[vala/0.34] vala: Don't transform an explicit "null" into a valid format-string

[Rico Tzschichholz <ricotz src gnome org>]

commit b75538ab4e1ab68b87f1d0bb3827c984adff6bd2
Author: Rico Tzschichholz <ricotz ubuntu com>
Date:   Wed Dec 6 16:11:10 2017 +0100

    vala: Don't transform an explicit "null" into a valid format-string
    
    https://bugzilla.gnome.org/show_bug.cgi?id=791215

 tests/Makefile.am            |    1 +
 tests/methods/bug791215.vala |    8 ++++++++
 vala/valamethodcall.vala     |    4 +++-
 3 files changed, 12 insertions(+), 1 deletions(-)
---
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 8befbd1..5a42d51 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -77,6 +77,7 @@ TESTS = \
        methods/bug737222.vala \
        methods/bug743877.vala \
        methods/bug771964.vala \
+       methods/bug791215.vala \
        methods/generics.vala \
        control-flow/break.vala \
        control-flow/expressions-conditional.vala \
diff --git a/tests/methods/bug791215.vala b/tests/methods/bug791215.vala
new file mode 100644
index 0000000..b5e0c09
--- /dev/null
+++ b/tests/methods/bug791215.vala
@@ -0,0 +1,8 @@
+[PrintfFormat]
+void foo_print (string? fmt, ...) {
+       assert (fmt == null);
+}
+
+void main () {
+       foo_print (null);
+}
diff --git a/vala/valamethodcall.vala b/vala/valamethodcall.vala
index 720b81c..68b689b 100644
--- a/vala/valamethodcall.vala
+++ b/vala/valamethodcall.vala
@@ -420,7 +420,9 @@ public class Vala.MethodCall : Expression {
                // printf arguments
                if (mtype is MethodType && ((MethodType) mtype).method_symbol.printf_format) {
                        StringLiteral format_literal = null;
-                       if (last_arg != null) {
+                       if (last_arg is NullLiteral) {
+                               // do not replace explicit null
+                       } else if (last_arg != null) {
                                // use last argument as format string
                                format_literal = last_arg as StringLiteral;
                                if (format_literal == null && args.size == params.size - 1) {