fix Issue 15672 - Casting from void[] to T[] is erroneously considere…

src/expression.d

@@ -11244,13 +11244,35 @@ extern (C++) final class CastExp : UnaExp
             {
                 Type tobn = tob.nextOf().toBasetype();
                 Type t1bn = t1b.nextOf().toBasetype();
+
+                /* From void[] to anything mutable is unsafe because:
+                 *  int*[] api;
+                 *  void[] av = api;
+                 *  int[] ai = cast(int[]) av;
+                 *  ai[0] = 7;
+                 *  *api[0] crash!
+                 */
+                if (t1bn.ty == Tvoid && tobn.isMutable() && ex.op != TOKarrayliteral)
+                    goto Lunsafe;
+
                 if (!tobn.hasPointers() && MODimplicitConv(t1bn.mod, tobn.mod))
                     goto Lsafe;
             }
             if (tob.ty == Tpointer && t1b.ty == Tpointer)
             {
                 Type tobn = tob.nextOf().toBasetype();
                 Type t1bn = t1b.nextOf().toBasetype();
+
+                /* From void* to anything mutable is unsafe because:
+                 *  int** ppi;
+                 *  void* pv = ppi;
+                 *  int* pi = cast(int*) pv;
+                 *  *pi = 7;
+                 *  **ppi crash!
+                 */
+                if (t1bn.ty == Tvoid && tobn.isMutable())
+                    goto Lunsafe;
+
                 // If the struct is opaque we don't know about the struct members and the cast becomes unsafe
                 bool sfwrd = tobn.ty == Tstruct && !(cast(TypeStruct)tobn).sym.members ||
                              t1bn.ty == Tstruct && !(cast(TypeStruct)t1bn).sym.members;

test/fail_compilation/test15672.d

@@ -0,0 +1,38 @@
+/*
+ * TEST_OUTPUT:
+---
+fail_compilation/test15672.d(15): Error: cast from void[] to byte[] not allowed in safe code
+fail_compilation/test15672.d(25): Error: cast from void* to byte* not allowed in safe code
+---
+*/
+// https://issues.dlang.org/show_bug.cgi?id=15672
+
+alias byte T;
+alias const(byte) CT;
+
+@safe T[] test1(void[] a)
+{
+    return cast(T[])a;
+}
+
+@safe CT[] test2(void[] a)
+{
+    return cast(CT[])a;
+}
+
+@safe T* test3(void* a)
+{
+    return cast(T*)a;
+}
+
+@safe CT* test4(void* a)
+{
+    return cast(CT*)a;
+}
+
+@safe T[] test5()
+{
+    return cast(T[])[];
+}
+
+

test/runnable/testsafe.d

@@ -12,7 +12,7 @@ void pointercast()
     static assert(!__traits(compiles, cast(int*)b));
     static assert(!__traits(compiles, cast(int*)b));
     static assert(!__traits(compiles, cast(short*)b));
-    static assert( __traits(compiles, cast(byte*)b));
+    static assert(!__traits(compiles, cast(byte*)b));
     static assert( __traits(compiles, cast(short*)a));
     static assert( __traits(compiles, cast(byte*)a));
 }
@@ -311,7 +311,7 @@ void arraycast()
     int[] x;
     void[] y = x;
     static assert( __traits(compiles, cast(void[])x));
-    static assert( __traits(compiles, cast(int[])y));
+    static assert(!__traits(compiles, cast(int[])y));
     static assert(!__traits(compiles, cast(int*[])y));
     static assert(!__traits(compiles, cast(void[][])y));