checker: fix generics with generic anon fn parameter (#10101)

vlib/v/checker/check_types.v

@@ -508,14 +508,12 @@ pub fn (mut c Checker) infer_fn_generic_types(f ast.Fn, mut call_expr ast.CallEx
 
 			if param.typ.has_flag(.generic) && param_type_sym.name == gt_name {
 				to_set = c.table.mktyp(arg.typ)
-				mut sym := c.table.get_type_symbol(arg.typ)
-				if mut sym.info is ast.FnType {
-					if !sym.info.is_anon {
-						sym.info.func.name = ''
-						idx := c.table.find_or_register_fn_type(c.mod, sym.info.func,
-							true, false)
-						to_set = ast.new_type(idx).derive(arg.typ)
-					}
+				sym := c.table.get_type_symbol(arg.typ)
+				if sym.info is ast.FnType {
+					mut func := sym.info.func
+					func.name = ''
+					idx := c.table.find_or_register_fn_type(c.mod, func, true, false)
+					to_set = ast.new_type(idx).derive(arg.typ)
 				}
 				if arg.expr.is_auto_deref_var() {
 					to_set = to_set.deref()

vlib/v/tests/generics_with_generics_fn_return_generics_fn_type_test.v

@@ -58,3 +58,48 @@ fn test_generics_with_generics_fn_return_type() {
 	f = mixed_v2(neg)
 	assert f(7) == -7
 }
+
+fn neg_a(a int) int {
+	return -a
+}
+
+fn neg_b(b int) int {
+	return -b
+}
+
+// assume that "generic" is a heavy function and should be shared
+// by *all* callers in this file
+[noinline]
+fn generic<T>(func T) T {
+	return func
+}
+
+fn test_generics_with_generics_fn_return_type_v2() {
+	mut f := neg_a
+	assert f(1) == -1
+
+	f = generic(neg_a)
+	assert f(2) == -2
+	f = generic(neg_b)
+	assert f(3) == -3
+
+	f = generic(fn (a int) int {
+		return -a
+	})
+	assert f(4) == -4
+	f = generic(fn (b int) int {
+		return -b
+	})
+	assert f(5) == -5
+
+	f = fn (a int) int {
+		return -a
+	}
+	f = generic(f)
+	assert f(6) == -6
+	f = fn (b int) int {
+		return -b
+	}
+	f = generic(f)
+	assert f(7) == -7
+}