cgen: fix generic resolver on non generic function (#18381)

vlib/v/gen/c/cgen.v

@@ -1876,7 +1876,7 @@ fn (mut g Gen) expr_with_tmp_var(expr ast.Expr, expr_typ ast.Type, ret_typ ast.T
 	}
 
 	stmt_str := g.go_before_stmt(0).trim_space()
-	styp := g.base_type(ret_typ)
+	mut styp := g.base_type(ret_typ)
 	g.empty_line = true
 
 	if g.table.sym(expr_typ).kind == .none_ {
@@ -1885,7 +1885,21 @@ fn (mut g Gen) expr_with_tmp_var(expr ast.Expr, expr_typ ast.Type, ret_typ ast.T
 		g.writeln(';')
 	} else {
 		mut is_ptr_to_ptr_assign := false
-		g.writeln('${g.typ(ret_typ)} ${tmp_var};')
+		if ret_typ.has_flag(.generic) {
+			if expr is ast.SelectorExpr && g.cur_concrete_types.len == 0 {
+				// resolve generic struct on selectorExpr inside non-generic function
+				if expr.expr is ast.Ident && (expr.expr as ast.Ident).obj is ast.Var {
+					if ((expr.expr as ast.Ident).obj as ast.Var).expr is ast.StructInit {
+						g.cur_concrete_types << (g.table.sym((expr.expr as ast.Ident).obj.typ).info as ast.Struct).concrete_types
+					}
+				}
+			}
+			styp = g.base_type(g.unwrap_generic(ret_typ))
+			ret_styp := g.typ(g.unwrap_generic(ret_typ)).replace('*', '_ptr')
+			g.writeln('${ret_styp} ${tmp_var};')
+		} else {
+			g.writeln('${g.typ(ret_typ)} ${tmp_var};')
+		}
 		if ret_typ.has_flag(.option) {
 			if expr_typ.has_flag(.option) && expr in [ast.StructInit, ast.ArrayInit, ast.MapInit] {
 				if expr is ast.StructInit && (expr as ast.StructInit).init_fields.len > 0 {
@@ -3693,8 +3707,9 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 			return
 		}
 	}
-	field_is_opt := node.expr is ast.Ident && (node.expr as ast.Ident).is_auto_heap()
-		&& (node.expr as ast.Ident).or_expr.kind != .absent && field_typ.has_flag(.option)
+	// var?.field_opt
+	field_is_opt := (node.expr is ast.Ident && (node.expr as ast.Ident).is_auto_heap()
+		&& (node.expr as ast.Ident).or_expr.kind != .absent && field_typ.has_flag(.option))
 	if field_is_opt {
 		g.write('((${g.base_type(field_typ)})')
 	}

vlib/v/gen/c/utils.v

@@ -36,6 +36,25 @@ fn (mut g Gen) unwrap_generic(typ ast.Type) ast.Type {
 					}
 				}
 			}
+		} else if typ.has_flag(.generic) && g.table.sym(typ).kind == .struct_ {
+			// resolve selector `a.foo` where `a` is struct[T] on non generic function	
+			sym := g.table.sym(typ)
+			if sym.info is ast.Struct {
+				if sym.info.generic_types.len > 0 {
+					generic_names := sym.info.generic_types.map(g.table.sym(it).name)
+					if t_typ := mut_table.resolve_generic_to_concrete(typ, generic_names,
+						sym.info.concrete_types)
+					{
+						return t_typ
+					}
+
+					if t_typ := mut_table.resolve_generic_to_concrete(typ, generic_names,
+						g.cur_concrete_types)
+					{
+						return t_typ
+					}
+				}
+			}
 		}
 	}
 	return typ

vlib/v/tests/generic_selector_test.v

@@ -0,0 +1,70 @@
+pub struct List[T] {
+mut:
+	head ?&Node[T]
+	size int
+}
+
+pub fn (mut l List[T]) prepend(mut node Node[T]) {
+	if head := l.head {
+		node.next = head
+		l.head = &node
+		l.size = l.size + 1
+	} else {
+		l.size = 1
+		l.head = &node
+	}
+}
+
+pub fn (mut l List[T]) append(mut node Node[T]) ?int {
+	if h := l.head {
+		_ := h
+	} else {
+		l.head = &node
+		l.size = 0
+		return l.size
+	}
+
+	mut curr_node := l.head
+	for {
+		if curr_node != none {
+			if next_node := curr_node?.next {
+				curr_node = next_node
+			} else {
+				curr_node?.next = &node
+				l.size = l.size + 1
+				break
+			}
+		}
+	}
+	return l.size
+}
+
+pub fn (mut l List[T]) find_last(node ?&Node[T]) ?&Node[T] {
+	if next := node?.next {
+		return l.find_last(next)
+	} else {
+		return node
+	}
+}
+
+[heap]
+pub struct Node[T] {
+mut:
+	data T
+	next ?&Node[T]
+}
+
+fn test_main() {
+	mut list := List[string]{}
+	list.prepend(mut Node{ data: 'zero' })
+	list.prepend(mut Node{ data: 'first' })
+	list.append(mut Node{ data: 'last' }) or { panic('unable to append linked list') }
+	list.append(mut Node{ data: 'very last' }) or { panic('unable to append linked list') }
+
+	assert list.find_last(list.head)? == Node{
+		data: 'very last'
+	}
+	assert list.head?.next?.next?.next? == Node{
+		data: 'very last'
+	}
+}