Unexpected result with -betterC

[DLearner]

The two fragments below compiled and ran as expected using dmd 
-betterC under Windows.

```
    string Scrn = "OPO NAM='DspVar1' POS='1,1' 
VAR=('IntVar1','I');E";


    printf("\nWR_Createtest entered.\n");


    OpStructFstPtr = WR_CreateFormatFile(Scrn);

```

```
OpStruct* WR_CreateFormatFile(string parm_Format) {

    import core.stdc.stdio: printf;
    import core.stdc.stdlib: malloc;


    OpStruct* FstOpStructPtr;


    if (parm_Format[0..3] == "OPO"[0..3]) {
       printf("OPO Found.\n");
    } else {
       printf("OPO NOT Found.\n");
    }

    if (parm_Format[$-1..$] == "E"[0..1]) {
       printf("E Found.\n");
    } else {
       printf("E NOT Found.\n");
    }

```

However, the docs say dynamic arrays are not allowed with 
betterC, and 'string' implies a dynamic array.
So I expected DMD to complain that my code was invalid.

Any ideas?