fix Issue 13899 - Unwanted warning for pure opApply

Add explicit cast(void) to discard return value of opApply.

src/statement.c

@@ -2501,6 +2501,7 @@ Statement *ForeachStatement::semantic(Scope *sc)
             if (!cases->dim)
             {
                 // Easy case, a clean exit from the loop
+                e = new CastExp(loc, e, Type::tvoid);   // Bugzilla 13899
                 s = new ExpStatement(loc, e);
             }
             else

test/compilable/warn3882.d

@@ -8,7 +8,7 @@ TEST_OUTPUT:
 @safe pure nothrow void strictVoidReturn(T)(T x) {}
 @safe pure nothrow void nonstrictVoidReturn(T)(ref T x) {}
 
-void main()
+void test3882()
 {
     int x = 3;
     strictVoidReturn(x);
@@ -64,3 +64,21 @@ void test12909()
     const(int[])[int] makeAA() { return null; }  // to make r-value
     makeAA().rehash();
 }
+
+/******************************************/
+// 13899
+
+const struct Foo13899
+{
+    int opApply(immutable int delegate(in ref int) pure nothrow dg) pure nothrow
+    {
+        return 1;
+    }
+}
+
+void test13899()
+{
+    foreach (x; Foo13899())
+    {
+    }
+}