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

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.

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.

7 files changed, 122 insertions, 0 deletions

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" }