fix Issue 12495 - CTFE slice cast can cause allocation

src/ctfeexpr.c

@@ -1793,11 +1793,23 @@ Expression *ctfeCast(Loc loc, Type *type, Type *to, Expression *e)
     {
         return paintTypeOntoLiteral(to, e);
     }
+
     Expression *r;
     if (e->type->equals(type) && type->equals(to))
-        r = e;          // necessary not to change e's address for pointer comparisons
+    {
+        // necessary not to change e's address for pointer comparisons
+        r = e;
+    }
+    else if (to->toBasetype()->ty == Tarray &&     type->toBasetype()->ty == Tarray &&
+             to->toBasetype()->nextOf()->size() == type->toBasetype()->nextOf()->size())
+    {
+        // Bugzilla 12495: Array reinterpret casts: eg. string to immutable(ubyte)[]
+        return paintTypeOntoLiteral(to, e);
+    }
     else
+    {
         r = Cast(type, to, e).copy();
+    }
     if (CTFEExp::isCantExp(r))
         error(loc, "cannot cast %s to %s at compile time", e->toChars(), to->toChars());
     if (e->op == TOKarrayliteral)

test/compilable/interpret3.d

@@ -7143,6 +7143,29 @@ static assert({
     return true;
 }());
 
+/**************************************************
+    12495 - cast from string to immutable(ubyte)[]
+**************************************************/
+
+string getStr12495()
+{
+    char[1] buf = void;                     // dummy starting point.
+    string s = cast(string)buf[0..0];       // empty slice, .ptr points mutable.
+    assert(buf.ptr == s.ptr);
+    s ~= 'a';                               // this should allocate.
+    assert(buf.ptr != s.ptr);
+    return s.idup;                          // this should allocate again, and
+                                            // definitly point immutable memory.
+}
+auto indexOf12495(string s)
+{
+    auto p1 = s.ptr;
+    auto p2 = (cast(immutable(ubyte)[])s).ptr;
+    assert(cast(void*)p1 == cast(void*)p2); // OK <- fails
+    return cast(void*)p2 - cast(void*)p1;   // OK <- "cannot subtract pointers ..."
+}
+static assert(indexOf12495(getStr12495()) == 0);
+
 /**************************************************
     13992 - Repainting pointer arithmetic result
 **************************************************/