cgen: fix C struct init when it has default expr (#21510)

vlib/v/gen/c/cgen.v

@@ -7126,15 +7126,18 @@ fn (mut g Gen) type_default(typ_ ast.Type) string {
 			} else {
 				'{'
 			}
-			if sym.language == .v {
+			if sym.language in [.c, .v] {
 				for field in info.fields {
 					field_sym := g.table.sym(field.typ)
 					if field.has_default_expr
 						|| field_sym.kind in [.array, .map, .string, .bool, .alias, .i8, .i16, .int, .i64, .u8, .u16, .u32, .u64, .f32, .f64, .char, .voidptr, .byteptr, .charptr, .struct_, .chan] {
+						if sym.language == .c && !field.has_default_expr {
+							continue
+						}
 						field_name := c_name(field.name)
 						if field.has_default_expr {
 							mut expr_str := ''
-							if g.table.sym(field.typ).kind in [.sum_type, .interface_] {
+							if field_sym.kind in [.sum_type, .interface_] {
 								expr_str = g.expr_string_with_cast(field.default_expr,
 									field.default_expr_typ, field.typ)
 							} else {

vlib/v/tests/c_structs/cstruct.h

@@ -25,10 +25,11 @@ typedef struct Foo {
 
 typedef struct Bar {
     int a;
+    float b;
 } Bar;
 
 ///
 
 typedef struct TestAlias {
     int a;
-};
+};

vlib/v/tests/c_structs/cstruct_default_value_test.v

@@ -0,0 +1,25 @@
+#include "@VMODROOT/cstruct.h"
+
+struct Foo {
+	a int = 3
+}
+
+struct C.Bar {
+	a int = 3
+	b f64
+}
+
+struct FooBar {
+	foo Foo
+	bar C.Bar
+}
+
+fn test_main() {
+	a := dump(Foo{})
+	b := dump(C.Bar{})
+	c := dump(FooBar{})
+
+	assert a.a == b.a
+	assert b.a == c.bar.a
+	assert b.b == c.bar.b
+}