Sema: add missing calls to wip_captures.finalize

Closes #13171

src/Sema.zig

@@ -28775,6 +28775,7 @@ fn semaBackingIntType(mod: *Module, struct_obj: *Module.Struct) CompileError!voi
 
         try sema.checkBackingIntType(&block, backing_int_src, backing_int_ty, fields_bit_sum);
         struct_obj.backing_int_ty = try backing_int_ty.copy(decl_arena_allocator);
+        try wip_captures.finalize();
     } else {
         var buf: Type.Payload.Bits = .{
             .base = .{ .tag = .int_unsigned },
@@ -29386,6 +29387,7 @@ fn semaStructFields(mod: *Module, struct_obj: *Module.Struct) CompileError!void
             }
         }
     }
+    try wip_captures.finalize();
 
     struct_obj.have_field_inits = true;
 }

test/behavior.zig

@@ -108,6 +108,7 @@ test {
     _ = @import("behavior/bugs/13112.zig");
     _ = @import("behavior/bugs/13128.zig");
     _ = @import("behavior/bugs/13164.zig");
+    _ = @import("behavior/bugs/13171.zig");
     _ = @import("behavior/byteswap.zig");
     _ = @import("behavior/byval_arg_var.zig");
     _ = @import("behavior/call.zig");

test/behavior/bugs/13171.zig

@@ -0,0 +1,16 @@
+const std = @import("std");
+const expect = std.testing.expect;
+
+fn BuildType(comptime T: type) type {
+    return struct {
+        val: union {
+            b: T,
+        },
+    };
+}
+
+test {
+    const TestStruct = BuildType(u32);
+    const c = TestStruct{ .val = .{ .b = 10 } };
+    try expect(c.val.b == 10);
+}