behavior: add coverage for extern struct field overalignment

test/behavior/struct.zig

@@ -2184,3 +2184,52 @@ test "pass a pointer to a comptime-only struct field to a function" {
     const s: struct { x: type } = .{ .x = u42 };
     try S.checkField(&s.x);
 }
+
+test "overaligned extern struct fields" {
+    const A = struct {
+        a: *anyopaque,
+        b: u64,
+        c: [1][]u8,
+        d: ?anyerror,
+    };
+
+    const B = union(enum) {
+        a: struct {
+            a: [2]usize,
+            b: C,
+        },
+        b: struct {
+            a: *anyopaque,
+            b: []const []u8,
+            c: C,
+        },
+        const C = union {
+            a: void,
+            b: *anyopaque,
+            c: anyerror!usize,
+        };
+    };
+
+    const D = extern struct {
+        a: u32,
+    };
+
+    const E = extern struct {
+        a: u32,
+        b: [2][@sizeOf(A)]u8 align(@alignOf(A)),
+        c: [2]u32,
+        d: [2][@sizeOf(B)]u8 align(@alignOf(B)),
+
+        fn cast(e: *@This()) *D {
+            e.a = 2;
+            return @ptrCast(e);
+        }
+    };
+
+    var e: E = undefined;
+    const d = e.cast();
+    try expect(d.a == 2);
+    try expect(std.mem.isAligned(@intFromPtr(&e.b), @alignOf(A)));
+    try expect(std.mem.isAligned(@intFromPtr(&e.c), @alignOf(u32)));
+    try expect(std.mem.isAligned(@intFromPtr(&e.d), @alignOf(B)));
+}