diff --git a/lib/std/meta.zig b/lib/std/meta.zig
index f1e59438ec2e418794230585ee2a3bb019a7aa22..14270ba9d86763f72a5a75eb5f846dee00759954 100644
--- a/lib/std/meta.zig
+++ b/lib/std/meta.zig
@@ -909,6 +909,15 @@ pub inline fn hasUniqueRepresentation(comptime T: type) bool {
             return @sizeOf(T) == sum_size;
         },
 
+        .@"union" => |info| {
+            if (info.layout == .@"packed") return @sizeOf(T) * 8 == @bitSizeOf(T);
+            inline for (info.field_types) |field_type| {
+                if (@sizeOf(field_type) != @sizeOf(T)) return false;
+                if (!hasUniqueRepresentation(field_type)) return false;
+            }
+            return true;
+        },
+
         .vector => |info| hasUniqueRepresentation(info.child) and
             @sizeOf(T) == @sizeOf(info.child) * info.len,
     };
@@ -978,6 +987,27 @@ test hasUniqueRepresentation {
 
     try testing.expect(!hasUniqueRepresentation(TestUnion4));
 
+    const TestUnion5 = extern union {
+        a: u32,
+        b: i32,
+    };
+
+    try testing.expect(hasUniqueRepresentation(TestUnion5));
+
+    const TestUnion6 = packed union(u7) {
+        a: u7,
+        b: i7,
+    };
+
+    try testing.expect(!hasUniqueRepresentation(TestUnion6));
+
+    const TestUnion7 = packed union(u8) {
+        a: u8,
+        b: i8,
+    };
+
+    try testing.expect(hasUniqueRepresentation(TestUnion7));
+
     inline for ([_]type{ u8, i16, u32, i64 }) |T| {
         try testing.expect(hasUniqueRepresentation(T));
         try testing.expect(hasUniqueRepresentation(enum(T) { _ }));
diff --git a/src/InternPool.zig b/src/InternPool.zig
index 9a0aa78e814291caacd7d2e852697f7b37e87586..be6e45441e55ec373e533643786528d0abeb310c 100644
--- a/src/InternPool.zig
+++ b/src/InternPool.zig
@@ -2147,9 +2147,11 @@ pub const Key = union(enum) {
         };
     };
 
-    pub const SpirvType = struct {
+    pub const SpirvType = extern struct {
         /// A `spirv_reify` instruction.
         zir_index: TrackedInst.Index,
+        /// Always 0.
+        padding: u32 = 0,
         /// A hash of this type's attributes generated by Sema.
         type_hash: u64,
     };
@@ -2591,7 +2593,6 @@ pub const Key = union(enum) {
         const KeyTag = @typeInfo(Key).@"union".tag_type.?;
         const seed = @intFromEnum(@as(KeyTag, key));
         return switch (key) {
-            // TODO: assert no padding in these types
             inline .ptr_type,
             .array_type,
             .vector_type,
@@ -2608,7 +2609,11 @@ pub const Key = union(enum) {
             .enum_tag,
             .inferred_error_set_type,
             .un,
-            => |x| Hash.hash(seed, asBytes(&x)),
+            => |x| {
+                _ = extern struct { is_extern: @TypeOf(x) };
+                comptime assert(std.meta.hasUniqueRepresentation(@TypeOf(x)));
+                return Hash.hash(seed, asBytes(&x));
+            },
 
             .int_type => |x| Hash.hash(seed + @intFromEnum(x.signedness), asBytes(&x.bits)),