fmt: keep generics for selective import as fn param (#10138)

vlib/v/ast/types.v

@@ -848,12 +848,6 @@ pub fn (mytable &Table) type_to_code(t Type) string {
 
 // import_aliases is a map of imported symbol aliases 'module.Type' => 'Type'
 pub fn (t &Table) type_to_str_using_aliases(typ Type, import_aliases map[string]string) string {
-	/*
-	if t.pref.is_verbose {
-	print_backtrace()
-	exit(0)
-	}
-	*/
 	sym := t.get_type_symbol(typ)
 	mut res := sym.name
 	match sym.kind {
@@ -957,6 +951,19 @@ pub fn (t &Table) type_to_str_using_aliases(typ Type, import_aliases map[string]
 				res = t.shorten_user_defined_typenames(res, import_aliases)
 			}
 		}
+		.generic_struct_inst {
+			info := sym.info as GenericStructInst
+			res = sym.name.all_before('<')
+			res += '<'
+			for i, ctyp in info.concrete_types {
+				res += t.get_type_symbol(ctyp).name
+				if i != info.concrete_types.len - 1 {
+					res += ', '
+				}
+			}
+			res += '>'
+			res = t.shorten_user_defined_typenames(res, import_aliases)
+		}
 		.void {
 			if typ.has_flag(.optional) {
 				return '?'

vlib/v/fmt/tests/generics_keep.vv

@@ -1,3 +1,9 @@
+import mymod { ImpNode }
+
+fn foobar_mymod<U>(inode ImpNode<U>) ImpNode<U> {
+	return ImpNode{}
+}
+
 fn simple<T>() T {
 	return T{}
 }
@@ -8,6 +14,18 @@ fn (_ Foo) simple<T>() T {
 	return T{}
 }
 
+struct NonGenericStruct {}
+
+fn use_as_generic(ngs NonGenericStruct<V>) NonGenericStruct<V> {
+	return NonGenericStruct{}
+}
+
+struct GenericStruct<A, B> {}
+
+fn proper_generics(gs GenericStruct<A, B>) GenericStruct<A, B> {
+	return gs
+}
+
 fn main() {
 	simple<int>()
 	Foo{}.simple<int>()

vlib/v/parser/parse_type.v

@@ -536,7 +536,8 @@ pub fn (mut p Parser) parse_generic_struct_inst_type(name string) ast.Type {
 	p.check(.gt)
 	p.in_generic_params = false
 	bs_name += '>'
-	if is_instance && concrete_types.len > 0 {
+	// fmt operates on a per-file basis, so is_instance might be not set correctly. Thus it's ignored.
+	if (is_instance || p.pref.is_fmt) && concrete_types.len > 0 {
 		mut gt_idx := p.table.find_type_idx(bs_name)
 		if gt_idx > 0 {
 			return ast.new_type(gt_idx)