
+++ gcc/testsuite/g++.dg/cpp1y/auto-fn15.C
@@ -22,6 +22,8 @@ template <class T>
 decltype(auto) h5(T t) { return t.i; }
 template <class T>
 decltype(auto) h6(T t) { return (t.i); }
+template <class T>
+decltype(auto) h7(T t) { return (i); }
 
 int main()
 {
@@ -48,4 +50,5 @@ int main()
   same_type<decltype(h4()),int&>();
   same_type<decltype(h5(a)),int>();
   same_type<decltype(h6(a)),int&>();
+  same_type<decltype(h7(a)),int&>();
 }
diff --git gcc/testsuite/g++.dg/cpp1z/static1.C gcc/testsuite/g++.dg/cpp1z/static1.C
index e69de29bb2d..cb872997c5a 100644
--- gcc/testsuite/g++.dg/cpp1z/static1.C
+++ gcc/testsuite/g++.dg/cpp1z/static1.C
@@ -0,0 +1,19 @@
+// PR c++/84582
+// { dg-options -std=c++17 }
+
+class C {
+  static inline const long b = 0;
+  static inline const unsigned c = (b);
+};
+class D {
+  static inline const long b = 0;
+  static inline const unsigned c = b;
+};
+template <class> class A {
+  static inline const long b = 0;
+  static inline const unsigned c = (b);
+};
+template <class> class B {
+  static inline const long b = 0;
+  static inline const unsigned c = b;
+};
diff --git gcc/testsuite/g++.dg/template/static37.C gcc/testsuite/g++.dg/template/static37.C
index e69de29bb2d..90bc65d2fbc 100644
--- gcc/testsuite/g++.dg/template/static37.C
+++ gcc/testsuite/g++.dg/template/static37.C
@@ -0,0 +1,18 @@
+// PR c++/84582
+
+class C {
+  static const long b = 0;
+  static const unsigned c = (b);
+};
+class D {
+  static const long b = 0;
+  static const unsigned c = b;
+};
+template <class> class A {
+  static const long b = 0;
+  static const unsigned c = (b);
+};
+template <class> class B {
+  static const long b = 0;
+  static const unsigned c = b;
+};

	Marek