From fc1752604c5cc740eae4ed6bc98d21a3f2e40fdd Mon Sep 17 00:00:00 2001
From: Walter Bright <walter@walterbright.com>
Date: Fri, 12 Aug 2022 00:18:30 -0700
Subject: [PATCH] fix Issue 20956 - [DIP1000] @safe defeated by closure
 capturing ref parameter

---
 compiler/src/dmd/func.d                    | 11 +++++++++++
 compiler/test/fail_compilation/test15544.d |  7 ++++---
 compiler/test/fail_compilation/test20956.d | 19 +++++++++++++++++++
 3 files changed, 34 insertions(+), 3 deletions(-)
 create mode 100644 compiler/test/fail_compilation/test20956.d

diff --git a/compiler/src/dmd/func.d b/compiler/src/dmd/func.d
index 4c0947415cd1..27f4d60d8b64 100644
--- a/compiler/src/dmd/func.d
+++ b/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;
         }
diff --git a/compiler/test/fail_compilation/test15544.d b/compiler/test/fail_compilation/test15544.d
index 91ac675c9595..613783a5528f 100644
--- a/compiler/test/fail_compilation/test15544.d
+++ b/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
 ---
 */
 
diff --git a/compiler/test/fail_compilation/test20956.d b/compiler/test/fail_compilation/test20956.d
new file mode 100644
index 000000000000..e6ba41aef0fb
--- /dev/null
+++ b/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);};
+}