parser: disallow [T] as fn return type (#26530)

Author: Delta456

vlib/v/parser/fn.v

@@ -762,6 +762,13 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	same_line := p.tok.line_nr == p.prev_tok.line_nr
 	if (p.tok.kind.is_start_of_type() && (same_line || p.tok.kind != .lsbr))
 		|| (same_line && p.tok.kind == .key_fn) {
+		// Disallow [T] as return type
+		if p.tok.kind == .lsbr && p.peek_tok.kind == .name && p.peek_tok.lit.len == 1
+			&& p.peek_tok.lit[0].is_capital() {
+			return_type_pos = return_type_pos.extend(p.peek_tok.pos()).extend(p.peek_token(2).pos())
+			p.error_with_pos('invalid generic return, use `${p.peek_tok.lit}` instead',
+				return_type_pos)
+		}
 		p.inside_fn_return = true
 		return_type = p.parse_type()
 		p.inside_fn_return = false

vlib/v/parser/tests/generic_return_invalid_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/generic_return_invalid_err.vv:3:16: error: invalid generic return, use `T` instead
+    1 | module main
+    2 | 
+    3 | fn foo[T](v T) [T] {
+      |                ~~~
+    4 |     return v
+    5 | }

vlib/v/parser/tests/generic_return_invalid_err.vv

@@ -0,0 +1,9 @@
+module main
+
+fn foo[T](v T) [T] {
+	return v
+}
+
+fn main() {
+	println(foo[string]('Hello World'))
+}