don't use memcmp() for small struct equality

Author: WalterBright

src/dmd/opover.d

@@ -1241,7 +1241,7 @@ Expression op_overload(Expression e, Scope* sc)
                     return;
 
                 import dmd.clone : needOpEquals;
-                if (!needOpEquals(sd))
+                if (!needOpEquals(sd) && t1.size() > Type.tvoidptr.size() * 2)
                 {
                     // Use bitwise equality.
                     auto op2 = e.op == TOK.equal ? TOK.identity : TOK.notIdentity;

test/compilable/test16284.d

@@ -0,0 +1,26 @@
+/* TEST_OUTPUT:
+---
+fail_compilation/test16284.d(23): Error: reinterpretation through overlapped field `s` is not allowed in CTFE
+fail_compilation/test16284.d(26):        called from here: `test()`
+fail_compilation/test16284.d(26):        while evaluating: `static assert(test())`
+---
+*/
+
+// https://issues.dlang.org/show_bug.cgi?id=16284
+
+struct S {}
+
+struct T
+{
+    union {int i; S s;}
+    this(uint dummy) { s = S.init; }
+}
+
+bool test()
+{
+    auto t1 = T(0);
+    auto t2 = T(0);
+    return t1 == t2;
+}
+
+static assert(test());

test/fail_compilation/test16284.d

@@ -728,7 +728,7 @@ bool test3789()
         auto ua2 = UnionA([1,2,3]);
         assert(ua1.u.x is ua2.u.x);
         assert(ua1.u.x != ua2.u.x);
-        assert(ua1 == ua2);
+        assert(ua1 != ua2);
         ua1.u.x = 1.0;
         ua2.u.x = 1.0;
         assert(ua1.u.x is ua2.u.x);
@@ -755,7 +755,7 @@ bool test3789()
         ub2.u.a = [1,2,3].dup;
         assert(ub1.u.a !is ub2.u.a);
         assert(ub1.u.a  == ub2.u.a);
-        assert(ub1 != ub2);
+        assert(ub1 == ub2);
         ub2.u.a = ub1.u.a;
         assert(ub1.u.a is ub2.u.a);
         assert(ub1.u.a == ub2.u.a);