Deprecate typesafe variadic class arguments (#20673)

changelog/dmd.deprecation-typesafe-variadic-class.dd

@@ -0,0 +1,40 @@
+Typesafe variadic class parameters have been deprecated
+
+This obscure feature allowed a limited form of implicit construction:
+
+---
+void check(bool x, Exception e...)
+{
+    if (!x)
+        throw e;
+}
+
+void main(string[] args)
+{
+    check(args.length > 1, "missing argument");
+}
+---
+
+However, few uses of this feature have been found, and one project was actually mistakenly using it instead of the more common Typesafe variadic array parameter.
+Considering D doesn't support implicit construction and already has a confusing amount of different variadic parameter forms, it was decided to remove this feature.
+
+As a corrective action, either call the constructor in the callee:
+
+---
+void check(string msg)
+{
+    if (!x)
+        throw new Exception(msg);
+}
+---
+
+Or let the caller construct the class instance:
+
+---
+void check(bool x, Exception e);
+
+void main(string[] args)
+{
+    check(args.length > 1, new Exception("missing argument"));
+}
+---

compiler/src/dmd/typesem.d

@@ -2493,10 +2493,17 @@ Type typeSemantic(Type type, Loc loc, Scope* sc)
                     errors = true;
                 }
 
-                const bool isTypesafeVariadic = i + 1 == dim &&
-                                                tf.parameterList.varargs == VarArg.typesafe &&
-                                                (t.isTypeDArray() || t.isTypeClass());
-                if (isTypesafeVariadic)
+                const bool isTypesafeVariadic = i + 1 == dim && tf.parameterList.varargs == VarArg.typesafe;
+                const bool isStackAllocatedVariadic = isTypesafeVariadic && (t.isTypeDArray() || t.isTypeClass());
+
+                if (isTypesafeVariadic && t.isTypeClass())
+                {
+                    // Deprecated in 2.111, kept as a legacy feature for compatibility (currently no plan to turn it into an error)
+                    .deprecation(loc, "typesafe variadic parameters with a `class` type (`%s %s...`) are deprecated",
+                        t.isTypeClass().sym.ident.toChars(), fparam.toChars());
+                }
+
+                if (isStackAllocatedVariadic)
                 {
                     /* typesafe variadic arguments are constructed on the stack, so must be `scope`
                      */
@@ -2518,7 +2525,7 @@ Type typeSemantic(Type type, Loc loc, Scope* sc)
                         }
                     }
 
-                    if (isTypesafeVariadic)
+                    if (isStackAllocatedVariadic)
                     {
                         /* This is because they can be constructed on the stack
                          * https://dlang.org/spec/function.html#typesafe_variadic_functions

compiler/test/fail_compilation/test22023.d

@@ -1,7 +1,6 @@
 /* TEST_OUTPUT:
 ---
 fail_compilation/test22023.d(102): Error: typesafe variadic function parameter `a` of type `int[]` cannot be marked `return`
-fail_compilation/test22023.d(107): Error: typesafe variadic function parameter `c` of type `test22023.C` cannot be marked `return`
 ---
 */
 
@@ -14,13 +13,3 @@ ref int f(return int[] a ...)
 {
     return a[2];
 }
-
-ref int g(return C c ...)
-{
-    return c.x;
-}
-
-class C
-{
-    int x;
-}

compiler/test/fail_compilation/test22818.d

@@ -1,6 +1,7 @@
 /* REQUIRED_ARGS: -preview=dip1000
  * TEST_OUTPUT:
 ---
+fail_compilation/test22818.d(102): Deprecation: typesafe variadic parameters with a `class` type (`C c...`) are deprecated
 fail_compilation/test22818.d(104): Error: scope parameter `c` may not be returned
 ---
 */

compiler/test/runnable/testv.d

@@ -83,39 +83,6 @@ void test3()
     printf("%d\n", i);
 }
 
-/*********************************************************/
-
-class Foo4
-{
-    int a;
-    float f;
-    double d;
-
-    this(int a, float f, double d)
-    {
-        this.a = a;
-        this.f = f;
-        this.d = d;
-    }
-}
-
-int sum4(Foo4 f ...)
-{
-    return cast(int)(f.a + f.f + f.d);
-}
-
-void test4()
-{
-    int i;
-    Foo4 foo = new Foo4(1, 2f, 3.0);
-
-    i = sum4(foo);
-    assert(i == 1+2+3);
-    i = sum4(4, 5f, 6.0);
-    assert(i == 4+5+6);
-
-    printf("%d\n", i);
-}
 
 /*********************************************************/
 
@@ -139,7 +106,6 @@ int main()
     test1();
     test2();
     test3();
-    test4();
     test5();
 
     printf("Success\n");