[PATCH] c++: Fixing the wording of () aggregate-init [PR92812]

Jason Merrill jason@redhat.com
Wed Aug 12 20:10:52 GMT 2020


On 8/11/20 7:34 PM, Marek Polacek wrote:
> P1975R0 tweaks the static_cast wording: it says that "An expression e can be
> explicitly converted to a type T if [...] T is an aggregate type having a first
> element x and there is an implicit conversion sequence from e to the type of
> x."  This already works for classes, e.g.:
> 
>    struct Aggr { int x; int y; };
>    Aggr a = static_cast<Aggr>(1);
> 
> for which we create TARGET_EXPR <D.2111, {.x=1}>.
> 
> The proposal also mentions "If T is ``array of unknown bound of U'',
> this direct-initialization defines the type of the expression as U[1]" which
> suggest that this should work for arrays (they're aggregates too, after all):
> 
>    int (&&r)[3] = static_cast<int[3]>(42);
>    int (&&r2)[1] = static_cast<int[]>(42);
> 
> So I handled that specifically in build_static_cast_1: wrap the
> expression in { } and initialize from that.  For the 'r' case above
> this creates TARGET_EXPR <D.2083, {42}>.
> 
> There are multiple things in play, as usual, so the tests test brace
> elision, narrowing, explicit constructors, and lifetime extension too.
> I think it's in line with what we discussed on the core reflector.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.

> gcc/cp/ChangeLog:
> 
> 	PR c++/92812
> 	* typeck.c (build_static_cast_1): Implement P1975R0 by allowing
> 	static_cast to aggregate type.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR c++/92812
> 	* g++.dg/cpp2a/paren-init27.C: New test.
> 	* g++.dg/cpp2a/paren-init28.C: New test.
> 	* g++.dg/cpp2a/paren-init29.C: New test.
> 	* g++.dg/cpp2a/paren-init30.C: New test.
> 	* g++.dg/cpp2a/paren-init31.C: New test.
> 	* g++.dg/cpp2a/paren-init32.C: New test.
> ---
>   gcc/cp/typeck.c                           | 14 +++++++++++++
>   gcc/testsuite/g++.dg/cpp2a/paren-init27.C | 24 +++++++++++++++++++++++
>   gcc/testsuite/g++.dg/cpp2a/paren-init28.C | 15 ++++++++++++++
>   gcc/testsuite/g++.dg/cpp2a/paren-init29.C | 15 ++++++++++++++
>   gcc/testsuite/g++.dg/cpp2a/paren-init30.C | 23 ++++++++++++++++++++++
>   gcc/testsuite/g++.dg/cpp2a/paren-init31.C | 10 ++++++++++
>   gcc/testsuite/g++.dg/cpp2a/paren-init32.C | 21 ++++++++++++++++++++
>   7 files changed, 122 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/paren-init27.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/paren-init28.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/paren-init29.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/paren-init30.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/paren-init31.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/paren-init32.C
> 
> diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
> index a557f3439a8..9166156a5d5 100644
> --- a/gcc/cp/typeck.c
> +++ b/gcc/cp/typeck.c
> @@ -7480,6 +7480,20 @@ build_static_cast_1 (location_t loc, tree type, tree expr, bool c_cast_p,
>        t.  */
>     result = perform_direct_initialization_if_possible (type, expr,
>   						      c_cast_p, complain);
> +  /* P1975 allows static_cast<Aggr>(42), as well as static_cast<T[5]>(42),
> +     which initialize the first element of the aggregate.  We need to handle
> +     the array case specifically.  */
> +  if (result == NULL_TREE
> +      && cxx_dialect >= cxx20
> +      && TREE_CODE (type) == ARRAY_TYPE)
> +    {
> +      /* Create { EXPR } and perform direct-initialization from it.  */
> +      tree e = build_constructor_single (init_list_type_node, NULL_TREE, expr);
> +      CONSTRUCTOR_IS_DIRECT_INIT (e) = true;
> +      CONSTRUCTOR_IS_PAREN_INIT (e) = true;
> +      result = perform_direct_initialization_if_possible (type, e, c_cast_p,
> +							  complain);
> +    }
>     if (result)
>       {
>         if (processing_template_decl)
> diff --git a/gcc/testsuite/g++.dg/cpp2a/paren-init27.C b/gcc/testsuite/g++.dg/cpp2a/paren-init27.C
> new file mode 100644
> index 00000000000..0b8cbe33b69
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/paren-init27.C
> @@ -0,0 +1,24 @@
> +// PR c++/92812
> +// P1975R0
> +// { dg-do run { target c++20 } }
> +// { dg-options "-Wall -Wextra" }
> +
> +struct Aggr { int x; int y; };
> +struct Base { int i; Base(int i_) : i{i_} { } };
> +struct BaseAggr : Base { };
> +struct X { };
> +struct AggrSDM { static X x; int i; int j; };
> +
> +int
> +main ()
> +{
> +  Aggr a = static_cast<Aggr>(42); // { dg-warning "missing initializer" }
> +  if (a.x != 42 || a.y != 0)
> +    __builtin_abort ();
> +  BaseAggr b = static_cast<BaseAggr>(42);
> +  if (b.i != 42)
> +    __builtin_abort ();
> +  AggrSDM s = static_cast<AggrSDM>(42); // { dg-warning "missing initializer" }
> +  if (s.i != 42 || s.j != 0)
> +    __builtin_abort ();
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp2a/paren-init28.C b/gcc/testsuite/g++.dg/cpp2a/paren-init28.C
> new file mode 100644
> index 00000000000..8c57dc8e155
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/paren-init28.C
> @@ -0,0 +1,15 @@
> +// PR c++/92812
> +// P1975R0
> +// { dg-do compile { target c++20 } }
> +
> +// In both cases the reference declarations lifetime-extend the array
> +// temporary.
> +int (&&r)[3] = static_cast<int[3]>(42);
> +int (&&r2)[1] = static_cast<int[]>(42);
> +
> +// Make sure we've lifetime-extended.
> +// { dg-final { scan-assembler "_ZGR1r_" } }
> +// { dg-final { scan-assembler "_ZGR2r2_" } }
> +
> +// Narrowing is probably OK here.
> +int (&&r3)[1] = static_cast<int[1]>(1.3);
> diff --git a/gcc/testsuite/g++.dg/cpp2a/paren-init29.C b/gcc/testsuite/g++.dg/cpp2a/paren-init29.C
> new file mode 100644
> index 00000000000..c39cd8cc0d0
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/paren-init29.C
> @@ -0,0 +1,15 @@
> +// PR c++/92812
> +// P1975R0
> +// { dg-do run { target c++20 } }
> +
> +int (&&r)[3] = static_cast<int[3]>(42);
> +int (&&r2)[1] = static_cast<int[]>(42);
> +
> +int
> +main ()
> +{
> +  if (r[0] != 42 || r[1] != 0 || r[2] != 0)
> +    __builtin_abort ();
> +  if (r2[0] != 42 || sizeof (r2) != sizeof (int))
> +    __builtin_abort ();
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp2a/paren-init30.C b/gcc/testsuite/g++.dg/cpp2a/paren-init30.C
> new file mode 100644
> index 00000000000..6b2e86f00f4
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/paren-init30.C
> @@ -0,0 +1,23 @@
> +// PR c++/92812
> +// P1975R0
> +// { dg-do compile { target c++20 } }
> +
> +struct S1 {
> +  int i;
> +  int j;
> +};
> +
> +struct S2 {
> +  S1 s[4];
> +};
> +
> +struct S3 {
> +  S2 s2;
> +};
> +
> +void
> +f ()
> +{
> +  // Brace elision not allowed.
> +  auto s3 = static_cast<S3>(1); // { dg-error "could not convert" }
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp2a/paren-init31.C b/gcc/testsuite/g++.dg/cpp2a/paren-init31.C
> new file mode 100644
> index 00000000000..3600c4993a5
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/paren-init31.C
> @@ -0,0 +1,10 @@
> +// PR c++/92812
> +// P1975R0
> +// { dg-do compile { target c++20 } }
> +// Test we don't lifetime-extend the int temporary.
> +
> +struct A { const int &r; };
> +A a(42);
> +auto a2 = static_cast<A>(42);
> +
> +// { dg-final { scan-assembler-not "_ZGR" } }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/paren-init32.C b/gcc/testsuite/g++.dg/cpp2a/paren-init32.C
> new file mode 100644
> index 00000000000..bf7833ebd5b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/paren-init32.C
> @@ -0,0 +1,21 @@
> +// PR c++/92812
> +// P1975R0
> +// { dg-do compile { target c++20 } }
> +
> +struct A { int i; };
> +struct A2 { int i; A2(int); };
> +struct A3 { int i; explicit A3(int); };
> +
> +struct X { A a; };
> +auto x = static_cast<X>(42); // { dg-error "could not convert" }
> +
> +struct X2 { A2 a; };
> +auto x2 = static_cast<X2>(42);
> +
> +struct X3 { A3 a; };
> +// Aggregate-initialization copy-initializes, so the explicit ctor
> +// isn't considered.
> +auto x3 = static_cast<X3>(42); // { dg-error "could not convert" }
> +
> +struct NonAggr { int i; virtual void foo (); };
> +auto x4 = static_cast<NonAggr>(42); // { dg-error "no matching" }
> 
> base-commit: ebd203778cc56df61cc3434fa01e77b4bae26362
> 



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