Fix #4993: allow to cast between for fat values

Author: kubo39

gen/llvmhelpers.cpp

@@ -673,6 +673,15 @@ DValue *DtoCastStruct(Loc loc, DValue *val, Type *to) {
     return new DLValue(to, lval);
   }
 
+  // A cast between fat values is possible only when the sizes match.
+  // https://github.com/ldc-developers/ldc/issues/4993
+  if (totype->ty == TY::Tsarray || totype->ty == TY::Tvector) {
+    if (size(totype) == size(val->type->toBasetype())) {
+      llvm::Value *lval = DtoLVal(val);
+      return new DLValue(to, lval);
+    }
+  }
+
   error(loc, "Internal Compiler Error: Invalid struct cast from `%s` to `%s`",
         val->type->toChars(), to->toChars());
   fatal();

tests/compilable/gh4993.d

@@ -0,0 +1,16 @@
+// RUN: %ldc -c %s
+
+struct Foo {}
+
+struct Bar
+{
+    int[4] v;
+}
+
+void main() {
+    Foo f;
+    auto x = cast(ubyte[Foo.sizeof])f;
+
+    Bar b;
+    auto y = cast(__vector(int[4]))b;
+}