[vala/0.48] vala: Property must be writable when invoking ReturnsModifiedPointer method

[Rico Tzschichholz <ricotz src gnome org>]

commit 65e8241435fd30f65498c6e7c46b1d0fa8a0d2c2
Author: Rico Tzschichholz <ricotz ubuntu com>
Date:   Sat Jul 4 09:07:06 2020 +0200

    vala: Property must be writable when invoking ReturnsModifiedPointer method
    
    Fixes https://gitlab.gnome.org/GNOME/vala/issues/1023

 tests/Makefile.am                                            |  1 +
 tests/semantic/property-method-returns-modified-pointer.test | 10 ++++++++++
 vala/valamethodcall.vala                                     |  8 +++++++-
 3 files changed, 18 insertions(+), 1 deletion(-)
---
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 2b6154616..5836ed15b 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -900,6 +900,7 @@ TESTS = \
        semantic/property-accessibility.test \
        semantic/property-construct.test \
        semantic/property-initializer-type.test \
+       semantic/property-method-returns-modified-pointer.test \
        semantic/property-override.test \
        semantic/property-override-class.test \
        semantic/property-override-interface.test \
diff --git a/tests/semantic/property-method-returns-modified-pointer.test 
b/tests/semantic/property-method-returns-modified-pointer.test
new file mode 100644
index 000000000..ae8ebbe6f
--- /dev/null
+++ b/tests/semantic/property-method-returns-modified-pointer.test
@@ -0,0 +1,10 @@
+Invalid Code
+
+class Foo {
+       public List<string> bar { get; }
+}
+
+void main () {
+       var foo = new Foo ();
+       foo.bar.append ("foo");
+}
diff --git a/vala/valamethodcall.vala b/vala/valamethodcall.vala
index baf704f45..2416e5b9f 100644
--- a/vala/valamethodcall.vala
+++ b/vala/valamethodcall.vala
@@ -527,7 +527,13 @@ public class Vala.MethodCall : Expression {
                                value_type.floating_reference = true;
                        }
                        if (m.returns_modified_pointer) {
-                               ((MemberAccess) call).inner.lvalue = true;
+                               unowned Expression inner = ((MemberAccess) call).inner;
+                               inner.lvalue = true;
+                               unowned Property? prop = inner.symbol_reference as Property;
+                               if (prop != null && (prop.set_accessor == null || 
!prop.set_accessor.writable)) {
+                                       error = true;
+                                       Report.error (inner.source_reference, "Property `%s' is 
read-only".printf (prop.get_full_name ()));
+                               }
                        }
                        // avoid passing possible null to ref_sink_function without checking
                        if (tree_can_fail && !value_type.nullable && value_type.floating_reference && 
ret_type is ObjectType) {