Fix Bugzilla 24622 - Modify const data with void[] concatenation/appe…

…nd (#16606)

compiler/src/dmd/expressionsem.d

@@ -11873,6 +11873,8 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
             (tb2.ty == Tarray || tb2.ty == Tsarray) &&
             (exp.e2.implicitConvTo(exp.e1.type) ||
              (tb2.nextOf().implicitConvTo(tb1next) &&
+             // Do not strip const(void)[]
+             (!global.params.fixImmutableConv || tb1next.ty != Tvoid) &&
               (tb2.nextOf().size(Loc.initial) == tb1next.size(Loc.initial)))))
         {
             // EXP.concatenateAssign
@@ -12622,7 +12624,9 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
             exp.type = tb.nextOf().arrayOf();
         if (exp.type.ty == Tarray && tb1next && tb2next && tb1next.mod != tb2next.mod)
         {
-            exp.type = exp.type.nextOf().toHeadMutable().arrayOf();
+            // Do not strip const(void)[]
+            if (!global.params.fixImmutableConv || tb.nextOf().ty != Tvoid)
+                exp.type = exp.type.nextOf().toHeadMutable().arrayOf();
         }
         if (Type tbn = tb.nextOf())
         {

compiler/test/fail_compilation/void_cat.d

@@ -0,0 +1,21 @@
+/*
+REQUIRED_ARGS: -preview=fixImmutableConv
+TEST_OUTPUT:
+---
+fail_compilation/void_cat.d(15): Error: cannot copy `const(void)[]` to `void[]`
+fail_compilation/void_cat.d(15):        Source data has incompatible type qualifier(s)
+fail_compilation/void_cat.d(15):        Use `cast(void[])` to force copy
+fail_compilation/void_cat.d(19): Error: cannot append type `const(void)[]` to type `void[]`
+---
+*/
+
+void g(int*[] a, const(int*)[] b) @system
+{
+    void[] va = a;
+    va[] = va.init ~ b; // a now contains b's data
+    *a[0] = 0; // modify const data
+
+    const(void)[] vb = b;
+    va ~= vb; // also leaks const pointers into void[]
+    // va could be copied into `a` via another void[]
+}