checker: add an error for $tmpl function type mismatches (#18826)

vlib/v/checker/return.v

@@ -179,6 +179,11 @@ fn (mut c Checker) return_stmt(mut node ast.Return) {
 			c.error('cannot use `${c.table.type_to_str(got_type)}` as ${c.error_type_name(exp_type)} in return argument',
 				pos)
 		}
+		if exprv is ast.ComptimeCall && exprv.method_name == 'tmpl'
+			&& c.table.final_sym(exp_type).kind != .string {
+			c.error('cannot use `string` as type `${c.table.type_to_str(exp_type)}` in return argument',
+				exprv.pos)
+		}
 		if node.exprs[expr_idxs[i]] !is ast.ComptimeCall {
 			got_type_sym := c.table.sym(got_type)
 			exp_type_sym := c.table.sym(exp_type)

vlib/v/checker/tests/template_type_mismatch_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/template_type_mismatch_err.vv:6:9: error: cannot use `string` as type `int` in return argument
+    4 | 
+    5 | fn return_item() int {
+    6 |     return $tmpl('./templates/template.md')
+      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+    7 | }

vlib/v/checker/tests/template_type_mismatch_err.vv

@@ -0,0 +1,7 @@
+fn main() {
+	println(return_item())
+}
+
+fn return_item() int {
+	return $tmpl('./templates/template.md')
+}

vlib/v/checker/tests/templates/template.md

@@ -0,0 +1 @@
+