diff --git a/compiler/compiler.go b/compiler/compiler.go
index 45e6c8a54b..c537b6c1c2 100644
--- a/compiler/compiler.go
+++ b/compiler/compiler.go
@@ -92,6 +92,7 @@ type compilerContext struct {
 	pkg              *types.Package
 	packageDir       string // directory for this package
 	runtimePkg       *types.Package
+	localTypeNames   typeutil.Map // *types.Named (synthetic local from generic instantiation) -> string
 }
 
 // newCompilerContext returns a new compiler context ready for use, most
@@ -300,6 +301,11 @@ func CompilePackage(moduleName string, pkg *loader.Package, ssaPkg *ssa.Package,
 	// Convert AST to SSA.
 	ssaPkg.Build()
 
+	// Assign names to function-local named types before compiling the
+	// package, so that types declared in different functions (or in
+	// different instantiations of a generic function) do not collide.
+	c.scanLocalTypes(ssaPkg)
+
 	// Initialize debug information.
 	if c.Debug {
 		c.cu = c.dibuilder.CreateCompileUnit(llvm.DICompileUnit{
diff --git a/compiler/interface.go b/compiler/interface.go
index 5f7e7e345b..8d6e2a1d97 100644
--- a/compiler/interface.go
+++ b/compiler/interface.go
@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"go/token"
 	"go/types"
+	"path/filepath"
 	"sort"
 	"strconv"
 	"strings"
@@ -165,7 +166,7 @@ func (c *compilerContext) getTypeCode(typ types.Type) llvm.Value {
 		}
 	}
 
-	typeCodeName, isLocal := getTypeCodeName(typ)
+	typeCodeName, isLocal := c.getTypeCodeName(typ)
 	globalName := "reflect/types.type:" + typeCodeName
 	var global llvm.Value
 	if isLocal {
@@ -580,20 +581,50 @@ var basicTypeNames = [...]string{
 // getTypeCodeName returns a name for this type that can be used in the
 // interface lowering pass to assign type codes as expected by the reflect
 // package. See getTypeCodeNum.
-func getTypeCodeName(t types.Type) (string, bool) {
+//
+// isLocal is true when the type is declared inside a function body.
+// Such types need a per-declaration (or per instantiation) suffix
+// because their printed names are not unique.
+//
+// Ordinary function-local types (TypeName.Parent() != nil) are
+// disambiguated lazily from their declaration position: every
+// declaration in the package has a distinct (file, line, column)
+// triple, and the position is taken un-//line-adjusted so it is
+// stable across builds. Such types are only nameable inside their
+// declaring package, so the name does not need to agree with anything
+// computed in another package.
+//
+// Synthetic locals (TypeName.Parent() == nil), produced by generic
+// instantiation, are pre-registered by scanLocalTypes because their
+// names must agree across packages that materialize the same instance.
+func (c *compilerContext) getTypeCodeName(t types.Type) (name string, isLocal bool) {
 	switch t := types.Unalias(t).(type) {
 	case *types.Named:
-		if t.Obj().Parent() != t.Obj().Pkg().Scope() {
-			return "named:" + t.String() + "$local", true
+		tn := t.Obj()
+		if tn.Pkg() == nil || tn.Parent() == tn.Pkg().Scope() {
+			// Package-scope or builtin: the printed name is unique.
+			return "named:" + t.String(), false
+		}
+		if tn.Parent() != nil {
+			// Ordinary function-local type. Use the un-//line-adjusted
+			// declaration position as the disambiguator.
+			pos := c.program.Fset.PositionFor(tn.Pos(), false)
+			return "named:" + t.String() + "$" + filepath.Base(pos.Filename) + ":" + strconv.Itoa(pos.Line) + ":" + strconv.Itoa(pos.Column), true
+		}
+		// Synthetic local from generic instantiation: must have been
+		// pre-registered by scanLocalTypes.
+		v := c.localTypeNames.At(t)
+		if v == nil {
+			panic("compiler: synthetic local type " + tn.Name() + " was not registered by scanLocalTypes")
 		}
-		return "named:" + t.String(), false
+		return "named:" + v.(string), true
 	case *types.Array:
-		s, isLocal := getTypeCodeName(t.Elem())
+		s, isLocal := c.getTypeCodeName(t.Elem())
 		return "array:" + strconv.FormatInt(t.Len(), 10) + ":" + s, isLocal
 	case *types.Basic:
 		return "basic:" + basicTypeNames[t.Kind()], false
 	case *types.Chan:
-		s, isLocal := getTypeCodeName(t.Elem())
+		s, isLocal := c.getTypeCodeName(t.Elem())
 		var dir string
 		switch t.Dir() {
 		case types.SendOnly:
@@ -613,7 +644,7 @@ func getTypeCodeName(t types.Type) (string, bool) {
 			if !token.IsExported(name) {
 				name = t.Method(i).Pkg().Path() + "." + name
 			}
-			s, local := getTypeCodeName(t.Method(i).Type())
+			s, local := c.getTypeCodeName(t.Method(i).Type())
 			if local {
 				isLocal = true
 			}
@@ -621,17 +652,17 @@ func getTypeCodeName(t types.Type) (string, bool) {
 		}
 		return "interface:" + "{" + strings.Join(methods, ",") + "}", isLocal
 	case *types.Map:
-		keyType, keyLocal := getTypeCodeName(t.Key())
-		elemType, elemLocal := getTypeCodeName(t.Elem())
+		keyType, keyLocal := c.getTypeCodeName(t.Key())
+		elemType, elemLocal := c.getTypeCodeName(t.Elem())
 		return "map:" + "{" + keyType + "," + elemType + "}", keyLocal || elemLocal
 	case *types.Pointer:
-		s, isLocal := getTypeCodeName(t.Elem())
+		s, isLocal := c.getTypeCodeName(t.Elem())
 		return "pointer:" + s, isLocal
 	case *types.Signature:
 		isLocal := false
 		params := make([]string, t.Params().Len())
 		for i := 0; i < t.Params().Len(); i++ {
-			s, local := getTypeCodeName(t.Params().At(i).Type())
+			s, local := c.getTypeCodeName(t.Params().At(i).Type())
 			if local {
 				isLocal = true
 			}
@@ -639,7 +670,7 @@ func getTypeCodeName(t types.Type) (string, bool) {
 		}
 		results := make([]string, t.Results().Len())
 		for i := 0; i < t.Results().Len(); i++ {
-			s, local := getTypeCodeName(t.Results().At(i).Type())
+			s, local := c.getTypeCodeName(t.Results().At(i).Type())
 			if local {
 				isLocal = true
 			}
@@ -647,7 +678,7 @@ func getTypeCodeName(t types.Type) (string, bool) {
 		}
 		return "func:" + "{" + strings.Join(params, ",") + "}{" + strings.Join(results, ",") + "}", isLocal
 	case *types.Slice:
-		s, isLocal := getTypeCodeName(t.Elem())
+		s, isLocal := c.getTypeCodeName(t.Elem())
 		return "slice:" + s, isLocal
 	case *types.Struct:
 		elems := make([]string, t.NumFields())
@@ -657,7 +688,7 @@ func getTypeCodeName(t types.Type) (string, bool) {
 			if t.Field(i).Embedded() {
 				embedded = "#"
 			}
-			s, local := getTypeCodeName(t.Field(i).Type())
+			s, local := c.getTypeCodeName(t.Field(i).Type())
 			if local {
 				isLocal = true
 			}
@@ -672,6 +703,226 @@ func getTypeCodeName(t types.Type) (string, bool) {
 	}
 }
 
+// scanLocalTypes assigns names to every synthetic *types.TypeName
+// (TypeName.Parent() == nil) reachable from this package and stores
+// them in c.localTypeNames.
+//
+// Synthetic TypeNames are produced by generic instantiation: two
+// instantiations of the same generic function (e.g. F[int] and
+// F[string]) produce TypeNames with the same printed name and the
+// same source position, so each is named with the enclosing
+// instance's RelString as prefix. RelString encodes the type
+// arguments, matching Go's runtime behavior, where F[int].Inner and
+// F[string].Inner are distinct types even when Inner does not mention
+// the type parameter.
+//
+// A given instance may be materialized by several packages (the body
+// of F[int] is compiled in every package that calls F[int]); its
+// reflect/types.type:* global has LinkOnceODRLinkage and is merged by
+// name at link time. The chosen name therefore depends only on
+// intrinsic SSA properties (RelString and the raw token.Pos used as a
+// sort key), so any package compiling the same instance produces the
+// same identifier.
+//
+// Ordinary function-local TypeNames (TypeName.Parent() != nil) are
+// not handled here: they are nameable only inside their declaring
+// package, and getTypeCodeName derives a stable per-declaration name
+// for them directly from their source position.
+func (c *compilerContext) scanLocalTypes(ssaPkg *ssa.Package) {
+	// Locate every generic instance reachable from this package
+	// (including instances declared in imported packages and any
+	// function reached through an instance subtree).
+	var instances []*ssa.Function
+	seen := map[*ssa.Function]bool{}
+	var walk func(fn *ssa.Function, inInstance bool)
+	walk = func(fn *ssa.Function, inInstance bool) {
+		if fn == nil || seen[fn] {
+			return
+		}
+		// fn belongs to an instance subtree if it is itself an
+		// instantiation or if we reached it from one.
+		//
+		// len(TypeArgs()) is used instead of fn.Origin() because
+		// Origin() may call Build() on fn's declaring package, which
+		// would defeat per-package compilation.
+		isInstanceRoot := len(fn.TypeArgs()) > 0
+		if !isInstanceRoot && !inInstance && fn.Pkg != ssaPkg {
+			return
+		}
+		if fn.Blocks == nil && fn.AnonFuncs == nil {
+			return
+		}
+		seen[fn] = true
+		isInInstance := inInstance || isInstanceRoot
+		if isInInstance {
+			instances = append(instances, fn)
+		}
+		for _, anon := range fn.AnonFuncs {
+			walk(anon, isInInstance)
+		}
+		var ops [10]*ssa.Value
+		for _, b := range fn.Blocks {
+			for _, instr := range b.Instrs {
+				for _, op := range instr.Operands(ops[:0]) {
+					if op == nil || *op == nil {
+						continue
+					}
+					if callee, ok := (*op).(*ssa.Function); ok {
+						walk(callee, isInInstance)
+					}
+				}
+			}
+		}
+	}
+	for _, member := range ssaPkg.Members {
+		switch m := member.(type) {
+		case *ssa.Function:
+			walk(m, false)
+		case *ssa.Type:
+			mset := c.program.MethodSets.MethodSet(m.Type())
+			for i := 0; i < mset.Len(); i++ {
+				walk(c.program.MethodValue(mset.At(i)), false)
+			}
+			pmset := c.program.MethodSets.MethodSet(types.NewPointer(m.Type()))
+			for i := 0; i < pmset.Len(); i++ {
+				walk(c.program.MethodValue(pmset.At(i)), false)
+			}
+		}
+	}
+
+	// Registration is first-writer-wins (a synthetic TypeName may be
+	// reachable from several instances), so visit instances in a
+	// deterministic order. Pos() is a defensive tiebreaker.
+	sort.Slice(instances, func(i, j int) bool {
+		ri, rj := instances[i].RelString(nil), instances[j].RelString(nil)
+		if ri != rj {
+			return ri < rj
+		}
+		return instances[i].Pos() < instances[j].Pos()
+	})
+	for _, fn := range instances {
+		c.registerSyntheticLocalTypes(fn)
+	}
+}
+
+// registerSyntheticLocalTypes walks every type reachable from fn's
+// body and records each synthetic *types.Named (TypeName.Parent() ==
+// nil) in c.localTypeNames. Each is named with fn.RelString as the
+// owning function plus a per-function counter assigned in source
+// order.
+//
+// First-writer-wins: a *types.Named already present in
+// c.localTypeNames is left alone, so a synthetic type reachable from
+// several instances keeps the name assigned by the first (in
+// scanLocalTypes' deterministic order).
+func (c *compilerContext) registerSyntheticLocalTypes(fn *ssa.Function) {
+	var found []*types.Named
+	seen := map[types.Type]bool{}
+	var visit func(t types.Type)
+	visit = func(t types.Type) {
+		if t == nil || seen[t] {
+			return
+		}
+		seen[t] = true
+		switch t := t.(type) {
+		case *types.Alias:
+			visit(types.Unalias(t))
+		case *types.Named:
+			tn := t.Obj()
+			if tn.Pkg() != nil && tn.Parent() == nil {
+				if c.localTypeNames.At(t) == nil {
+					// Reserve the slot so later calls within this
+					// scanLocalTypes invocation skip it; the final
+					// name is filled in after sorting, before any
+					// getTypeCodeName lookups happen.
+					c.localTypeNames.Set(t, "")
+					found = append(found, t)
+				}
+			}
+			targs := t.TypeArgs()
+			for i := 0; i < targs.Len(); i++ {
+				visit(targs.At(i))
+			}
+			visit(t.Underlying())
+		case *types.Pointer:
+			visit(t.Elem())
+		case *types.Slice:
+			visit(t.Elem())
+		case *types.Array:
+			visit(t.Elem())
+		case *types.Chan:
+			visit(t.Elem())
+		case *types.Map:
+			visit(t.Key())
+			visit(t.Elem())
+		case *types.Struct:
+			for i := 0; i < t.NumFields(); i++ {
+				visit(t.Field(i).Type())
+			}
+		case *types.Signature:
+			if p := t.Params(); p != nil {
+				for i := 0; i < p.Len(); i++ {
+					visit(p.At(i).Type())
+				}
+			}
+			if r := t.Results(); r != nil {
+				for i := 0; i < r.Len(); i++ {
+					visit(r.At(i).Type())
+				}
+			}
+		case *types.Tuple:
+			for i := 0; i < t.Len(); i++ {
+				visit(t.At(i).Type())
+			}
+		case *types.Interface:
+			// A synthetic local type can be reachable only through a
+			// local interface's method signature, so descend into
+			// them. getTypeCodeName encodes those signatures into
+			// the interface's identifier, and the seen map breaks
+			// cycles formed by methods that mention the interface
+			// itself.
+			for i := 0; i < t.NumMethods(); i++ {
+				visit(t.Method(i).Type())
+			}
+		}
+	}
+	for _, p := range fn.Params {
+		visit(p.Type())
+	}
+	for _, fv := range fn.FreeVars {
+		visit(fv.Type())
+	}
+	for _, l := range fn.Locals {
+		visit(l.Type())
+	}
+	var ops [10]*ssa.Value
+	for _, b := range fn.Blocks {
+		for _, instr := range b.Instrs {
+			if v, ok := instr.(ssa.Value); ok {
+				visit(v.Type())
+			}
+			for _, op := range instr.Operands(ops[:0]) {
+				if op != nil && *op != nil {
+					visit((*op).Type())
+				}
+			}
+		}
+	}
+	if len(found) == 0 {
+		return
+	}
+	// Sort by raw token.Pos: this gives a total order on declarations
+	// that is stable across builds and unaffected by //line directives
+	// (which only adjust the human-facing position from Fset.Position).
+	sort.Slice(found, func(i, j int) bool {
+		return found[i].Obj().Pos() < found[j].Obj().Pos()
+	})
+	enclosing := fn.RelString(nil)
+	for i, named := range found {
+		c.localTypeNames.Set(named, enclosing+"."+named.Obj().Name()+"$"+strconv.Itoa(i+1))
+	}
+}
+
 // getTypeMethodSet returns a reference (GEP) to a global method set. This
 // method set should be unreferenced after the interface lowering pass.
 func (c *compilerContext) getTypeMethodSet(typ types.Type) llvm.Value {
@@ -769,7 +1020,7 @@ func (b *builder) createTypeAssert(expr *ssa.TypeAssert) llvm.Value {
 			commaOk = b.createInterfaceTypeAssert(intf, actualTypeNum)
 		}
 	} else {
-		name, _ := getTypeCodeName(expr.AssertedType)
+		name, _ := b.getTypeCodeName(expr.AssertedType)
 		globalName := "reflect/types.typeid:" + name
 		assertedTypeCodeGlobal := b.mod.NamedGlobal(globalName)
 		if assertedTypeCodeGlobal.IsNil() {
@@ -857,7 +1108,7 @@ func (c *compilerContext) getMethodSetValue(methods []*types.Func) llvm.Value {
 		if !token.IsExported(name) {
 			name = method.Pkg().Path() + "." + name
 		}
-		s, _ := getTypeCodeName(method.Type())
+		s, _ := c.getTypeCodeName(method.Type())
 		globalName := "reflect/types.signature:" + name + ":" + s
 		value := c.mod.NamedGlobal(globalName)
 		if value.IsNil() {
@@ -901,7 +1152,7 @@ func (c *compilerContext) getMethodSetValue(methods []*types.Func) llvm.Value {
 // thunk is declared, not defined: it will be defined by the interface lowering
 // pass.
 func (c *compilerContext) getInvokeFunction(instr *ssa.CallCommon) llvm.Value {
-	s, _ := getTypeCodeName(instr.Value.Type().Underlying())
+	s, _ := c.getTypeCodeName(instr.Value.Type().Underlying())
 	fnName := s + "." + instr.Method.Name() + "$invoke"
 	llvmFn := c.mod.NamedFunction(fnName)
 	if llvmFn.IsNil() {
@@ -926,7 +1177,7 @@ func (c *compilerContext) getInvokeFunction(instr *ssa.CallCommon) llvm.Value {
 // by the interface lowering pass as a type-ID comparison chain, avoiding the
 // need for runtime.typeImplementsMethodSet at compile time.
 func (b *builder) createInterfaceTypeAssert(intf *types.Interface, actualType llvm.Value) llvm.Value {
-	s, _ := getTypeCodeName(intf)
+	s, _ := b.getTypeCodeName(intf)
 	fnName := s + ".$typeassert"
 	llvmFn := b.mod.NamedFunction(fnName)
 	if llvmFn.IsNil() {
diff --git a/main_test.go b/main_test.go
index 55eb678910..f7d827eb58 100644
--- a/main_test.go
+++ b/main_test.go
@@ -76,6 +76,8 @@ func TestBuild(t *testing.T) {
 		"init_multi.go",
 		"interface.go",
 		"json.go",
+		"localtypes/",
+		"localtypes.go",
 		"map.go",
 		"math.go",
 		"oldgo/",
diff --git a/testdata/localtypes.go b/testdata/localtypes.go
new file mode 100644
index 0000000000..0ea9f80c7b
--- /dev/null
+++ b/testdata/localtypes.go
@@ -0,0 +1,229 @@
+package main
+
+import "reflect"
+
+type checker = func(any) bool
+
+func siblingA() (any, checker) {
+	type Foo struct{ V int }
+	return Foo{V: 1}, func(x any) bool { _, ok := x.(Foo); return ok }
+}
+
+func siblingB() (any, checker) {
+	type Foo struct{ V int }
+	return Foo{V: 2}, func(x any) bool { _, ok := x.(Foo); return ok }
+}
+
+func nestedScopes() (any, any, any, checker, checker, checker) {
+	var av, bv, cv any
+	var ac, bc, cc checker
+	{
+		type Bar struct{ X int }
+		av = Bar{X: 10}
+		ac = func(x any) bool { _, ok := x.(Bar); return ok }
+	}
+	{
+		type Bar struct{ X int }
+		bv = Bar{X: 20}
+		bc = func(x any) bool { _, ok := x.(Bar); return ok }
+	}
+	{
+		type Bar struct{ X int }
+		cv = Bar{X: 30}
+		cc = func(x any) bool { _, ok := x.(Bar); return ok }
+	}
+	return av, bv, cv, ac, bc, cc
+}
+
+func genericWithLocals[T any]() (any, any, checker, checker) {
+	type UsesT struct{ V T }
+	type NoT struct{ V int }
+	var z T
+	return UsesT{V: z}, NoT{V: 42},
+		func(x any) bool { _, ok := x.(UsesT); return ok },
+		func(x any) bool { _, ok := x.(NoT); return ok }
+}
+
+func siblingClosures() (any, any, checker, checker) {
+	var av, bv any
+	var ac, bc checker
+	func() {
+		type C struct{ V int }
+		av = C{V: 1}
+		ac = func(x any) bool { _, ok := x.(C); return ok }
+	}()
+	func() {
+		type C struct{ V int }
+		bv = C{V: 2}
+		bc = func(x any) bool { _, ok := x.(C); return ok }
+	}()
+	return av, bv, ac, bc
+}
+
+func closureInGenericUsesT[T any]() (any, checker) {
+	var v any
+	var c checker
+	func() {
+		type X struct{ V T }
+		var z T
+		v = X{V: z}
+		c = func(x any) bool { _, ok := x.(X); return ok }
+	}()
+	return v, c
+}
+
+func closureInGenericNoT[T any]() (any, checker) {
+	var v any
+	var c checker
+	func() {
+		type X struct{ V int }
+		v = X{V: 7}
+		c = func(x any) bool { _, ok := x.(X); return ok }
+	}()
+	return v, c
+}
+
+func siblingClosuresInGeneric[T any]() (any, any, checker, checker) {
+	var av, bv any
+	var ac, bc checker
+	func() {
+		type C struct{ V T }
+		var z T
+		av = C{V: z}
+		ac = func(x any) bool { _, ok := x.(C); return ok }
+	}()
+	func() {
+		type C struct{ V T }
+		var z T
+		bv = C{V: z}
+		bc = func(x any) bool { _, ok := x.(C); return ok }
+	}()
+	return av, bv, ac, bc
+}
+
+func doublyNestedInGeneric[T any]() (any, checker) {
+	var v any
+	var c checker
+	func() {
+		func() {
+			type Y struct{ V T }
+			var z T
+			v = Y{V: z}
+			c = func(x any) bool { _, ok := x.(Y); return ok }
+		}()
+	}()
+	return v, c
+}
+
+func expect(name string, ok bool) {
+	if ok {
+		println("ok:", name)
+	} else {
+		println("BUG:", name)
+	}
+}
+
+// issue5180Copy1 and issue5180CopyIgnoreNilMembers are the original
+// repro from https://github.com/tinygo-org/tinygo/issues/5180. Each
+// declares its own local Foo with a different field type, then uses
+// reflect.New to construct a value via the runtime type and asserts it
+// back. Without distinct local-type names the two Foos collide and the
+// second assertion panics.
+func issue5180Copy1() bool {
+	type Foo struct{ A int }
+	f1 := &Foo{}
+	dst := reflect.New(reflect.TypeOf(f1).Elem()).Interface()
+	_, ok := dst.(*Foo)
+	return ok
+}
+
+func issue5180CopyIgnoreNilMembers() (ok bool) {
+	type Foo struct{ A *int }
+	defer func() {
+		if r := recover(); r != nil {
+			ok = false
+		}
+	}()
+	f1 := &Foo{}
+	dst := reflect.New(reflect.TypeOf(f1).Elem()).Interface()
+	_, ok = dst.(*Foo)
+	return ok
+}
+
+func main() {
+	expect("issue5180 TestCopy1", issue5180Copy1())
+	expect("issue5180 TestCopyIgnoreNilMembers", issue5180CopyIgnoreNilMembers())
+
+	// Two siblings with same-named local types are distinct.
+	aV, aC := siblingA()
+	bV, bC := siblingB()
+	expect("siblingA.Foo accepts own", aC(aV))
+	expect("siblingB.Foo accepts own", bC(bV))
+	expect("siblingA.Foo rejects siblingB.Foo", !aC(bV))
+	expect("siblingB.Foo rejects siblingA.Foo", !bC(aV))
+
+	// Three sibling-scope locals in one function are mutually distinct.
+	n1V, n2V, n3V, n1C, n2C, n3C := nestedScopes()
+	expect("nestedScopes.Bar#1 accepts own", n1C(n1V))
+	expect("nestedScopes.Bar#2 accepts own", n2C(n2V))
+	expect("nestedScopes.Bar#3 accepts own", n3C(n3V))
+	expect("nestedScopes.Bar#1 rejects #2", !n1C(n2V))
+	expect("nestedScopes.Bar#2 rejects #3", !n2C(n3V))
+	expect("nestedScopes.Bar#1 rejects #3", !n1C(n3V))
+
+	// Generic instantiations are distinct from each other but match
+	// themselves across calls.
+	iU, iN, iUC, iNC := genericWithLocals[int]()
+	sU, sN, sUC, sNC := genericWithLocals[string]()
+	iU2, iN2, _, _ := genericWithLocals[int]()
+	expect("genericWithLocals[int].UsesT accepts own", iUC(iU))
+	expect("genericWithLocals[int].NoT accepts own", iNC(iN))
+	expect("genericWithLocals[string].UsesT accepts own", sUC(sU))
+	expect("genericWithLocals[string].NoT accepts own", sNC(sN))
+	expect("genericWithLocals[int].UsesT rejects [string].UsesT", !iUC(sU))
+	expect("genericWithLocals[int].NoT rejects [string].NoT", !iNC(sN))
+	expect("genericWithLocals[string].UsesT rejects [int].UsesT", !sUC(iU))
+	expect("genericWithLocals[string].NoT rejects [int].NoT", !sNC(iN))
+	expect("genericWithLocals[int].UsesT matches across calls", iUC(iU2))
+	expect("genericWithLocals[int].NoT matches across calls", iNC(iN2))
+
+	// Sibling closures in a non-generic function.
+	scA, scB, scAC, scBC := siblingClosures()
+	expect("siblingClosures.C#1 accepts own", scAC(scA))
+	expect("siblingClosures.C#2 accepts own", scBC(scB))
+	expect("siblingClosures.C#1 rejects C#2", !scAC(scB))
+	expect("siblingClosures.C#2 rejects C#1", !scBC(scA))
+
+	// Closure inside generic function, type uses T.
+	cgi, cgiC := closureInGenericUsesT[int]()
+	cgs, cgsC := closureInGenericUsesT[string]()
+	expect("closureInGenericUsesT[int].X accepts own", cgiC(cgi))
+	expect("closureInGenericUsesT[string].X accepts own", cgsC(cgs))
+	expect("closureInGenericUsesT[int].X rejects [string].X", !cgiC(cgs))
+	expect("closureInGenericUsesT[string].X rejects [int].X", !cgsC(cgi))
+
+	// Closure inside generic function, type does not use T.
+	cni, cniC := closureInGenericNoT[int]()
+	cns, cnsC := closureInGenericNoT[string]()
+	expect("closureInGenericNoT[int].X accepts own", cniC(cni))
+	expect("closureInGenericNoT[string].X accepts own", cnsC(cns))
+	expect("closureInGenericNoT[int].X rejects [string].X", !cniC(cns))
+	expect("closureInGenericNoT[string].X rejects [int].X", !cnsC(cni))
+
+	// Sibling closures inside a generic function.
+	sgIA, sgIB, sgIAC, sgIBC := siblingClosuresInGeneric[int]()
+	sgSA, _, sgSAC, _ := siblingClosuresInGeneric[string]()
+	expect("siblingClosuresInGeneric[int].C#1 accepts own", sgIAC(sgIA))
+	expect("siblingClosuresInGeneric[int].C#2 accepts own", sgIBC(sgIB))
+	expect("siblingClosuresInGeneric[int].C#1 rejects C#2", !sgIAC(sgIB))
+	expect("siblingClosuresInGeneric[int].C#1 rejects [string].C#1", !sgIAC(sgSA))
+	expect("siblingClosuresInGeneric[string].C#1 rejects [int].C#1", !sgSAC(sgIA))
+
+	// Doubly-nested closure inside a generic function.
+	dni, dniC := doublyNestedInGeneric[int]()
+	dns, dnsC := doublyNestedInGeneric[string]()
+	expect("doublyNestedInGeneric[int].Y accepts own", dniC(dni))
+	expect("doublyNestedInGeneric[string].Y accepts own", dnsC(dns))
+	expect("doublyNestedInGeneric[int].Y rejects [string].Y", !dniC(dns))
+	expect("doublyNestedInGeneric[string].Y rejects [int].Y", !dnsC(dni))
+}
diff --git a/testdata/localtypes.txt b/testdata/localtypes.txt
new file mode 100644
index 0000000000..f9253c32d1
--- /dev/null
+++ b/testdata/localtypes.txt
@@ -0,0 +1,43 @@
+ok: issue5180 TestCopy1
+ok: issue5180 TestCopyIgnoreNilMembers
+ok: siblingA.Foo accepts own
+ok: siblingB.Foo accepts own
+ok: siblingA.Foo rejects siblingB.Foo
+ok: siblingB.Foo rejects siblingA.Foo
+ok: nestedScopes.Bar#1 accepts own
+ok: nestedScopes.Bar#2 accepts own
+ok: nestedScopes.Bar#3 accepts own
+ok: nestedScopes.Bar#1 rejects #2
+ok: nestedScopes.Bar#2 rejects #3
+ok: nestedScopes.Bar#1 rejects #3
+ok: genericWithLocals[int].UsesT accepts own
+ok: genericWithLocals[int].NoT accepts own
+ok: genericWithLocals[string].UsesT accepts own
+ok: genericWithLocals[string].NoT accepts own
+ok: genericWithLocals[int].UsesT rejects [string].UsesT
+ok: genericWithLocals[int].NoT rejects [string].NoT
+ok: genericWithLocals[string].UsesT rejects [int].UsesT
+ok: genericWithLocals[string].NoT rejects [int].NoT
+ok: genericWithLocals[int].UsesT matches across calls
+ok: genericWithLocals[int].NoT matches across calls
+ok: siblingClosures.C#1 accepts own
+ok: siblingClosures.C#2 accepts own
+ok: siblingClosures.C#1 rejects C#2
+ok: siblingClosures.C#2 rejects C#1
+ok: closureInGenericUsesT[int].X accepts own
+ok: closureInGenericUsesT[string].X accepts own
+ok: closureInGenericUsesT[int].X rejects [string].X
+ok: closureInGenericUsesT[string].X rejects [int].X
+ok: closureInGenericNoT[int].X accepts own
+ok: closureInGenericNoT[string].X accepts own
+ok: closureInGenericNoT[int].X rejects [string].X
+ok: closureInGenericNoT[string].X rejects [int].X
+ok: siblingClosuresInGeneric[int].C#1 accepts own
+ok: siblingClosuresInGeneric[int].C#2 accepts own
+ok: siblingClosuresInGeneric[int].C#1 rejects C#2
+ok: siblingClosuresInGeneric[int].C#1 rejects [string].C#1
+ok: siblingClosuresInGeneric[string].C#1 rejects [int].C#1
+ok: doublyNestedInGeneric[int].Y accepts own
+ok: doublyNestedInGeneric[string].Y accepts own
+ok: doublyNestedInGeneric[int].Y rejects [string].Y
+ok: doublyNestedInGeneric[string].Y rejects [int].Y
diff --git a/testdata/localtypes/go.mod b/testdata/localtypes/go.mod
new file mode 100644
index 0000000000..fe996758fa
--- /dev/null
+++ b/testdata/localtypes/go.mod
@@ -0,0 +1,3 @@
+module github.com/tinygo-org/tinygo/testdata/localtypes
+
+go 1.21
diff --git a/testdata/localtypes/lib/lib.go b/testdata/localtypes/lib/lib.go
new file mode 100644
index 0000000000..404328a192
--- /dev/null
+++ b/testdata/localtypes/lib/lib.go
@@ -0,0 +1,28 @@
+package lib
+
+type Checker = func(any) bool
+
+func GenericWithLocals[T any]() (any, any, Checker, Checker) {
+	type UsesT struct{ V T }
+	type NoT struct{ V int }
+	var z T
+	return UsesT{V: z}, NoT{V: 42},
+		func(x any) bool { _, ok := x.(UsesT); return ok },
+		func(x any) bool { _, ok := x.(NoT); return ok }
+}
+
+func SiblingClosures() (any, any, Checker, Checker) {
+	var av, bv any
+	var ac, bc Checker
+	func() {
+		type Foo struct{ V int }
+		av = Foo{V: 1}
+		ac = func(x any) bool { _, ok := x.(Foo); return ok }
+	}()
+	func() {
+		type Foo struct{ V int }
+		bv = Foo{V: 2}
+		bc = func(x any) bool { _, ok := x.(Foo); return ok }
+	}()
+	return av, bv, ac, bc
+}
diff --git a/testdata/localtypes/lib2/lib2.go b/testdata/localtypes/lib2/lib2.go
new file mode 100644
index 0000000000..5aecb2316b
--- /dev/null
+++ b/testdata/localtypes/lib2/lib2.go
@@ -0,0 +1,5 @@
+package lib2
+
+import "github.com/tinygo-org/tinygo/testdata/localtypes/lib"
+
+func IntPair() (any, any, lib.Checker, lib.Checker) { return lib.GenericWithLocals[int]() }
diff --git a/testdata/localtypes/main.go b/testdata/localtypes/main.go
new file mode 100644
index 0000000000..880e6dc29a
--- /dev/null
+++ b/testdata/localtypes/main.go
@@ -0,0 +1,40 @@
+package main
+
+import (
+	"github.com/tinygo-org/tinygo/testdata/localtypes/lib"
+	"github.com/tinygo-org/tinygo/testdata/localtypes/lib2"
+)
+
+func expect(name string, ok bool) {
+	if ok {
+		println("ok:", name)
+	} else {
+		println("BUG:", name)
+	}
+}
+
+func main() {
+	mainU, mainN, mainUC, mainNC := lib.GenericWithLocals[int]()
+	libU, libN, libUC, libNC := lib2.IntPair()
+
+	// Same instance compiled in two packages: each package's checker
+	// must accept the other package's value.
+	expect("main GenericWithLocals[int].UsesT accepts lib2's value", mainUC(libU))
+	expect("main GenericWithLocals[int].NoT accepts lib2's value", mainNC(libN))
+	expect("lib2 GenericWithLocals[int].UsesT accepts main's value", libUC(mainU))
+	expect("lib2 GenericWithLocals[int].NoT accepts main's value", libNC(mainN))
+
+	// Different instantiations: distinct types.
+	stringU, stringN, stringUC, stringNC := lib.GenericWithLocals[string]()
+	expect("GenericWithLocals[int].UsesT rejects [string].UsesT", !mainUC(stringU))
+	expect("GenericWithLocals[int].NoT rejects [string].NoT", !mainNC(stringN))
+	expect("GenericWithLocals[string].UsesT rejects [int].UsesT", !stringUC(mainU))
+	expect("GenericWithLocals[string].NoT rejects [int].NoT", !stringNC(mainN))
+
+	// Sibling closures inside one function (declared in lib).
+	scA, scB, scAC, scBC := lib.SiblingClosures()
+	expect("lib.SiblingClosures.Foo#1 accepts own", scAC(scA))
+	expect("lib.SiblingClosures.Foo#2 accepts own", scBC(scB))
+	expect("lib.SiblingClosures.Foo#1 rejects Foo#2", !scAC(scB))
+	expect("lib.SiblingClosures.Foo#2 rejects Foo#1", !scBC(scA))
+}
diff --git a/testdata/localtypes/out.txt b/testdata/localtypes/out.txt
new file mode 100644
index 0000000000..52266e57b3
--- /dev/null
+++ b/testdata/localtypes/out.txt
@@ -0,0 +1,12 @@
+ok: main GenericWithLocals[int].UsesT accepts lib2's value
+ok: main GenericWithLocals[int].NoT accepts lib2's value
+ok: lib2 GenericWithLocals[int].UsesT accepts main's value
+ok: lib2 GenericWithLocals[int].NoT accepts main's value
+ok: GenericWithLocals[int].UsesT rejects [string].UsesT
+ok: GenericWithLocals[int].NoT rejects [string].NoT
+ok: GenericWithLocals[string].UsesT rejects [int].UsesT
+ok: GenericWithLocals[string].NoT rejects [int].NoT
+ok: lib.SiblingClosures.Foo#1 accepts own
+ok: lib.SiblingClosures.Foo#2 accepts own
+ok: lib.SiblingClosures.Foo#1 rejects Foo#2
+ok: lib.SiblingClosures.Foo#2 rejects Foo#1