[vala/staging: 2/2] vala: Non-nullable value-type in coalesce expression needs to be copied

[Rico Tzschichholz <ricotz src gnome org>]

commit 6cfa112524d13f878d02149475079e34cd366116
Author: Jeremy Philippe <jeremy philippe gmail com>
Date:   Wed Jan 8 13:20:31 2020 +0100

    vala: Non-nullable value-type in coalesce expression needs to be copied
    
    The code generated by the coalesce expression could lead to stale
    pointers to the stack if the right-side expression is an immediate value
    (such as an integer literal or a struct).
    
    Fixes https://gitlab.gnome.org/GNOME/vala/issues/893

 tests/Makefile.am                            |  1 +
 tests/control-flow/coalesce-right-value.vala | 17 +++++++++++++++++
 vala/valabinaryexpression.vala               |  6 ++++++
 3 files changed, 24 insertions(+)
---
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 2c66135bf..314b01439 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -166,6 +166,7 @@ TESTS = \
        control-flow/break.vala \
        control-flow/break-invalid.test \
        control-flow/coalesce-reference-transfer.vala \
+       control-flow/coalesce-right-value.vala \
        control-flow/continue-invalid.test \
        control-flow/double-catch.test \
        control-flow/expressions-conditional.vala \
diff --git a/tests/control-flow/coalesce-right-value.vala b/tests/control-flow/coalesce-right-value.vala
new file mode 100644
index 000000000..ca64126d7
--- /dev/null
+++ b/tests/control-flow/coalesce-right-value.vala
@@ -0,0 +1,17 @@
+struct Foo {
+       public int i;
+}
+
+void main () {
+       {
+               int? null_int = null;
+               int i = null_int ?? 42;
+               assert (i == 42);
+       }
+       {
+               Foo? null_foo = null;
+               Foo right_foo = { 42 };
+               Foo foo = null_foo ?? right_foo;
+               assert (foo.i == 42);
+       }
+}
diff --git a/vala/valabinaryexpression.vala b/vala/valabinaryexpression.vala
index fe559e1d5..927ca5413 100644
--- a/vala/valabinaryexpression.vala
+++ b/vala/valabinaryexpression.vala
@@ -234,6 +234,12 @@ public class Vala.BinaryExpression : Expression {
                                local_type = right.value_type.copy ();
                        }
 
+                       if (local_type != null && right.value_type is ValueType && 
!right.value_type.nullable) {
+                               // immediate values in the right expression must always be boxed,
+                               // otherwise the local variable may receive a stale pointer to the stack
+                               local_type.value_owned = true;
+                       }
+
                        var local = new LocalVariable (local_type, get_temp_name (), left, source_reference);
                        var decl = new DeclarationStatement (local, source_reference);