c++: Detect long double -> double narrowing [PR94590]

This PR points out that we don't detect long double -> double narrowing
when long double happens to have the same precision as double; on x86_64
this can be achieved by -mlong-double-64.

[dcl.init.list]gcc-mirror#7.2 specifically says "from long double to double or float,
or from double to float", but check_narrowing only checks

  TYPE_PRECISION (type) < TYPE_PRECISION (ftype)

so we need to handle the other cases too, e.g. by same_type_p as in
the following patch.

	PR c++/94590 - Detect long double -> double narrowing.
	* typeck2.c (check_narrowing): Detect long double -> double
	narrowing even when double and long double have the same
	precision.  Make it handle conversions to float too.

	* g++.dg/cpp0x/Wnarrowing18.C: New test.

Author: mpolacek

gcc/cp/ChangeLog

@@ -1,3 +1,10 @@
+2020-05-07  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94590 - Detect long double -> double narrowing.
+	* typeck2.c (check_narrowing): Detect long double -> double
+	narrowing even when double and long double have the same
+	precision.  Make it handle conversions to float too.
+
 2020-05-07  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/94255

gcc/cp/typeck2.c

@@ -1008,10 +1008,17 @@ check_narrowing (tree type, tree init, tsubst_flags_t complain,
 	      || !int_fits_type_p (init, type)))
 	ok = false;
     }
+  /* [dcl.init.list]#7.2: "from long double to double or float, or from
+      double to float".  */
   else if (TREE_CODE (ftype) == REAL_TYPE
 	   && TREE_CODE (type) == REAL_TYPE)
     {
-      if (TYPE_PRECISION (type) < TYPE_PRECISION (ftype))
+      if ((same_type_p (ftype, long_double_type_node)
+	   && (same_type_p (type, double_type_node)
+	       || same_type_p (type, float_type_node)))
+	  || (same_type_p (ftype, double_type_node)
+	      && same_type_p (type, float_type_node))
+	  || (TYPE_PRECISION (type) < TYPE_PRECISION (ftype)))
 	{
 	  if (TREE_CODE (init) == REAL_CST)
 	    {

gcc/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-05-07  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94590 - Detect long double -> double narrowing.
+	* g++.dg/cpp0x/Wnarrowing18.C: New test.
+
 2020-05-07  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/94255

gcc/testsuite/g++.dg/cpp0x/Wnarrowing18.C

@@ -0,0 +1,15 @@
+// PR c++/94590 - Detect long double -> double narrowing.
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-mlong-double-64" { target x86_64-*-* i?86-*-* } }
+
+int
+main ()
+{
+  using T = long double;
+  extern long double ld;
+  extern T ld2;
+  extern const T ld3;
+  double d{ld}; // { dg-error "narrowing conversion" }
+  double d2{ld2}; // { dg-error "narrowing conversion" }
+  double d3{ld3}; // { dg-error "narrowing conversion" }
+}