compiler: check for nil receiver in value method

    
    We already dereference the pointer to copy the value, but if the
    method does not use the value then the pointer dereference may be
    optimized away.  Do an explicit nil check so that we get the panic
    that is required.
    
    Fixes golang/go#19806
    
    Reviewed-on: https://go-review.googlesource.com/91275

	* go.go-torture/execute/printnil.go: New test.

From-SVN: r257280

gcc/go/gofrontend/MERGE

-65eaa9003db4effc9c5ffe9c955e9534ba5d7d15
+71758f9ca1804743afe178f0e2fca489e0217474
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/gogo.cc

@@ -5610,7 +5610,7 @@ Function::build(Gogo* gogo, Named_object* named_function)
                   Expression::make_var_reference(parm_no, loc);
               parm_ref =
                   Expression::make_dereference(parm_ref,
-                                               Expression::NIL_CHECK_DEFAULT,
+                                               Expression::NIL_CHECK_NEEDED,
                                                loc);
 	      if ((*p)->var_value()->is_in_heap())
 		parm_ref = Expression::make_heap_expression(parm_ref, loc);

gcc/testsuite/go.go-torture/execute/printnil.go

+// printnil checks that fmt correctly handles a nil pointer receiver
+// for a value method at all optimization levels.
+package main
+
+import "fmt"
+
+type MyType struct {
+	val int
+}
+
+func (t MyType) String() string {
+	return "foobar"
+}
+
+func main() {
+	if got := fmt.Sprintf("%s", (*MyType)(nil)); got != "<nil>" {
+		panic(got)
+	}
+}