PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.

This patch implements C++20 P1331, allowing trivial default initialization in
constexpr contexts.

	* c-cppbuiltin.c (c_cpp_builtins): Adjust the value of __cpp_constexpr.

	* class.c (trivial_default_constructor_is_constexpr): Return true in
	C++20.
	* constexpr.c (cx_check_missing_mem_inits): Allow missing field
	initializers in C++20.
	(cxx_eval_call_expression): Don't clear CONSTRUCTOR_NO_CLEARING for
	constexpr constructors in C++20.
	(reduced_constant_expression_p): Don't set FIELD for union and array
	types.  Skip empty class fields without initializers.
	* decl.c (check_for_uninitialized_const_var): Permit trivial default
	initialization in constexpr.
	(next_initializable_field): Don't skip vptr fields.
	* method.c (walk_field_subobs): Still consider a constructor that
	doesn't initialize all the members constexpr.

	* g++.dg/cpp0x/constexpr-array6.C: Adjust dg-error.
	* g++.dg/cpp0x/constexpr-ctor.C: Likewise.
	* g++.dg/cpp0x/constexpr-diag3.C: Likewise.
	* g++.dg/cpp0x/constexpr-diag4.C: Likewise.
	* g++.dg/cpp0x/constexpr-ex3.C: Likewise.
	* g++.dg/cpp0x/constexpr-template2.C: Likewise.
	* g++.dg/cpp0x/constexpr-union2.C: Likewise.
	* g++.dg/cpp0x/lambda/lambda-mangle.C: Rip out a piece of code ...
	* g++.dg/cpp0x/lambda/lambda-mangle6.C: ... and put it here.
	* g++.dg/cpp0x/pr79118.C: Adjust dg-error.
	* g++.dg/cpp1y/constexpr-83921-3.C: Likewise.
	* g++.dg/cpp1y/constexpr-neg1.C: Likewise.
	* g++.dg/cpp1z/constexpr-lambda12.C: Likewise.
	* g++.dg/cpp1z/feat-cxx1z.C: Use -std=c++17.
	* g++.dg/cpp2a/constexpr-init1.C: New test.
	* g++.dg/cpp2a/constexpr-init2.C: New test.
	* g++.dg/cpp2a/constexpr-init3.C: New test.
	* g++.dg/cpp2a/constexpr-init4.C: New test.
	* g++.dg/cpp2a/constexpr-init5.C: New test.
	* g++.dg/cpp2a/constexpr-init6.C: New test.
	* g++.dg/cpp2a/constexpr-init7.C: New test.
	* g++.dg/cpp2a/constexpr-init8.C: New test.
	* g++.dg/cpp2a/constexpr-init9.C: New test.
	* g++.dg/cpp2a/constexpr-init10.C: New test.
	* g++.dg/cpp2a/constexpr-init11.C: New test.
	* g++.dg/cpp2a/constexpr-init12.C: New test.
	* g++.dg/cpp2a/constexpr-init13.C: New test.
	* g++.dg/cpp2a/constexpr-init14.C: New test.
	* g++.dg/cpp2a/constexpr-init15.C: New test.
	* g++.dg/cpp2a/constexpr-try5.C: Adjust dg-error.
	* g++.dg/cpp2a/feat-cxx2a.C: Test __cpp_constexpr.
	* g++.dg/cpp2a/lambda-mangle.C: New test.
	* g++.dg/debug/dwarf2/pr44641.C: Skip for c++2a.
	* g++.dg/ext/stmtexpr21.C: Adjust dg-error.

Co-Authored-By: Jakub Jelinek <jakub@redhat.com>

From-SVN: r279019

5 files changed, 80 insertions, 19 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 4e88ed8..b5c2d1b 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,4 +1,22 @@
 2019-12-05  Marek Polacek  <polacek@redhat.com>
+	    Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.
+	* class.c (trivial_default_constructor_is_constexpr): Return true in
+	C++20.
+	* constexpr.c (cx_check_missing_mem_inits): Allow missing field
+	initializers in C++20.
+	(cxx_eval_call_expression): Don't clear CONSTRUCTOR_NO_CLEARING for
+	constexpr constructors in C++20.
+	(reduced_constant_expression_p): Don't set FIELD for union and array
+	types.  Skip empty class fields without initializers.
+	* decl.c (check_for_uninitialized_const_var): Permit trivial default
+	initialization in constexpr.
+	(next_initializable_field): Don't skip vptr fields.
+	* method.c (walk_field_subobs): Still consider a constructor that
+	doesn't initialize all the members constexpr.
+
+2019-12-05  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/92271 - make __is_same alias for __is_same_as.
 	* cxx-pretty-print.c (pp_cxx_trait_expression) <case CPTK_IS_SAME_AS>:
diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index f36f75f..d8bb449 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -5288,8 +5288,14 @@ trivial_default_constructor_is_constexpr (tree t)
   /* A defaulted trivial default constructor is constexpr
      if there is nothing to initialize.  */
   gcc_assert (!TYPE_HAS_COMPLEX_DFLT (t));
-  /* A class with a vptr doesn't have a trivial default ctor.  */
-  return is_really_empty_class (t, /*ignore_vptr*/true);
+  /* A class with a vptr doesn't have a trivial default ctor.
+     In C++20, a class can have transient uninitialized members, e.g.:
+
+       struct S { int i; constexpr S() = default; };
+
+     should work.  */
+  return (cxx_dialect >= cxx2a
+	  || is_really_empty_class (t, /*ignore_vptr*/true));
 }
 
 /* Returns true iff class T has a constexpr default constructor.  */
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 3706334..19e09c7 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -767,6 +767,10 @@ massage_constexpr_body (tree fun, tree body)
 static bool
 cx_check_missing_mem_inits (tree ctype, tree body, bool complain)
 {
+  /* We allow uninitialized bases/fields in C++20.  */
+  if (cxx_dialect >= cxx2a)
+    return false;
+
   unsigned nelts = 0;
   
   if (body)
@@ -815,7 +819,7 @@ cx_check_missing_mem_inits (tree ctype, tree body, bool complain)
 	    continue;
 	  if (ANON_AGGR_TYPE_P (TREE_TYPE (field)))
 	    {
-	      /* Recurse to check the anonummous aggregate member.  */
+	      /* Recurse to check the anonymous aggregate member.  */
 	      bad |= cx_check_missing_mem_inits
 		(TREE_TYPE (field), NULL_TREE, complain);
 	      if (bad && !complain)
@@ -2179,15 +2183,26 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 	entry->result = result;
     }
 
-  /* The result of a constexpr function must be completely initialized.  */
-  if (TREE_CODE (result) == CONSTRUCTOR)
+  /* The result of a constexpr function must be completely initialized.
+
+     However, in C++20, a constexpr constructor doesn't necessarily have
+     to initialize all the fields, so we don't clear CONSTRUCTOR_NO_CLEARING
+     in order to detect reading an unitialized object in constexpr instead
+     of value-initializing it.  (reduced_constant_expression_p is expected to
+     take care of clearing the flag.)  */
+  if (TREE_CODE (result) == CONSTRUCTOR
+      && (cxx_dialect < cxx2a
+	  || !DECL_CONSTRUCTOR_P (fun)))
     clear_no_implicit_zero (result);
 
   pop_cx_call_context ();
   return result;
 }
 
-/* FIXME speed this up, it's taking 16% of compile time on sieve testcase.  */
+/* Return true if T is a valid constant initializer.  If a CONSTRUCTOR
+   initializes all the members, the CONSTRUCTOR_NO_CLEARING flag will be
+   cleared.
+   FIXME speed this up, it's taking 16% of compile time on sieve testcase.  */
 
 bool
 reduced_constant_expression_p (tree t)
@@ -2209,6 +2224,12 @@ reduced_constant_expression_p (tree t)
 	  if (TREE_CODE (TREE_TYPE (t)) == VECTOR_TYPE)
 	    /* An initialized vector would have a VECTOR_CST.  */
 	    return false;
+	  else if (cxx_dialect >= cxx2a
+		   /* An ARRAY_TYPE doesn't have any TYPE_FIELDS.  */
+		   && (TREE_CODE (TREE_TYPE (t)) == ARRAY_TYPE
+		       /* A union only initializes one member.  */
+		       || TREE_CODE (TREE_TYPE (t)) == UNION_TYPE))
+	    field = NULL_TREE;
 	  else
 	    field = next_initializable_field (TYPE_FIELDS (TREE_TYPE (t)));
 	}
@@ -2222,14 +2243,20 @@ reduced_constant_expression_p (tree t)
 	    return false;
 	  if (field)
 	    {
-	      if (idx != field)
-		return false;
+	      /* Empty class field may or may not have an initializer.  */
+	      for (; idx != field;
+		   field = next_initializable_field (DECL_CHAIN (field)))
+		if (!is_really_empty_class (TREE_TYPE (field),
+					    /*ignore_vptr*/false))
+		  return false;
 	      field = next_initializable_field (DECL_CHAIN (field));
 	    }
 	}
-      if (field)
-	return false;
-      else if (CONSTRUCTOR_NO_CLEARING (t))
+      /* There could be a non-empty field at the end.  */
+      for (; field; field = next_initializable_field (DECL_CHAIN (field)))
+	if (!is_really_empty_class (TREE_TYPE (field), /*ignore_vptr*/false))
+	  return false;
+      if (CONSTRUCTOR_NO_CLEARING (t))
 	/* All the fields are initialized.  */
 	CONSTRUCTOR_NO_CLEARING (t) = false;
       return true;
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 7897327..a44f172 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -5858,8 +5858,12 @@ check_for_uninitialized_const_var (tree decl, bool constexpr_context_p,
      7.1.6 */
   if (VAR_P (decl)
       && !TYPE_REF_P (type)
-      && (constexpr_context_p
-	  || CP_TYPE_CONST_P (type) || var_in_constexpr_fn (decl))
+      && (CP_TYPE_CONST_P (type)
+	  /* C++20 permits trivial default initialization in constexpr
+	     context (P1331R2).  */
+	  || (cxx_dialect < cxx2a
+	      && (constexpr_context_p
+		  || var_in_constexpr_fn (decl))))
       && !DECL_NONTRIVIALLY_INITIALIZED_P (decl))
     {
       tree field = default_init_uninitialized_part (type);
@@ -5868,7 +5872,7 @@ check_for_uninitialized_const_var (tree decl, bool constexpr_context_p,
 
       bool show_notes = true;
 
-      if (!constexpr_context_p)
+      if (!constexpr_context_p || cxx_dialect >= cxx2a)
 	{
 	  if (CP_TYPE_CONST_P (type))
 	    {
@@ -5938,7 +5942,11 @@ next_initializable_field (tree field)
 	 && (TREE_CODE (field) != FIELD_DECL
 	     || DECL_UNNAMED_BIT_FIELD (field)
 	     || (DECL_ARTIFICIAL (field)
-		 && !(cxx_dialect >= cxx17 && DECL_FIELD_IS_BASE (field)))))
+		 /* In C++17, don't skip base class fields.  */
+		 && !(cxx_dialect >= cxx17 && DECL_FIELD_IS_BASE (field))
+		 /* Don't skip vptr fields.  We might see them when we're
+		    called from reduced_constant_expression_p.  */
+		 && !DECL_VIRTUAL_P (field))))
     field = DECL_CHAIN (field);
 
   return field;
diff --git a/gcc/cp/method.c b/gcc/cp/method.c
index a707940..d2aed47 100644
--- a/gcc/cp/method.c
+++ b/gcc/cp/method.c
@@ -1985,10 +1985,12 @@ walk_field_subobs (tree fields, special_function_kind sfk, tree fnname,
 	  if (bad && deleted_p)
 	    *deleted_p = true;
 
-	  /* For an implicitly-defined default constructor to be constexpr,
-	     every member must have a user-provided default constructor or
-	     an explicit initializer.  */
-	  if (constexpr_p && !CLASS_TYPE_P (mem_type)
+	  /* Before C++20, for an implicitly-defined default constructor to
+	     be constexpr, every member must have a user-provided default
+	     constructor or an explicit initializer.  */
+	  if (constexpr_p
+	      && cxx_dialect < cxx2a
+	      && !CLASS_TYPE_P (mem_type)
 	      && TREE_CODE (DECL_CONTEXT (field)) != UNION_TYPE)
 	    {
 	      *constexpr_p = false;