cgen: fix comptime generic arg resolution (allow several different st…

…ruct fields in generic functions using comptime) (fix #18110) (#21682)

vlib/v/checker/fn.v

@@ -1595,10 +1595,10 @@ fn (mut c Checker) resolve_comptime_args(func ast.Fn, node_ ast.CallExpr, concre
 			} else {
 				func.params[offset + i]
 			}
-			k++
 			if !param.typ.has_flag(.generic) {
 				continue
 			}
+			k++
 			param_typ := param.typ
 			if call_arg.expr is ast.Ident {
 				if call_arg.expr.obj is ast.Var {

vlib/v/gen/c/fn.v

@@ -1186,10 +1186,10 @@ fn (mut g Gen) resolve_comptime_args(func ast.Fn, mut node_ ast.CallExpr, concre
 			} else {
 				func.params[offset + i]
 			}
-			k++
 			if !param.typ.has_flag(.generic) {
 				continue
 			}
+			k++
 			param_typ := param.typ
 			if mut call_arg.expr is ast.Ident {
 				if mut call_arg.expr.obj is ast.Var {

vlib/v/tests/generic_comptime_arg_test.v

@@ -0,0 +1,53 @@
+import toml
+
+struct Parent {
+	name   string
+	child1 Child1
+	child2 Child2
+}
+
+struct Child1 {
+	name string
+}
+
+struct Child2 {
+	age int
+}
+
+fn decode[T](toml_str string) !T {
+	doc := toml.parse_text(toml_str)!
+
+	return decode_struct(doc.to_any(), &T{})
+}
+
+// `T` param serves here as a workaround to allow to infer types of nested struct fields.
+fn decode_struct[T](doc toml.Any, typ &T) T {
+	mut res := T{}
+	$for field in T.fields {
+		val := doc.value(field.name)
+		$if field.typ is string {
+			res.$(field.name) = val.string()
+		} $else $if field.typ is int {
+			res.$(field.name) = val.int()
+		} $else $if field.is_struct {
+			typ_ := typ.$(field.name)
+			res.$(field.name) = decode_struct(val, &typ_)
+		}
+	}
+	return res
+}
+
+fn test_main() {
+	toml_str := 'name = "John"
+	child1 = { name = "abc" }
+	child2 = { age = 5 }'
+
+	a := dump(decode[Parent](toml_str)!)
+	assert a.name == 'John'
+	assert a.child1 == Child1{
+		name: 'abc'
+	}
+	assert a.child2 == Child2{
+		age: 5
+	}
+}