checker: check unknown generic type (#7954)

vlib/v/checker/checker.v

@@ -1658,6 +1658,12 @@ pub fn (mut c Checker) call_fn(mut call_expr ast.CallExpr) table.Type {
 		// builtin C.m*, C.s* only - temp
 		c.warn('function `$f.name` must be called from an `unsafe` block', call_expr.pos)
 	}
+	if f.is_generic {
+		sym := c.table.get_type_symbol(call_expr.generic_type)
+		if sym.kind == .placeholder {
+			c.error('unknown type `$sym.name`', call_expr.generic_list_pos)
+		}
+	}
 	if f.is_generic && f.return_type.has_flag(.generic) {
 		rts := c.table.get_type_symbol(f.return_type)
 		if rts.kind == .struct_ {

vlib/v/checker/tests/unknown_generic_type.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/unknown_generic_type.vv:5:13: error: unknown type `Foo`
+    3 |
+    4 | fn main() {
+    5 |     x := decode<Foo>('{"name": "test"}')?
+      |                ~~~~~
+    6 |     println(x)
+    7 | }

vlib/v/checker/tests/unknown_generic_type.vv

@@ -0,0 +1,7 @@
+fn decode<T>(raw_data string) ?T {
+}
+
+fn main() {
+	x := decode<Foo>('{"name": "test"}')?
+	println(x)
+}