Fix bugzilla 24733 - ImportC: #pragma pack(pop) restores wrong alig…

…nment (#16821)

compiler/src/dmd/cparse.d

@@ -5813,7 +5813,7 @@ final class CParser(AST) : Parser!AST
                 if (len == 1)   // stack is now empty
                     packalign.setDefault();
                 else
-                    packalign = (*this.packs)[len - 1];
+                    packalign = (*this.packs)[len - 2];
                 return closingParen();
             }
             while (n.value == TOK.comma)        // #pragma pack ( pop ,

compiler/test/compilable/test24733.c

@@ -0,0 +1,21 @@
+struct S1 {unsigned char c;int i;};
+#pragma pack(push, 2)
+struct S2 {unsigned char c;int i;};
+#pragma pack(push, 1)
+struct S3 {unsigned char c;int i;};
+#pragma pack(pop)
+struct S4 {unsigned char c;int i;};
+#pragma pack(pop)
+struct S5 {unsigned char c;int i;};
+
+_Static_assert(_Alignof(struct S1) == 4, "alignof S1");
+_Static_assert(_Alignof(struct S2) == 2, "alignof S2");
+_Static_assert(_Alignof(struct S3) == 1, "alignof S3");
+_Static_assert(_Alignof(struct S4) == 2, "alignof S4");
+_Static_assert(_Alignof(struct S5) == 4, "alignof S5");
+
+_Static_assert(sizeof(struct S1) == 8, "sizeof S1");
+_Static_assert(sizeof(struct S2) == 6, "sizeof S2");
+_Static_assert(sizeof(struct S3) == 5, "sizeof S3");
+_Static_assert(sizeof(struct S4) == 6, "sizeof S4");
+_Static_assert(sizeof(struct S5) == 8, "sizeof S5");