c++: improve reference binding diagnostic [PR91849]

Here we were complaining about binding the lvalue reference to the rvalue
result of converting from float to int, but didn't mention that conversion.
Talk about the type of the initializer instead.

gcc/cp/ChangeLog:

	PR c++/91849
	* call.c (convert_like_internal): Improve reference diagnostic.

gcc/testsuite/ChangeLog:

	PR c++/91849
	* g++.dg/conversion/pr66211.C: Adjust diagnostic.
	* g++.dg/conversion/ref7.C: New test.

gcc/cp/call.c

@@ -7898,8 +7898,19 @@ convert_like_internal (conversion *convs, tree expr, tree fn, int argnum,
                         "lvalue of type %qI", totype, extype);
 	    else if (!TYPE_REF_IS_RVALUE (ref_type) && !lvalue_p (expr)
 		     && !CP_TYPE_CONST_NON_VOLATILE_P (TREE_TYPE (ref_type)))
+	      {
+		conversion *next = next_conversion (convs);
+		if (next->kind == ck_std)
+		  {
+		    next = next_conversion (next);
+		    error_at (loc, "cannot bind non-const lvalue reference of "
+			      "type %qH to a value of type %qI",
+			      totype, next->type);
+		  }
+		else
 		  error_at (loc, "cannot bind non-const lvalue reference of "
 			    "type %qH to an rvalue of type %qI", totype, extype);
+	      }
 	    else if (!reference_compatible_p (TREE_TYPE (totype), extype))
 	      {
 		/* If we're converting from T[] to T[N], don't talk

gcc/testsuite/g++.dg/conversion/pr66211.C

@@ -7,5 +7,5 @@ int main()
 {
   int x = 0;
   double y = 1;
-  f(1 > 0 ? x : y); // { dg-error "cannot bind .* to an rvalue" }
+  f(1 > 0 ? x : y); // { dg-error "cannot bind non-const lvalue reference of type .int&. to a value of type .double" }
 }

gcc/testsuite/g++.dg/conversion/ref7.C

@@ -0,0 +1,17 @@
+// PR c++/91849
+
+struct A { operator float(); };
+
+void
+g ()
+{
+  float f = 1.f;
+  int &r = f;			// { dg-error "float" }
+  int &r2 = A();		// { dg-error "float" }
+}
+
+void
+g2 ()
+{
+  int &r = 1.f;			// { dg-error "float" }
+}