Implement P0892R2, explicit(bool).

	* c-cppbuiltin.c (c_cpp_builtins): Define __cpp_explicit_bool.

	* call.c (add_template_candidate_real): Return if the declaration is
	explicit and we're only looking for non-converting constructor.
	* cp-tree.h (lang_decl_fn): Add has_dependent_explicit_spec_p bit.
	(DECL_HAS_DEPENDENT_EXPLICIT_SPEC_P): New macro.
	(cp_decl_specifier_seq): Add explicit_specifier field.
	(build_explicit_specifier, store_explicit_specifier): Declare.
	* decl.c (grokdeclarator): Call store_explicit_specifier.
	(build_explicit_specifier): New function.
	* parser.c (cp_parser_function_specifier_opt) <case RID_EXPLICIT>:
	Parse C++20 explicit(bool).
	* pt.c (store_explicit_specifier, lookup_explicit_specifier): New.
	(tsubst_function_decl): Handle explicit(dependent-expr).

	* g++.dg/cpp2a/explicit1.C: New test.
	* g++.dg/cpp2a/explicit10.C: New test.
	* g++.dg/cpp2a/explicit11.C: New test.
	* g++.dg/cpp2a/explicit12.C: New test.
	* g++.dg/cpp2a/explicit13.C: New test.
	* g++.dg/cpp2a/explicit2.C: New test.
	* g++.dg/cpp2a/explicit3.C: New test.
	* g++.dg/cpp2a/explicit4.C: New test.
	* g++.dg/cpp2a/explicit5.C: New test.
	* g++.dg/cpp2a/explicit6.C: New test.
	* g++.dg/cpp2a/explicit7.C: New test.
	* g++.dg/cpp2a/explicit8.C: New test.
	* g++.dg/cpp2a/explicit9.C: New test.

	* testsuite/20_util/any/cons/explicit.cc: Adjust dg-error.
	* testsuite/20_util/pair/cons/explicit_construct.cc: Likewise.
	* testsuite/20_util/tuple/cons/explicit_construct.cc: Likewise.

From-SVN: r265641

22 files changed, 616 insertions, 4 deletions

diff --git a/gcc/c-family/ChangeLog b/gcc/c-family/ChangeLog
index 64e0988..ace36fb 100644
--- a/gcc/c-family/ChangeLog
+++ b/gcc/c-family/ChangeLog
@@ -1,3 +1,8 @@
+2018-10-30  Marek Polacek  <polacek@redhat.com>
+
+	Implement P0892R2, explicit(bool).
+	* c-cppbuiltin.c (c_cpp_builtins): Define __cpp_explicit_bool.
+
 2018-10-29  David Malcolm  <dmalcolm@redhat.com>
 
 	* name-hint.h (name_hint::take_deferred): New member function.
diff --git a/gcc/c-family/c-cppbuiltin.c b/gcc/c-family/c-cppbuiltin.c
index 96a6b4d..b085cf92 100644
--- a/gcc/c-family/c-cppbuiltin.c
+++ b/gcc/c-family/c-cppbuiltin.c
@@ -955,7 +955,7 @@ c_cpp_builtins (cpp_reader *pfile)
 	}
       if (cxx_dialect > cxx14)
 	{
-	  /* Set feature test macros for C++1z.  */
+	  /* Set feature test macros for C++17.  */
 	  cpp_define (pfile, "__cpp_unicode_characters=201411");
 	  cpp_define (pfile, "__cpp_static_assert=201411");
 	  cpp_define (pfile, "__cpp_namespace_attributes=201411");
@@ -975,6 +975,11 @@ c_cpp_builtins (cpp_reader *pfile)
 	  cpp_define (pfile, "__cpp_structured_bindings=201606");
 	  cpp_define (pfile, "__cpp_variadic_using=201611");
 	}
+      if (cxx_dialect > cxx17)
+	{
+	  /* Set feature test macros for C++2a.  */
+	  cpp_define (pfile, "__cpp_explicit_bool=201806");
+	}
       if (flag_concepts)
 	cpp_define (pfile, "__cpp_concepts=201507");
       if (flag_tm)
diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 75fcc8a..d76d317 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,19 @@
+2018-10-30  Marek Polacek  <polacek@redhat.com>
+
+	Implement P0892R2, explicit(bool).
+	* call.c (add_template_candidate_real): Return if the declaration is
+	explicit and we're only looking for non-converting constructor.
+	* cp-tree.h (lang_decl_fn): Add has_dependent_explicit_spec_p bit.
+	(DECL_HAS_DEPENDENT_EXPLICIT_SPEC_P): New macro.
+	(cp_decl_specifier_seq): Add explicit_specifier field.
+	(build_explicit_specifier, store_explicit_specifier): Declare.
+	* decl.c (grokdeclarator): Call store_explicit_specifier.
+	(build_explicit_specifier): New function.
+	* parser.c (cp_parser_function_specifier_opt) <case RID_EXPLICIT>:
+	Parse C++20 explicit(bool).
+	* pt.c (store_explicit_specifier, lookup_explicit_specifier): New.
+	(tsubst_function_decl): Handle explicit(dependent-expr).
+
 2018-10-30  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	* decl.c (grokdeclarator): Use declarator->id_loc in diagnostic
diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index a7dce2e..6f40156 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -3251,6 +3251,12 @@ add_template_candidate_real (struct z_candidate **candidates, tree tmpl,
       goto fail;
     }
 
+  /* Now the explicit specifier might have been deduced; check if this
+     declaration is explicit.  If it is and we're ignoring non-converting
+     constructors, don't add this function to the set of candidates.  */
+  if ((flags & LOOKUP_ONLYCONVERTING) && DECL_NONCONVERTING_P (fn))
+    return NULL;
+
   if (DECL_CONSTRUCTOR_P (fn) && nargs == 2)
     {
       tree arg_types = FUNCTION_FIRST_USER_PARMTYPE (fn);
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 8454cb4..c9427a0 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -2587,7 +2587,8 @@ struct GTY(()) lang_decl_fn {
   unsigned this_thunk_p : 1;
   unsigned hidden_friend_p : 1;
   unsigned omp_declare_reduction_p : 1;
-  unsigned spare : 13;
+  unsigned has_dependent_explicit_spec_p : 1;
+  unsigned spare : 12;
 
   /* 32-bits padding on 64-bit host.  */
 
@@ -3033,6 +3034,12 @@ struct GTY(()) lang_decl {
 #define DECL_PURE_VIRTUAL_P(NODE) \
   (LANG_DECL_FN_CHECK (NODE)->pure_virtual)
 
+/* Nonzero for FUNCTION_DECL means that this member function (either
+   a constructor or a conversion function) has an explicit specifier
+   with a value-dependent expression.  */
+#define DECL_HAS_DEPENDENT_EXPLICIT_SPEC_P(NODE) \
+  (LANG_DECL_FN_CHECK (NODE)->has_dependent_explicit_spec_p)
+
 /* True (in a FUNCTION_DECL) if NODE is a virtual function that is an
    invalid overrider for a function from a base class.  Once we have
    complained about an invalid overrider we avoid complaining about it
@@ -5748,6 +5755,8 @@ struct cp_decl_specifier_seq {
   /* If non-NULL, a built-in type that the user attempted to redefine
      to some other type.  */
   tree redefined_builtin_type;
+  /* The explicit-specifier, if any.  */
+  tree explicit_specifier;
   /* The storage class specified -- or sc_none if no storage class was
      explicitly specified.  */
   cp_storage_class storage_class;
@@ -6375,6 +6384,7 @@ extern tree cxx_maybe_build_cleanup		(tree, tsubst_flags_t);
 extern bool check_array_designated_initializer  (constructor_elt *,
 						 unsigned HOST_WIDE_INT);
 extern bool check_for_uninitialized_const_var   (tree, bool, tsubst_flags_t);
+extern tree build_explicit_specifier		(tree, tsubst_flags_t);
 
 /* in decl2.c */
 extern void record_mangling			(tree, bool);
@@ -6772,6 +6782,7 @@ extern bool dguide_name_p			(tree);
 extern bool deduction_guide_p			(const_tree);
 extern bool copy_guide_p			(const_tree);
 extern bool template_guide_p			(const_tree);
+extern void store_explicit_specifier		(tree, tree);
 
 /* in repo.c */
 extern void init_repo				(void);
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 496ed98..11320b6 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -12382,6 +12382,9 @@ grokdeclarator (const cp_declarator *declarator,
 	       is called a converting constructor.  */
 	    if (explicitp == 2)
 	      DECL_NONCONVERTING_P (decl) = 1;
+
+	    if (declspecs->explicit_specifier)
+	      store_explicit_specifier (decl, declspecs->explicit_specifier);
 	  }
 	else if (!staticp && !dependent_type_p (type)
 		 && !COMPLETE_TYPE_P (complete_type (type))
@@ -16562,4 +16565,20 @@ require_deduced_type (tree decl, tsubst_flags_t complain)
   return true;
 }
 
+/* Create a representation of the explicit-specifier with
+   constant-expression of EXPR.  COMPLAIN is as for tsubst.  */
+
+tree
+build_explicit_specifier (tree expr, tsubst_flags_t complain)
+{
+  if (processing_template_decl && value_dependent_expression_p (expr))
+    /* Wait for instantiation, tsubst_function_decl will handle it.  */
+    return expr;
+
+  expr = build_converted_constant_expr (boolean_type_node, expr, complain);
+  expr = instantiate_non_dependent_expr (expr);
+  expr = cxx_constant_value (expr);
+  return expr;
+}
+
 #include "gt-cp-decl.h"
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index c4bda7f..206ceb0 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -13897,6 +13897,9 @@ cp_parser_storage_class_specifier_opt (cp_parser* parser)
      virtual
      explicit
 
+   C++2A Extension:
+     explicit(constant-expression)
+
    Returns an IDENTIFIER_NODE corresponding to the keyword used.
    Updates DECL_SPECS, if it is non-NULL.  */
 
@@ -13923,8 +13926,53 @@ cp_parser_function_specifier_opt (cp_parser* parser,
       break;
 
     case RID_EXPLICIT:
-      set_and_check_decl_spec_loc (decl_specs, ds_explicit, token);
-      break;
+      {
+	tree id = cp_lexer_consume_token (parser->lexer)->u.value;
+	/* If we see '(', it's C++20 explicit(bool).  */
+	tree expr;
+	if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_PAREN))
+	  {
+	    matching_parens parens;
+	    parens.consume_open (parser);
+
+	    /* New types are not allowed in an explicit-specifier.  */
+	    const char *saved_message
+	      = parser->type_definition_forbidden_message;
+	    parser->type_definition_forbidden_message
+	      = G_("types may not be defined in explicit-specifier");
+
+	    if (cxx_dialect < cxx2a)
+	      pedwarn (token->location, 0,
+		       "%<explicit(bool)%> only available with -std=c++2a "
+		       "or -std=gnu++2a");
+
+	    /* Parse the constant-expression.  */
+	    expr = cp_parser_constant_expression (parser);
+
+	    /* Restore the saved message.  */
+	    parser->type_definition_forbidden_message = saved_message;
+	    parens.require_close (parser);
+	  }
+	else
+	  /* The explicit-specifier explicit without a constant-expression is
+	     equivalent to the explicit-specifier explicit(true).  */
+	  expr = boolean_true_node;
+
+	/* [dcl.fct.spec]
+	   "the constant-expression, if supplied, shall be a contextually
+	   converted constant expression of type bool."  */
+	expr = build_explicit_specifier (expr, tf_warning_or_error);
+	/* We could evaluate it -- mark the decl as appropriate.  */
+	if (expr == boolean_true_node)
+	  set_and_check_decl_spec_loc (decl_specs, ds_explicit, token);
+	else if (expr == boolean_false_node)
+	  /* Don't mark the decl as explicit.  */;
+	else if (decl_specs)
+	  /* The expression was value-dependent.  Remember it so that we can
+	     substitute it later.  */
+	  decl_specs->explicit_specifier = expr;
+	return id;
+      }
 
     default:
       return NULL_TREE;
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index f290cb3..fc6cf9895 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -12803,6 +12803,28 @@ tsubst_default_arguments (tree fn, tsubst_flags_t complain)
 						    complain);
 }
 
+/* Hash table mapping a FUNCTION_DECL to its dependent explicit-specifier.  */
+static GTY((cache)) tree_cache_map *explicit_specifier_map;
+
+/* Store a pair to EXPLICIT_SPECIFIER_MAP.  */
+
+void
+store_explicit_specifier (tree v, tree t)
+{
+  if (!explicit_specifier_map)
+    explicit_specifier_map = tree_cache_map::create_ggc (37);
+  DECL_HAS_DEPENDENT_EXPLICIT_SPEC_P (v) = true;
+  explicit_specifier_map->put (v, t);
+}
+
+/* Lookup an element in EXPLICIT_SPECIFIER_MAP.  */
+
+static tree
+lookup_explicit_specifier (tree v)
+{
+  return *explicit_specifier_map->get (v);
+}
+
 /* Subroutine of tsubst_decl for the case when T is a FUNCTION_DECL.  */
 
 static tree
@@ -12943,6 +12965,17 @@ tsubst_function_decl (tree t, tree args, tsubst_flags_t complain,
     DECL_INITIAL (r) = NULL_TREE;
   DECL_CONTEXT (r) = ctx;
 
+  /* Handle explicit(dependent-expr).  */
+  if (DECL_HAS_DEPENDENT_EXPLICIT_SPEC_P (t))
+    {
+      tree spec = lookup_explicit_specifier (t);
+      spec = tsubst_copy_and_build (spec, args, complain, in_decl,
+				    /*function_p=*/false,
+				    /*i_c_e_p=*/true);
+      spec = build_explicit_specifier (spec, complain);
+      DECL_NONCONVERTING_P (r) = (spec == boolean_true_node);
+    }
+
   /* OpenMP UDRs have the only argument a reference to the declared
      type.  We want to diagnose if the declared type is a reference,
      which is invalid, but as references to references are usually
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 491e581..cab5927 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,20 @@
+2018-10-30  Marek Polacek  <polacek@redhat.com>
+
+	Implement P0892R2, explicit(bool).
+	* g++.dg/cpp2a/explicit1.C: New test.
+	* g++.dg/cpp2a/explicit10.C: New test.
+	* g++.dg/cpp2a/explicit11.C: New test.
+	* g++.dg/cpp2a/explicit12.C: New test.
+	* g++.dg/cpp2a/explicit13.C: New test.
+	* g++.dg/cpp2a/explicit2.C: New test.
+	* g++.dg/cpp2a/explicit3.C: New test.
+	* g++.dg/cpp2a/explicit4.C: New test.
+	* g++.dg/cpp2a/explicit5.C: New test.
+	* g++.dg/cpp2a/explicit6.C: New test.
+	* g++.dg/cpp2a/explicit7.C: New test.
+	* g++.dg/cpp2a/explicit8.C: New test.
+	* g++.dg/cpp2a/explicit9.C: New test.
+
 2018-10-30  Segher Boessenkool  <segher@kernel.crashing.org>
 
 	PR rtl-optimization/87708
diff --git a/gcc/testsuite/g++.dg/cpp2a/explicit1.C b/gcc/testsuite/g++.dg/cpp2a/explicit1.C
new file mode 100644
index 0000000..b39f90f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/explicit1.C
@@ -0,0 +1,63 @@
+// P0892R2
+// { dg-do compile }
+// { dg-options "-std=c++2a" }
+
+constexpr int fn0 () { return 0; }
+constexpr int fn1 () { return 1; }
+
+struct S {
+  explicit(true) S(int);
+  explicit(1 == 0) S(int, int);
+  explicit(fn0()) S(int, int, int);
+  explicit(fn1()) S(int, int, int, int);
+};
+
+struct X {
+  static const bool value = true;
+  static constexpr bool foo () { return 1; }
+};
+
+struct T {
+  explicit(true ? 1 : throw 1) T(int);
+  explicit(true || true ? 1 : throw 1) T(int, int);
+  explicit(X::value) T(int, int, int);
+  explicit(X::foo ()) T(int, int, int, int);
+};
+
+struct W {
+  constexpr operator bool() { return true; };
+};
+
+struct W2 {
+  constexpr operator bool() { return false; };
+};
+
+struct U {
+  explicit(W()) U(int);
+  explicit(W2()) U(int, int);
+};
+
+int
+main ()
+{
+  S s1 = { 1 }; // { dg-error "converting" }
+  S s1x{ 1 };
+  S s2 = { 2, 3 };
+  S s3 = { 4, 5, 6 };
+  S s4 = { 7, 8, 9, 10 }; // { dg-error "converting" }
+  S s4x{ 7, 8, 9, 10 };
+
+  T t1 = { 1 }; // { dg-error "converting" }
+  T t2 = { 1, 2 }; // { dg-error "converting" }
+  T t3 = { 1, 2, 3 }; // { dg-error "converting" }
+  T t4 = { 1, 2, 3, 4 }; // { dg-error "converting" }
+  T t5{ 1 };
+  T t6{ 1, 2 };
+  T t7{ 1, 2, 3 };
+  T t8{ 1, 2, 3, 4 };
+
+  U u1 = { 1 }; // { dg-error "converting" }
+  U u2{ 1 };
+  U u3 = { 1, 2 };
+  U u4 { 1, 2 };
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/explicit10.C b/gcc/testsuite/g++.dg/cpp2a/explicit10.C
new file mode 100644
index 0000000..c870155
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/explicit10.C
@@ -0,0 +1,32 @@
+// P0892R2
+// { dg-do compile }
+// { dg-options "-std=c++2a" }
+
+#include <type_traits>
+
+class A {};
+class B : public A {};
+class C {};
+class D { public: operator C() { return c; }  C c; };
+
+template <typename T1, typename T2>
+struct S {
+  explicit(!std::is_convertible_v<T1, T2>)
+  S(T1, T2) { }
+};
+
+void
+foo ()
+{
+  A a;
+  B b;
+  C c;
+  D d;
+
+  S<int, int> s{ 1, 2 };
+  S<int, int> s2 = { 1, 2 };
+  S<B*, A*> s3 = { &b, &a };
+  S<A*, B*> s4 = { &a, &b }; // { dg-error "converting" }
+  S<B*, C*> s5 = { &b, &c }; // { dg-error "converting" }
+  S<D, C> s6 = { d, c };
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/explicit11.C b/gcc/testsuite/g++.dg/cpp2a/explicit11.C
new file mode 100644
index 0000000..ad1bed5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/explicit11.C
@@ -0,0 +1,29 @@
+// P0892R2
+// { dg-do compile }
+// { dg-options "-std=c++2a -pedantic" }
+
+template<typename T>
+struct A {
+  explicit A(const T&, ...) noexcept;
+  A(T&&, ...);
+};
+
+int i;
+A a1 = { i, i }; // { dg-error "deduction|cannot" }
+A a2{ i, i };
+A a3{ 0, i };
+A a4 = { 0, i };
+
+template<typename T> A(const T&, const T&) -> A<T&>;
+template<typename T> explicit A(T&&, T&&) -> A<T>;
+
+A a5 = { 0, 1 }; // { dg-error "deduction|ambiguous" }
+A a6{ 0, 1 };
+
+template<typename T>
+struct B {
+  template<typename U> using TA = T;
+  template<typename U> B(U, TA<U>);
+};
+
+B b{(int *)0, (char *)0};
diff --git a/gcc/testsuite/g++.dg/cpp2a/explicit12.C b/gcc/testsuite/g++.dg/cpp2a/explicit12.C
new file mode 100644
index 0000000..6db3157
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/explicit12.C
@@ -0,0 +1,23 @@
+// P0892R2
+// { dg-do compile }
+// { dg-options "-std=c++2a" }
+
+template<typename> struct A {
+  template<typename T, int N = 0>
+  explicit(N) operator T();
+};
+
+template<typename> struct B {
+  template<typename T, int N = 1>
+  explicit(N) operator T();
+};
+
+void
+bar ()
+{
+  A<int> a;
+  int i = a;
+
+  B<int> b;
+  int j = b; // { dg-error "cannot convert" }
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/explicit13.C b/gcc/testsuite/g++.dg/cpp2a/explicit13.C
new file mode 100644
index 0000000..4747ebd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/explicit13.C
@@ -0,0 +1,35 @@
+// P0892R2
+// { dg-do compile }
+// { dg-options "-std=c++2a" }
+
+template<int M = 0> struct A {
+  template<typename T, int N = 0>
+  explicit(N + M) operator T();
+};
+
+template<int M = 1> struct B {
+  template<typename T, int N = 1>
+  explicit(N * M) operator T();
+};
+
+void
+bar ()
+{
+  A a;
+  int i = a;
+
+  A<0> a0;
+  int i0 = a0;
+
+  A<1> a1;
+  int i1 = a1; // { dg-error "cannot convert" }
+
+  B b;
+  int j = b; // { dg-error "cannot convert" }
+
+  B<0> b0;
+  int j0 = b0;
+
+  B<1> b1;
+  int j1 = b1; // { dg-error "cannot convert" }
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/explicit2.C b/gcc/testsuite/g++.dg/cpp2a/explicit2.C
new file mode 100644
index 0000000..7d1748c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/explicit2.C
@@ -0,0 +1,25 @@
+// P0892R2
+// { dg-do compile }
+// { dg-options "-std=c++2a" }
+
+int foo() { return 42; }
+int g;
+
+struct S {
+  explicit(foo()) S(int); // { dg-error "call to" }
+  explicit(int) S(int, int); // { dg-error "expected" }
+  explicit(false ? 1 : throw 1) S(int, int, int); // { dg-error "not a constant" }
+};
+
+struct S2 {
+  explicit(true) S2();
+  explicit(false) S2(); // { dg-error "cannot be overloaded" }
+};
+
+int
+main ()
+{
+  S s1 = { 1 };
+  S s2 = { 1, 2 }; // { dg-error "could not convert" }
+  S s3 = { 1, 2, 3 };
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/explicit3.C b/gcc/testsuite/g++.dg/cpp2a/explicit3.C
new file mode 100644
index 0000000..7c495a3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/explicit3.C
@@ -0,0 +1,24 @@
+// P0892R2
+// { dg-do compile }
+// { dg-options "-std=c++2a" }
+
+#include <type_traits>
+
+template <typename T1, typename T2>
+struct pair {
+    template <typename U1=T1, typename U2=T2,
+        std::enable_if_t<
+            std::is_constructible_v<T1, U1> &&
+            std::is_constructible_v<T2, U2>
+        , int> = 0>
+    explicit(!std::is_convertible_v<U1, T1> ||
+        !std::is_convertible_v<U2, T2>)
+    constexpr pair(U1&&, U2&&) { }
+};
+
+void
+foo ()
+{
+  pair<int, int> p{1, 2};
+  pair<int, int> p2 = {1, 2};
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/explicit4.C b/gcc/testsuite/g++.dg/cpp2a/explicit4.C
new file mode 100644
index 0000000..822a1f1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/explicit4.C
@@ -0,0 +1,41 @@
+// P0892R2
+// { dg-do compile }
+// { dg-options "-std=c++2a" }
+
+template<int T = 1>
+struct S {
+  explicit(T) S(int);
+  explicit(!T) S(int, int);
+};
+
+template<typename T, int N>
+struct S2 {
+  explicit(N) S2(T);
+};
+
+template<typename T>
+struct S3 {
+  explicit((T) 1.0) S3(int);
+};
+
+int
+main ()
+{
+  S<> s1 = { 1 }; // { dg-error "converting" }
+  S<true> s2 = { 1 }; // { dg-error "converting" }
+  S<false> s3 = { 1 };
+  S<> s4{ 1 };
+  S<true> s5{ 1 };
+  S<> s6 = { 1, 2 };
+  S<true> s7 = { 1, 2 };
+  S<false> s8 = { 1, 2 }; // { dg-error "converting" }
+  S<false> s9{ 1, 2 };
+
+  const int x = 1;
+  S<x> s10 = { 1 }; // { dg-error "converting" }
+  S<x> s11{ 2 };
+
+  S2<int, true> s12 = { 1 }; // { dg-error "converting" }
+
+  S3<int> s13 = { 1 }; // { dg-error "converting" }
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/explicit5.C b/gcc/testsuite/g++.dg/cpp2a/explicit5.C
new file mode 100644
index 0000000..70a106f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/explicit5.C
@@ -0,0 +1,71 @@
+// P0892R2
+// { dg-do compile }
+// { dg-options "-std=c++2a" }
+
+constexpr int fn0 () { return 0; }
+constexpr int fn1 () { return 1; }
+
+struct S0 {
+  explicit(false) operator int();
+  explicit(1 == 0) operator double();
+  explicit(fn0()) operator char();
+};
+
+struct S1 {
+  explicit(true) operator int();
+  explicit(1 == 1) operator double();
+  explicit(fn1()) operator char();
+};
+
+struct X {
+  static const bool value = true;
+  static constexpr bool foo () { return 1; }
+};
+
+struct T {
+  explicit(true ? 1 : throw 1) operator int();
+  explicit(true || true ? 1 : throw 1) operator double();
+  explicit(X::value) operator char();
+  explicit(X::foo ()) operator long();
+};
+
+struct W {
+  constexpr operator bool() { return true; };
+};
+
+struct W2 {
+  constexpr operator bool() { return false; };
+};
+
+struct U1 {
+  explicit(W()) operator int();
+};
+
+struct U2 {
+  explicit(W2()) operator int();
+};
+
+int
+main ()
+{
+  S0 s0;
+  S1 s1;
+  int i0 = s0;
+  int i1 = s1; // { dg-error "cannot convert" }
+  double d0 = s0;
+  double d1 = s1; // { dg-error "cannot convert" }
+  char c0 = s0;
+  char c1 = s1; // { dg-error "cannot convert" }
+
+  T t;
+  int i2 = t; // { dg-error "cannot convert" }
+  double d2 = t; // { dg-error "cannot convert" }
+  char c2 = t; // { dg-error "cannot convert" }
+  long l1 = t; // { dg-error "cannot convert" }
+
+  U1 u1;
+  int i3 = u1; // { dg-error "cannot convert" }
+
+  U2 u2;
+  int i4 = u2;
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/explicit6.C b/gcc/testsuite/g++.dg/cpp2a/explicit6.C
new file mode 100644
index 0000000..1013468
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/explicit6.C
@@ -0,0 +1,41 @@
+// P0892R2
+// { dg-do compile }
+// { dg-options "-std=c++2a" }
+
+template<int T = 1>
+struct S {
+  explicit(T) operator int();
+};
+
+template<typename T, int N>
+struct R {
+  explicit(N) operator T();
+};
+
+template<typename T>
+struct U {
+  explicit((T) 1.0) operator T();
+};
+
+int
+main ()
+{
+  S s;
+  int i1 = s; // { dg-error "cannot convert" }
+  S<true> s2;
+  int i2 = s2; // { dg-error "cannot convert" }
+  S<false> s3;
+  int i3 = s3;
+  int i4{s};
+  int i5{s2};
+  int i6{s3};
+
+  R<int, true> r;
+  int i7 = r; // { dg-error "cannot convert" }
+  R<int, false> r2;
+  int i8 = r2;
+
+  U<int> u;
+  int i9 = u; // { dg-error "cannot convert" }
+  int i10{u};
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/explicit7.C b/gcc/testsuite/g++.dg/cpp2a/explicit7.C
new file mode 100644
index 0000000..dfa4e13
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/explicit7.C
@@ -0,0 +1,22 @@
+// P0892R2
+// { dg-do compile }
+// { dg-options "-std=c++2a" }
+
+template<typename T>
+struct B {
+  static const T value = true;
+};
+
+struct X {
+  template<typename T>
+  explicit(B<T>::value) operator T();
+};
+
+int
+main ()
+{
+  X x;
+  int i = x.operator int();
+  int i3 = x; // { dg-error "cannot convert" }
+  int i2{x};
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/explicit8.C b/gcc/testsuite/g++.dg/cpp2a/explicit8.C
new file mode 100644
index 0000000..bf2f9ed
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/explicit8.C
@@ -0,0 +1,24 @@
+// P0892R2
+// { dg-do compile }
+// { dg-options "-std=c++2a" }
+
+struct X {
+  template<typename T, int N = 1>
+  explicit(N) operator T();
+};
+
+int
+main ()
+{
+  X x;
+  int i = x; // { dg-error "cannot convert" }
+  int i2{x};
+  double d = x; // { dg-error "cannot convert" }
+  double d2{x};
+  char c = x; // { dg-error "cannot convert" }
+  char c2{x};
+  long l = x; // { dg-error "cannot convert" }
+  long l2{x};
+  int *p = x; // { dg-error "cannot convert" }
+  int *p2{x};
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/explicit9.C b/gcc/testsuite/g++.dg/cpp2a/explicit9.C
new file mode 100644
index 0000000..6568e5c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/explicit9.C
@@ -0,0 +1,22 @@
+// P0892R2
+// { dg-do compile }
+// { dg-options "-std=c++2a -fconcepts" }
+
+#include <type_traits>
+
+template <typename T1, typename T2>
+struct pair {
+    template <typename U1=T1, typename U2=T2>
+        requires std::is_constructible_v<T1, U1> &&
+            std::is_constructible_v<T2, U2>
+    explicit(!std::is_convertible_v<U1, T1> ||
+        !std::is_convertible_v<U2, T2>)
+    constexpr pair(U1&&, U2&&) { }
+};
+
+void
+foo ()
+{
+  pair<int, int> p{1, 2};
+  pair<int, int> p2 = {1, 2};
+}