fix Issue 20956 - [DIP1000] @safe defeated by closure capturing ref p…

…arameter

compiler/src/dmd/func.d

@@ -2197,6 +2197,17 @@ extern (C++) class FuncDeclaration : Declaration
         }
         else
         {
+            foreach (v; closureVars)
+            {
+                if (v.isRef() &&
+                    (global.params.useDIP1000 == FeatureState.enabled && isSafe()))
+                {
+                    error("cannot close over `ref` variable `%s`", v.toChars());
+                    if (global.gag)     // need not report supplemental errors
+                        return true;
+                }
+            }
+
             printGCUsage(loc, "using closure causes GC allocation");
             return false;
         }

compiler/test/fail_compilation/test15544.d

@@ -2,8 +2,9 @@
 REQUIRED_ARGS: -preview=dip1000
 TEST_OUTPUT:
 ---
-fail_compilation/test15544.d(20): Error: reference to local `this` assigned to non-scope `_del` in @safe code
-fail_compilation/test15544.d(22): Error: reference to local `this` assigned to non-scope `_del` in @safe code
+fail_compilation/test15544.d(21): Error: reference to local `this` assigned to non-scope `_del` in @safe code
+fail_compilation/test15544.d(23): Error: reference to local `this` assigned to non-scope `_del` in @safe code
+fail_compilation/test15544.d(18): Error: function `test15544.S.test` cannot close over `ref` variable `this`
 ---
 */
 
@@ -26,7 +27,7 @@ struct S {
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/test15544.d(46): Error: reference to local `y` assigned to non-scope `dg` in @safe code
+fail_compilation/test15544.d(47): Error: reference to local `y` assigned to non-scope `dg` in @safe code
 ---
 */
 

compiler/test/fail_compilation/test17423.d

@@ -1,8 +1,9 @@
 /* REQUIRED_ARGS: -preview=dip1000
 TEST_OUTPUT:
 ---
-fail_compilation/test17423.d(27): Error: reference to local `this` assigned to non-scope parameter `dlg` calling `opApply`
-fail_compilation/test17423.d(16):        which is not `scope` because of `this.myDlg = dlg`
+fail_compilation/test17423.d(28): Error: reference to local `this` assigned to non-scope parameter `dlg` calling `opApply`
+fail_compilation/test17423.d(17):        which is not `scope` because of `this.myDlg = dlg`
+fail_compilation/test17423.d(27): Error: constructor `test17423.Foo.this` cannot close over `ref` variable `this`
 ---
 */
 

compiler/test/fail_compilation/test20956.d

@@ -0,0 +1,19 @@
+/* REQUIRED_ARGS: -preview=dip1000
+TEST_OUTPUT:
+---
+fail_compilation/test20956.d(105): Error: function `test20956.forwardDg` cannot close over `ref` variable `c`
+---
+ */
+
+// https://issues.dlang.org/show_bug.cgi?id=20956
+
+#line 100
+
+@safe:
+
+alias DG = void delegate() @safe;
+
+DG forwardDg(ref int c)
+{
+    return () {assert(c == 42);};
+}