checker: check generic fn declaration (#15104)

vlib/v/checker/fn.v

@@ -108,6 +108,17 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 		} else if return_sym.kind == .array_fixed {
 			c.error('fixed array cannot be returned by function', node.return_type_pos)
 		}
+		// Ensure each generic type of the parameter was declared in the function's definition
+		if node.return_type.has_flag(.generic) {
+			generic_names := c.table.generic_type_names(node.return_type)
+			for name in generic_names {
+				if name !in node.generic_names {
+					fn_generic_names := node.generic_names.join(', ')
+					c.error('generic type name `$name` is not mentioned in fn `$node.name<$fn_generic_names>`',
+						node.return_type_pos)
+				}
+			}
+		}
 	} else {
 		for mut a in node.attrs {
 			if a.kind == .comptime_define {
@@ -207,6 +218,7 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 					}
 				}
 			}
+			// Ensure each generic type of the parameter was declared in the function's definition
 			if param.typ.has_flag(.generic) {
 				generic_names := c.table.generic_type_names(param.typ)
 				for name in generic_names {

vlib/v/checker/tests/generic_fn_decl_err.out

@@ -0,0 +1,35 @@
+vlib/v/checker/tests/generic_fn_decl_err.vv:19:29: error: generic type name `P` is not mentioned in fn `create1<U>`
+   17 | }
+   18 |
+   19 | fn (r Db) create1<U>(u U, p P) {
+      |                             ^
+   20 |     println('Yo')
+   21 | }
+vlib/v/checker/tests/generic_fn_decl_err.vv:23:29: error: generic type name `P` is not mentioned in fn `create2<U>`
+   21 | }
+   22 |
+   23 | fn (r Db) create2<U>(u U, p &P) {
+      |                             ~~
+   24 |     println('Yo')
+   25 | }
+vlib/v/checker/tests/generic_fn_decl_err.vv:27:29: error: generic type name `P` is not mentioned in fn `create3<U>`
+   25 | }
+   26 |
+   27 | fn (r Db) create3<U>(u U, p []P) {
+      |                             ~~~
+   28 |     println('Yo')
+   29 | }
+vlib/v/checker/tests/generic_fn_decl_err.vv:31:27: error: generic type name `P` is not mentioned in fn `create4<U>`
+   29 | }
+   30 |
+   31 | fn (r Db) create4<U>(u U) P {
+      |                           ^
+   32 |     return P{}
+   33 | }
+vlib/v/checker/tests/generic_fn_decl_err.vv:35:27: error: generic type name `P` is not mentioned in fn `create5<U>`
+   33 | }
+   34 |
+   35 | fn (r Db) create5<U>(u U) []P {
+      |                           ~~~
+   36 |     return [P{}]
+   37 | }

vlib/v/checker/tests/generic_fn_decl_err.vv

@@ -0,0 +1,37 @@
+module main
+
+struct Db {}
+struct User {}
+struct Post {}
+
+fn main() {
+	r := Db{}
+	u := User{}
+	p := Post{}
+
+	r.create1(u, p)
+	r.create2(u, &p)
+	r.create3(u, [p])
+	r.create4(u)
+	r.create5(u)
+}
+
+fn (r Db) create1<U>(u U, p P) {
+	println('Yo')
+}
+
+fn (r Db) create2<U>(u U, p &P) {
+	println('Yo')
+}
+
+fn (r Db) create3<U>(u U, p []P) {
+	println('Yo')
+}
+
+fn (r Db) create4<U>(u U) P {
+	return P{}
+}
+
+fn (r Db) create5<U>(u U) []P {
+	return [P{}]
+}

vlib/v/checker/tests/generic_fn_decl_err_a.out

@@ -1,7 +0,0 @@
-vlib/v/checker/tests/generic_fn_decl_err_a.vv:17:28: error: generic type name `P` is not mentioned in fn `create<U>`
-   15 | }
-   16 |
-   17 | fn (r Db) create<U>(u U, p P) {
-      |                            ^
-   18 |     println('Yo')
-   19 | }

vlib/v/checker/tests/generic_fn_decl_err_a.vv

@@ -1,19 +0,0 @@
-module main
-
-struct Db {}
-
-struct User {}
-
-struct Post {}
-
-fn main() {
-	r := Db{}
-	u := User{}
-	p := Post{}
-
-	r.create(u, p)
-}
-
-fn (r Db) create<U>(u U, p P) {
-	println('Yo')
-}