ast: fix concrete fn type that returns pair type as generic type (#17780)

vlib/v/ast/table.v

@@ -2105,7 +2105,7 @@ pub fn (mut t Table) generic_insts_to_concrete() {
 					}
 					mut fields := parent_info.fields.clone()
 					if parent_info.generic_types.len == info.concrete_types.len {
-						generic_names := parent_info.generic_types.map(t.sym(it).name)
+						generic_names := t.get_generic_names(parent_info.generic_types)
 						for i in 0 .. fields.len {
 							if fields[i].typ.has_flag(.generic) {
 								if fields[i].typ.idx() != info.parent_idx {
@@ -2151,7 +2151,7 @@ pub fn (mut t Table) generic_insts_to_concrete() {
 					}
 					if parent_info.generic_types.len == info.concrete_types.len {
 						mut fields := parent_info.fields.clone()
-						generic_names := parent_info.generic_types.map(t.sym(it).name)
+						generic_names := t.get_generic_names(parent_info.generic_types)
 						for i in 0 .. fields.len {
 							if t_typ := t.resolve_generic_to_concrete(fields[i].typ, generic_names,
 								info.concrete_types)
@@ -2209,7 +2209,7 @@ pub fn (mut t Table) generic_insts_to_concrete() {
 					if parent_info.generic_types.len == info.concrete_types.len {
 						mut fields := parent_info.fields.clone()
 						mut variants := parent_info.variants.clone()
-						generic_names := parent_info.generic_types.map(t.sym(it).name)
+						generic_names := t.get_generic_names(parent_info.generic_types)
 						for i in 0 .. fields.len {
 							if t_typ := t.resolve_generic_to_concrete(fields[i].typ, generic_names,
 								info.concrete_types)
@@ -2250,11 +2250,12 @@ pub fn (mut t Table) generic_insts_to_concrete() {
 					// TODO: Cache function's generic types (parameters and return type) like Struct and Interface etc. do?
 					mut parent_info := parent.info as FnType
 					mut function := parent_info.func
-					mut generic_names := []string{cap: function.params.len + 1}
+					mut generic_types := []Type{cap: function.params.len + 1}
-					generic_names << function.params.filter(it.typ.has_flag(.generic)).map(t.sym(it.typ).name)
+					generic_types << function.params.filter(it.typ.has_flag(.generic)).map(it.typ)
 					if function.return_type.has_flag(.generic) {
-						generic_names << t.sym(function.return_type).name
+						generic_types << function.return_type
 					}
+					generic_names := t.get_generic_names(generic_types)
 					for mut param in function.params {
 						if param.typ.has_flag(.generic) {
 							if t_typ := t.resolve_generic_to_concrete(param.typ, generic_names,
@@ -2264,11 +2265,11 @@ pub fn (mut t Table) generic_insts_to_concrete() {
 							}
 						}
 					}
-					mut return_type := function.return_type
+					if function.return_type.has_flag(.generic) {
-					if return_type.has_flag(.generic) {
+						if t_typ := t.resolve_generic_to_concrete(function.return_type,
-						if return_type.idx() != info.parent_idx {
+							generic_names, info.concrete_types)
-							return_type = t.unwrap_generic_type(return_type, generic_names,
+						{
-								info.concrete_types)
+							function.return_type = t_typ
 						}
 					}
 					sym.info = FnType{
@@ -2284,6 +2285,30 @@ pub fn (mut t Table) generic_insts_to_concrete() {
 	}
 }
 
+// Extracts all type names from given types, notice that MultiReturn will be decompose
+// and will not included in returned string
+pub fn (t &Table) get_generic_names(generic_types []Type) []string {
+	mut generic_names := []string{cap: generic_types.len}
+	for typ in generic_types {
+		if !typ.has_flag(.generic) {
+			continue
+		}
+
+		sym := t.sym(typ)
+		info := sym.info
+
+		match info {
+			MultiReturn {
+				generic_names << t.get_generic_names(info.types)
+			}
+			else {
+				generic_names << sym.name
+			}
+		}
+	}
+	return generic_names
+}
+
 pub fn (t &Table) is_comptime_type(x Type, y ComptimeType) bool {
 	x_kind := t.type_kind(x)
 	match y.kind {

vlib/v/tests/concrete_type_as_generic_type_test.v

@@ -2,6 +2,8 @@ type Fn = fn (T)
 
 type FnReturn = fn (T) R
 
+type FnMultiReturn = fn (I) (O, R)
+
 fn func_fn_concrete() Fn[string] {
 	return fn (_s string) {}
 }
@@ -23,7 +25,13 @@ fn func_fn_return_dynamic[T, R]() FnReturn[T, R] {
 	}
 }
 
-// vfmt will erase explicit generic type (bug)
+fn func_fn_multi_return_concrete() FnMultiReturn[string, string, string] {
+	return fn (s string) (string, string) {
+		return s[..1], s[1..]
+	}
+}
+
+// vfmt will erase explicit generic type (bug reported in #17773)
 // vfmt off
 
 fn test_concrete_function_type_as_generic_type() {
@@ -31,6 +39,11 @@ fn test_concrete_function_type_as_generic_type() {
 	func_fn_dynamic[string]()('V')
 
 	assert func_fn_return_dynamic[string, int]()('100') == 100
+	
+	s1, s2 := func_fn_multi_return_concrete()('VLang')
+
+	assert s1 == 'V'
+	assert s2 == 'Lang'
 }
 
 // vfmt on