[C++] Implement D1957R0, T* to bool should be considered narrowing.

[Jason Merrill]

This paper was delayed until the February meeting in Prague so that we could
get a better idea of what the impact on existing code would actually be.  To
that end, I'm implementing it now.

Tested x86_64-pc-linux-gnu, applying to trunk.

	* typeck2.c (check_narrowing): Treat pointer->bool as a narrowing
	conversion with -std=c++2a.
---
 gcc/cp/typeck2.c                        |  5 +++
 gcc/testsuite/g++.dg/cpp0x/initlist92.C | 51 +++++++------------------
 2 files changed, 18 insertions(+), 38 deletions(-)

diff --git a/gcc/cp/typeck2.c b/gcc/cp/typeck2.c
index 7884d423a59..9fb36fd1ed3 100644
--- a/gcc/cp/typeck2.c
+++ b/gcc/cp/typeck2.c
@@ -1018,6 +1018,11 @@ check_narrowing (tree type, tree init, tsubst_flags_t complain,
 	    ok = true;
 	}
     }
+  else if (TREE_CODE (type) == BOOLEAN_TYPE
+	   && (TYPE_PTR_P (ftype) || TYPE_PTRMEM_P (ftype)))
+    /* This hasn't actually made it into C++20 yet, but let's add it now to get
+       an idea of the impact.  */
+    ok = (cxx_dialect < cxx2a);
 
   bool almost_ok = ok;
   if (!ok && !CONSTANT_CLASS_P (init) && (complain & tf_warning_or_error))
diff --git a/gcc/testsuite/g++.dg/cpp0x/initlist92.C b/gcc/testsuite/g++.dg/cpp0x/initlist92.C
index 81a63182f0e..319264ae274 100644
--- a/gcc/testsuite/g++.dg/cpp0x/initlist92.C
+++ b/gcc/testsuite/g++.dg/cpp0x/initlist92.C
@@ -1,26 +1,13 @@
 // PR c++/64665, DR 1467 
-// { dg-do run { target c++11 } }
+// { dg-do compile { target c++11 } }
 
 #include <string>
-#include <cassert>
 
-bool Test1(bool) 
-{
-  return true;
-}
-bool Test1(std::string)
-{
-  return false;
-}
+bool Test1(bool);
+bool Test1(std::string) = delete;
 
-bool Test2(int)
-{
-  return false;
-}
-bool Test2(std::initializer_list<int>)
-{
-  return true;
-}
+bool Test2(int) = delete;
+bool Test2(std::initializer_list<int>);
 
 struct S 
 { 
@@ -28,28 +15,16 @@ struct S
 private:
     int a;
 };
-bool Test3(int)
-{
-  return true;
-}
-bool Test3(S)
-{
-  return false;
-}
+bool Test3(int);
+bool Test3(S) = delete;
 
-bool Test4(bool) 
-{
-  return false;
-}
-bool Test4(std::initializer_list<std::string>)
-{
-  return true;
-}
+bool Test4(bool) = delete;
+bool Test4(std::initializer_list<std::string>);
 
 int main () 
 {
-  assert ( Test1({"false"}) );
-  assert ( Test2({123}) );
-  assert ( Test3({456}) );
-  assert ( Test4({"false"}) );
+  ( Test1({"false"}) );	// { dg-error "narrowing" "" { target c++2a } }
+  ( Test2({123}) );
+  ( Test3({456}) );
+  ( Test4({"false"}) );
 }

base-commit: 9b0807d9fe86b69112c4b1b65a923d685640b094
-- 
2.18.1