From ebf629dc4124a6207b18028cf89dc6910d25eaf3 Mon Sep 17 00:00:00 2001
From: CreeperFace <165158232+dy-tea@users.noreply.github.com>
Date: Mon, 10 Nov 2025 07:18:17 +0000
Subject: [PATCH] ast,checker: improve type checking for sumtypes with generics
 (fix #25690) (#25699)

---
 vlib/v/ast/table.v                            | 86 +++++++++++++++----
 vlib/v/ast/types.v                            |  1 +
 vlib/v/checker/check_types.v                  | 46 ++++++++++
 vlib/v/checker/fn.v                           | 27 ++++++
 vlib/v/checker/struct.v                       | 47 ++++++++--
 vlib/v/parser/struct.v                        |  2 +
 .../sumtypes/sumtype_type_coercion_test.v     | 20 +++++
 7 files changed, 208 insertions(+), 21 deletions(-)
 create mode 100644 vlib/v/tests/sumtypes/sumtype_type_coercion_test.v

diff --git a/vlib/v/ast/table.v b/vlib/v/ast/table.v
index 696ebd748..2ee7da778 100644
--- a/vlib/v/ast/table.v
+++ b/vlib/v/ast/table.v
@@ -937,6 +937,9 @@ pub fn (mut t Table) register_sym(sym TypeSymbol) int {
 		...sym
 	}
 	t.type_symbols[idx].idx = idx
+	if t.type_symbols[idx].ngname == '' {
+		t.type_symbols[idx].ngname = strip_generic_params(sym.name)
+	}
 	t.type_idxs[sym_name] = idx
 	return idx
 }
@@ -960,6 +963,11 @@ pub fn (t &Table) known_type(name string) bool {
 	return t.type_idxs[name] != 0 || t.parsing_type == name || name in ['i32', 'byte']
 }
 
+@[inline]
+pub fn strip_generic_params(name string) string {
+	return name.all_before('[')
+}
+
 // start_parsing_type open the scope during the parsing of a type
 // where the type name must include the module prefix
 pub fn (mut t Table) start_parsing_type(type_name string) {
@@ -1178,6 +1186,7 @@ pub fn (mut t Table) find_or_register_chan(elem_type Type, is_mut bool) int {
 		kind:       .chan
 		name:       name
 		cname:      cname
+		ngname:     strip_generic_params(name)
 		info:       Chan{
 			elem_type: elem_type
 			is_mut:    is_mut
@@ -1199,6 +1208,7 @@ pub fn (mut t Table) find_or_register_map(key_type Type, value_type Type) int {
 		kind:       .map
 		name:       name
 		cname:      cname
+		ngname:     strip_generic_params(name)
 		info:       Map{
 			key_type:   key_type
 			value_type: value_type
@@ -1220,6 +1230,7 @@ pub fn (mut t Table) find_or_register_thread(return_type Type) int {
 		kind:       .thread
 		name:       name
 		cname:      cname
+		ngname:     strip_generic_params(name)
 		info:       Thread{
 			return_type: return_type
 		}
@@ -1242,6 +1253,7 @@ pub fn (mut t Table) find_or_register_promise(return_type Type) int {
 		kind:       .struct
 		name:       name
 		cname:      cname
+		ngname:     strip_generic_params(name)
 		info:       Struct{
 			concrete_types: [return_type, t.type_idxs['JS.Any']]
 		}
@@ -1265,6 +1277,7 @@ pub fn (mut t Table) find_or_register_array(elem_type Type) int {
 		kind:       .array
 		name:       name
 		cname:      cname
+		ngname:     strip_generic_params(name)
 		info:       Array{
 			nr_dims:   1
 			elem_type: elem_type
@@ -1292,10 +1305,11 @@ pub fn (mut t Table) find_or_register_array_fixed(elem_type Type, size int, size
 	cname := prefix + t.array_fixed_cname(elem_type, size)
 	// register
 	array_fixed_type := TypeSymbol{
-		kind:  .array_fixed
-		name:  name
-		cname: cname
-		info:  ArrayFixed{
+		kind:   .array_fixed
+		name:   name
+		cname:  cname
+		ngname: strip_generic_params(name)
+		info:   ArrayFixed{
 			elem_type: elem_type
 			size:      size
 			size_expr: size_expr
@@ -1328,10 +1342,11 @@ pub fn (mut t Table) find_or_register_multi_return(mr_typs []Type) int {
 		return existing_idx
 	}
 	multireg_sym := TypeSymbol{
-		kind:  .multi_return
-		name:  name
-		cname: cname
-		info:  MultiReturn{
+		kind:   .multi_return
+		name:   name
+		cname:  cname
+		ngname: strip_generic_params(name)
+		info:   MultiReturn{
 			types: mr_typs
 		}
 	}
@@ -1354,11 +1369,12 @@ pub fn (mut t Table) find_or_register_fn_type(f Fn, is_anon bool, has_decl bool)
 		return existing_idx
 	}
 	return t.register_sym(
-		kind:  .function
-		name:  name
-		cname: cname
-		mod:   f.mod
-		info:  FnType{
+		kind:   .function
+		name:   name
+		cname:  cname
+		ngname: strip_generic_params(name)
+		mod:    f.mod
+		info:   FnType{
 			is_anon:  anon
 			has_decl: has_decl
 			func:     f
@@ -1366,6 +1382,44 @@ pub fn (mut t Table) find_or_register_fn_type(f Fn, is_anon bool, has_decl bool)
 	)
 }
 
+pub fn (mut t Table) find_or_register_generic_inst(parent_typ Type, concrete_types []Type) int {
+	parent_sym := t.sym(parent_typ)
+	if parent_sym.info !is Struct {
+		return 0
+	}
+	struct_info := parent_sym.info as Struct
+	if struct_info.generic_types.len == 0 || concrete_types.len != struct_info.generic_types.len {
+		return 0
+	}
+	mut inst_name := parent_sym.ngname + '['
+	mut inst_cname := parent_sym.cname + '_T_'
+	for i, ct in concrete_types {
+		ct_sym := t.sym(ct)
+		inst_name += ct_sym.name
+		inst_cname += ct_sym.cname
+		if i < concrete_types.len - 1 {
+			inst_name += ', '
+			inst_cname += '_T_'
+		}
+	}
+	inst_name += ']'
+	existing_idx := t.type_idxs[inst_name]
+	if existing_idx > 0 {
+		return existing_idx
+	}
+	return t.register_sym(
+		kind:   .generic_inst
+		name:   inst_name
+		cname:  inst_cname
+		ngname: parent_sym.ngname
+		mod:    parent_sym.mod
+		info:   GenericInst{
+			parent_idx:     parent_typ.idx()
+			concrete_types: concrete_types
+		}
+	)
+}
+
 pub fn (mut t Table) add_placeholder_type(name string, cname string, language Language) int {
 	mut modname := ''
 	if name.contains('.') {
@@ -1375,6 +1429,7 @@ pub fn (mut t Table) add_placeholder_type(name string, cname string, language La
 		kind:       .placeholder
 		name:       name
 		cname:      util.no_dots(cname).replace_each(['&', ''])
+		ngname:     strip_generic_params(name)
 		language:   language
 		mod:        modname
 		is_pub:     true
@@ -1893,7 +1948,7 @@ pub fn (mut t Table) convert_generic_type(generic_type Type, generic_names []str
 			if sym.info.is_generic {
 				mut nrt := '${sym.name}['
 				mut rnrt := '${sym.rname}['
-				mut cnrt := '${sym.cname}['
+				mut cnrt := '${sym.cname}_T_'
 				mut t_generic_names := generic_names.clone()
 				mut t_to_types := to_types.clone()
 				if sym.generic_types.len > 0 && sym.generic_types.len == sym.info.generic_types.len
@@ -1930,7 +1985,7 @@ pub fn (mut t Table) convert_generic_type(generic_type Type, generic_names []str
 						if i != sym.info.generic_types.len - 1 {
 							nrt += ', '
 							rnrt += ', '
-							cnrt += ', '
+							cnrt += '_'
 						}
 					} else {
 						return none
@@ -1938,7 +1993,6 @@ pub fn (mut t Table) convert_generic_type(generic_type Type, generic_names []str
 				}
 				nrt += ']'
 				rnrt += ']'
-				cnrt += ']'
 				mut idx := t.type_idxs[nrt]
 				if idx == 0 {
 					idx = t.type_idxs[rnrt]
diff --git a/vlib/v/ast/types.v b/vlib/v/ast/types.v
index a575906c0..c7928c8e5 100644
--- a/vlib/v/ast/types.v
+++ b/vlib/v/ast/types.v
@@ -115,6 +115,7 @@ pub mut:
 	name          string // the internal & source name of the type, i.e. `[5]int`.
 	cname         string // the name with no dots for use in the generated C code
 	rname         string // the raw name
+	ngname        string // the name without generic parameters
 	methods       []Fn
 	generic_types []Type
 	mod           string
diff --git a/vlib/v/checker/check_types.v b/vlib/v/checker/check_types.v
index 72959277c..9dccd0e2e 100644
--- a/vlib/v/checker/check_types.v
+++ b/vlib/v/checker/check_types.v
@@ -460,6 +460,21 @@ fn (mut c Checker) check_basic(got ast.Type, expected ast.Type) bool {
 	if c.table.sumtype_has_variant(expected, ast.mktyp(got), false) {
 		return true
 	}
+	if exp_sym.kind == .placeholder && c.expected_type != ast.void_type {
+		base_type := c.table.find_type(exp_sym.ngname)
+		if base_type != 0 {
+			base_sym := c.table.sym(base_type)
+			if base_sym.kind == .sum_type && base_sym.info is ast.SumType {
+				base_info := base_sym.info as ast.SumType
+				for variant in base_info.variants {
+					variant_sym := c.table.sym(variant)
+					if variant_sym.ngname == got_sym.ngname {
+						return true
+					}
+				}
+			}
+		}
+	}
 	// struct
 	if exp_sym.kind == .struct && got_sym.kind == .struct {
 		if c.table.type_to_str(expected) == c.table.type_to_str(got) {
@@ -950,6 +965,37 @@ fn (mut c Checker) infer_struct_generic_types(typ ast.Type, node ast.StructInit)
 							}
 						}
 					}
+				} else if field_sym.info is ast.SumType {
+					for t in node.init_fields {
+						if ft.name == t.name && t.typ != 0 {
+							init_sym := c.table.sym(t.typ)
+							for variant in field_sym.info.variants {
+								variant_sym := c.table.sym(variant)
+								if variant_sym.name == init_sym.name {
+									if variant_sym.info is ast.Struct
+										&& variant_sym.info.generic_types.len > 0 {
+										if init_sym.info is ast.Struct
+											&& init_sym.info.concrete_types.len > 0 {
+											concrete_types << ast.mktyp(init_sym.info.concrete_types[0])
+											continue gname
+										}
+									} else {
+										for init_field in node.init_fields {
+											if init_field.name != t.name && init_field.typ != 0 {
+												field := sym.info.fields.filter(it.name == init_field.name)
+												if field.len > 0 {
+													if c.table.sym(field[0].typ).name == gt_name {
+														concrete_types << ast.mktyp(init_field.typ)
+														continue gname
+													}
+												}
+											}
+										}
+									}
+								}
+							}
+						}
+					}
 				}
 			}
 			c.error('could not infer generic type `${gt_name}` in generic struct `${sym.name}[${generic_names.join(', ')}]`',
diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index e9385ffa6..fa0ad9135 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -1569,6 +1569,33 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 				node.args[i].typ = call_arg.expr.obj.typ
 			}
 		}
+		// sumtype coercion
+		param_type_sym := c.table.sym(param.typ)
+		if param_type_sym.kind == .placeholder {
+			base_type := c.table.find_type(param_type_sym.ngname)
+			if base_type != 0 {
+				base_sym := c.table.sym(base_type)
+				if base_sym.kind == .sum_type && base_sym.info is ast.SumType {
+					base_info := base_sym.info as ast.SumType
+					arg_typ_sym := c.table.sym(arg_typ)
+					for variant in base_info.variants {
+						variant_sym := c.table.sym(variant)
+						variant_base_name := variant_sym.ngname
+						if variant_base_name == arg_typ_sym.ngname {
+							node.args[i].expr = ast.CastExpr{
+								expr:    call_arg.expr
+								typ:     param.typ
+								typname: c.table.type_to_str(param.typ)
+								pos:     call_arg.expr.pos()
+							}
+							node.args[i].typ = param.typ
+							arg_typ = param.typ
+							break
+						}
+					}
+				}
+			}
+		}
 		arg_typ_sym := c.table.sym(arg_typ)
 		if param.typ.has_flag(.generic) {
 			if arg_typ_sym.kind == .none && !param.typ.has_flag(.option) {
diff --git a/vlib/v/checker/struct.v b/vlib/v/checker/struct.v
index a34899ca5..55a096d75 100644
--- a/vlib/v/checker/struct.v
+++ b/vlib/v/checker/struct.v
@@ -700,6 +700,14 @@ fn (mut c Checker) struct_init(mut node ast.StructInit, is_field_zero_struct_ini
 				mut exp_type := ast.no_type
 				inited_fields << field_name
 				exp_type = field_info.typ
+				if c.inside_generic_struct_init && exp_type.has_flag(.generic) {
+					generic_names := c.cur_struct_generic_types.map(c.table.sym(it).name)
+					if unwrapped := c.table.convert_generic_type(exp_type, generic_names,
+						c.cur_struct_concrete_types)
+					{
+						exp_type = unwrapped
+					}
+				}
 				exp_type_sym := c.table.sym(exp_type)
 				c.expected_type = exp_type
 				got_type = c.expr(mut init_field.expr)
@@ -783,9 +791,35 @@ or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.',
 					}
 				} else if got_type != ast.void_type && got_type_sym.kind != .placeholder
 					&& !exp_type.has_flag(.generic) {
-					c.check_expected(c.unwrap_generic(got_type), c.unwrap_generic(exp_type)) or {
-						c.error('cannot assign to field `${field_info.name}`: ${err.msg()}',
-							init_field.pos)
+					mut needs_sum_type_cast := false
+					if exp_type_sym.kind == .placeholder {
+						base_type := c.table.find_type(exp_type_sym.ngname)
+						if base_type != 0 {
+							base_sym := c.table.sym(base_type)
+							if base_sym.kind == .sum_type && base_sym.info is ast.SumType {
+								base_info := base_sym.info as ast.SumType
+								for variant in base_info.variants {
+									if c.table.sym(variant).ngname == got_type_sym.ngname {
+										needs_sum_type_cast = true
+										break
+									}
+								}
+							}
+						}
+					}
+					if needs_sum_type_cast {
+						init_field.expr = ast.CastExpr{
+							expr:    init_field.expr
+							typ:     exp_type
+							typname: c.table.type_to_str(exp_type)
+							pos:     init_field.expr.pos()
+						}
+						init_field.typ = exp_type
+					} else {
+						c.check_expected(c.unwrap_generic(got_type), c.unwrap_generic(exp_type)) or {
+							c.error('cannot assign to field `${field_info.name}`: ${err.msg()}',
+								init_field.pos)
+						}
 					}
 				}
 				if exp_type.has_flag(.shared_f) {
@@ -936,8 +970,11 @@ or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.',
 		if struct_sym.info.concrete_types.len == 0 {
 			concrete_types := c.infer_struct_generic_types(node.typ, node)
 			if concrete_types.len > 0 {
-				generic_names := struct_sym.info.generic_types.map(c.table.sym(it).name)
-				node.typ = c.table.unwrap_generic_type(node.typ, generic_names, concrete_types)
+				idx := c.table.find_or_register_generic_inst(node.typ, concrete_types)
+				if idx > 0 {
+					node.typ = ast.new_type(idx)
+					c.table.generic_insts_to_concrete()
+				}
 			}
 		} else if struct_sym.info.generic_types.len == struct_sym.info.concrete_types.len {
 			parent_type := struct_sym.info.parent_type
diff --git a/vlib/v/parser/struct.v b/vlib/v/parser/struct.v
index 29e856f60..d2063528e 100644
--- a/vlib/v/parser/struct.v
+++ b/vlib/v/parser/struct.v
@@ -433,6 +433,7 @@ fn (mut p Parser) struct_decl(is_anon bool) ast.StructDecl {
 		language:   language
 		name:       name
 		cname:      util.no_dots(name)
+		ngname:     ast.strip_generic_params(name)
 		mod:        p.mod
 		info:       ast.Struct{
 			scoped_name:   scoped_name
@@ -727,6 +728,7 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 		kind:     .interface
 		name:     interface_name
 		cname:    util.no_dots(interface_name)
+		ngname:   ast.strip_generic_params(interface_name)
 		mod:      p.mod
 		info:     ast.Interface{
 			types:         []
diff --git a/vlib/v/tests/sumtypes/sumtype_type_coercion_test.v b/vlib/v/tests/sumtypes/sumtype_type_coercion_test.v
new file mode 100644
index 000000000..ea95d4549
--- /dev/null
+++ b/vlib/v/tests/sumtypes/sumtype_type_coercion_test.v
@@ -0,0 +1,20 @@
+struct Empty {}
+
+struct Node[T] {
+	value T
+	next  Chain[T]
+}
+
+type Chain[T] = Empty | Node[T]
+
+fn get[T](chain Chain[T]) T {
+	return match chain {
+		Empty { 0 }
+		Node[T] { chain.value }
+	}
+}
+
+fn test_main() {
+	chain := Node{0.2, Empty{}}
+	assert get(chain) == 0.2
+}
-- 
2.39.5