diff --git a/internal/gcimporter/gcimporter_test.go b/internal/gcimporter/gcimporter_test.go
index 95cc36c..b93cd2a 100644
--- a/internal/gcimporter/gcimporter_test.go
+++ b/internal/gcimporter/gcimporter_test.go
@@ -190,6 +190,7 @@
 			"nested.go":     "fails to compile",              // TODO(rfindley): investigate this.
 			"issue47631.go": "can not handle local type declarations",
 			"issue55101.go": "fails to compile",
+			"issue50259.go": "compiler crashes if GODEBUG=gotypesalias=1", // TODO(adonovan): delete when #66550 is fixed.
 		}
 	}
 
diff --git a/internal/gcimporter/iexport.go b/internal/gcimporter/iexport.go
index 638fc1d..683bd73 100644
--- a/internal/gcimporter/iexport.go
+++ b/internal/gcimporter/iexport.go
@@ -21,6 +21,7 @@
 	"sort"
 	"strconv"
 	"strings"
+	"unsafe"
 
 	"golang.org/x/tools/go/types/objectpath"
 	"golang.org/x/tools/internal/aliases"
@@ -464,7 +465,7 @@
 
 	switch obj := obj.(type) {
 	case *types.Var:
-		w.tag('V')
+		w.tag(varTag)
 		w.pos(obj.Pos())
 		w.typ(obj.Type(), obj.Pkg())
 
@@ -482,9 +483,9 @@
 
 		// Function.
 		if sig.TypeParams().Len() == 0 {
-			w.tag('F')
+			w.tag(funcTag)
 		} else {
-			w.tag('G')
+			w.tag(genericFuncTag)
 		}
 		w.pos(obj.Pos())
 		// The tparam list of the function type is the declaration of the type
@@ -500,7 +501,7 @@
 		w.signature(sig)
 
 	case *types.Const:
-		w.tag('C')
+		w.tag(constTag)
 		w.pos(obj.Pos())
 		w.value(obj.Type(), obj.Val())
 
@@ -508,7 +509,7 @@
 		t := obj.Type()
 
 		if tparam, ok := aliases.Unalias(t).(*types.TypeParam); ok {
-			w.tag('P')
+			w.tag(typeParamTag)
 			w.pos(obj.Pos())
 			constraint := tparam.Constraint()
 			if p.version >= iexportVersionGo1_18 {
@@ -523,8 +524,13 @@
 		}
 
 		if obj.IsAlias() {
-			w.tag('A')
+			w.tag(aliasTag)
 			w.pos(obj.Pos())
+			if alias, ok := t.(*aliases.Alias); ok {
+				// Preserve materialized aliases,
+				// even of non-exported types.
+				t = aliasRHS(alias)
+			}
 			w.typ(t, obj.Pkg())
 			break
 		}
@@ -536,9 +542,9 @@
 		}
 
 		if named.TypeParams().Len() == 0 {
-			w.tag('T')
+			w.tag(typeTag)
 		} else {
-			w.tag('U')
+			w.tag(genericTypeTag)
 		}
 		w.pos(obj.Pos())
 
@@ -548,7 +554,7 @@
 			w.tparamList(obj.Name(), named.TypeParams(), obj.Pkg())
 		}
 
-		underlying := obj.Type().Underlying()
+		underlying := named.Underlying()
 		w.typ(underlying, obj.Pkg())
 
 		if types.IsInterface(t) {
@@ -739,7 +745,10 @@
 		}()
 	}
 	switch t := t.(type) {
-	// TODO(adonovan): support types.Alias.
+	case *aliases.Alias:
+		// TODO(adonovan): support parameterized aliases, following *types.Named.
+		w.startType(aliasType)
+		w.qualifiedType(t.Obj())
 
 	case *types.Named:
 		if targs := t.TypeArgs(); targs.Len() > 0 {
@@ -1322,3 +1331,19 @@
 func internalErrorf(format string, args ...interface{}) error {
 	return internalError(fmt.Sprintf(format, args...))
 }
+
+// aliasRHS removes exactly one Alias constructor.
+func aliasRHS(alias *aliases.Alias) types.Type {
+	// TODO(adonovan): if proposal #66559 is accepted, this will
+	// become Alias.RHS(alias). In the meantime, we must punch
+	// through the drywall.
+	type go123Alias struct {
+		_   *types.TypeName
+		_   *types.TypeParamList
+		RHS types.Type
+		_   types.Type
+	}
+	var raw *go123Alias
+	*(**aliases.Alias)(unsafe.Pointer(&raw)) = alias
+	return raw.RHS
+}
diff --git a/internal/gcimporter/iexport_go118_test.go b/internal/gcimporter/iexport_go118_test.go
index c748fb3..3aed235 100644
--- a/internal/gcimporter/iexport_go118_test.go
+++ b/internal/gcimporter/iexport_go118_test.go
@@ -123,6 +123,12 @@
 				t.Fatal(err)
 			}
 
+			// TODO(adonovan): delete when #66550 is fixed.
+			if strings.Contains(os.Getenv("GODEBUG"), "gotypesalias=1") &&
+				entry.Name() == "issue50259.go" {
+				t.Skip("Skipping test of defined<->alias cycle under gotypesaliases=1 (#66550)")
+			}
+
 			if !bytes.HasPrefix(src, []byte("// run")) && !bytes.HasPrefix(src, []byte("// compile")) {
 				// We're bypassing the logic of run.go here, so be conservative about
 				// the files we consider in an attempt to make this test more robust to
diff --git a/internal/gcimporter/iexport_test.go b/internal/gcimporter/iexport_test.go
index 4ee79da..0da2599 100644
--- a/internal/gcimporter/iexport_test.go
+++ b/internal/gcimporter/iexport_test.go
@@ -32,6 +32,7 @@
 	"golang.org/x/tools/go/buildutil"
 	"golang.org/x/tools/go/gcexportdata"
 	"golang.org/x/tools/go/loader"
+	"golang.org/x/tools/internal/aliases"
 	"golang.org/x/tools/internal/gcimporter"
 	"golang.org/x/tools/internal/testenv"
 	"golang.org/x/tools/internal/typeparams/genericfeatures"
@@ -145,6 +146,7 @@
 		t.Errorf("Loaded only %d packages, want at least %d", numPkgs, want)
 	}
 
+	// TODO(adonovan): opt: parallelize this slow loop.
 	for _, pkg := range sorted {
 		if exportdata, err := iexport(conf.Fset, version, pkg); err != nil {
 			t.Error(err)
@@ -333,14 +335,15 @@
 		if xalias, yalias := x.IsAlias(), y.(*types.TypeName).IsAlias(); xalias != yalias {
 			return fmt.Errorf("mismatching IsAlias(): %s vs %s", x, y)
 		}
+
 		// equalType does not recurse into the underlying types of named types, so
 		// we must pass the underlying type explicitly here. However, in doing this
 		// we may skip checking the features of the named types themselves, in
 		// situations where the type name is not referenced by the underlying or
 		// any other top-level declarations. Therefore, we must explicitly compare
 		// named types here, before passing their underlying types into equalType.
-		xn, _ := xt.(*types.Named)
-		yn, _ := yt.(*types.Named)
+		xn, _ := aliases.Unalias(xt).(*types.Named)
+		yn, _ := aliases.Unalias(yt).(*types.Named)
 		if (xn == nil) != (yn == nil) {
 			return fmt.Errorf("mismatching types: %T vs %T", xt, yt)
 		}
diff --git a/internal/gcimporter/iimport.go b/internal/gcimporter/iimport.go
index 4d50eb8..2732121 100644
--- a/internal/gcimporter/iimport.go
+++ b/internal/gcimporter/iimport.go
@@ -80,6 +80,20 @@
 	typeParamType
 	instanceType
 	unionType
+	aliasType
+)
+
+// Object tags
+const (
+	varTag          = 'V'
+	funcTag         = 'F'
+	genericFuncTag  = 'G'
+	constTag        = 'C'
+	aliasTag        = 'A'
+	genericAliasTag = 'B'
+	typeParamTag    = 'P'
+	typeTag         = 'T'
+	genericTypeTag  = 'U'
 )
 
 // IImportData imports a package from the serialized package data
@@ -324,7 +338,7 @@
 	}
 
 	// SetConstraint can't be called if the constraint type is not yet complete.
-	// When type params are created in the 'P' case of (*importReader).obj(),
+	// When type params are created in the typeParamTag case of (*importReader).obj(),
 	// the associated constraint type may not be complete due to recursion.
 	// Therefore, we defer calling SetConstraint there, and call it here instead
 	// after all types are complete.
@@ -546,25 +560,29 @@
 	pos := r.pos()
 
 	switch tag {
-	case 'A':
+	case aliasTag:
 		typ := r.typ()
+		// TODO(adonovan): support generic aliases:
+		// if tag == genericAliasTag {
+		// 	tparams := r.tparamList()
+		// 	alias.SetTypeParams(tparams)
+		// }
+		r.declare(aliases.NewAlias(pos, r.currPkg, name, typ))
 
-		r.declare(types.NewTypeName(pos, r.currPkg, name, typ))
-
-	case 'C':
+	case constTag:
 		typ, val := r.value()
 
 		r.declare(types.NewConst(pos, r.currPkg, name, typ, val))
 
-	case 'F', 'G':
+	case funcTag, genericFuncTag:
 		var tparams []*types.TypeParam
-		if tag == 'G' {
+		if tag == genericFuncTag {
 			tparams = r.tparamList()
 		}
 		sig := r.signature(nil, nil, tparams)
 		r.declare(types.NewFunc(pos, r.currPkg, name, sig))
 
-	case 'T', 'U':
+	case typeTag, genericTypeTag:
 		// Types can be recursive. We need to setup a stub
 		// declaration before recursing.
 		obj := types.NewTypeName(pos, r.currPkg, name, nil)
@@ -572,7 +590,7 @@
 		// Declare obj before calling r.tparamList, so the new type name is recognized
 		// if used in the constraint of one of its own typeparams (see #48280).
 		r.declare(obj)
-		if tag == 'U' {
+		if tag == genericTypeTag {
 			tparams := r.tparamList()
 			named.SetTypeParams(tparams)
 		}
@@ -604,7 +622,7 @@
 			}
 		}
 
-	case 'P':
+	case typeParamTag:
 		// We need to "declare" a typeparam in order to have a name that
 		// can be referenced recursively (if needed) in the type param's
 		// bound.
@@ -637,7 +655,7 @@
 		// completely set up all types in ImportData.
 		r.p.later = append(r.p.later, setConstraintArgs{t: t, constraint: constraint})
 
-	case 'V':
+	case varTag:
 		typ := r.typ()
 
 		r.declare(types.NewVar(pos, r.currPkg, name, typ))
@@ -854,7 +872,7 @@
 		errorf("unexpected kind tag in %q: %v", r.p.ipath, k)
 		return nil
 
-	case definedType:
+	case aliasType, definedType:
 		pkg, name := r.qualifiedIdent()
 		r.p.doDecl(pkg, name)
 		return pkg.Scope().Lookup(name).(*types.TypeName).Type()
diff --git a/internal/gcimporter/ureader_yes.go b/internal/gcimporter/ureader_yes.go
index f4edc46..b3be452 100644
--- a/internal/gcimporter/ureader_yes.go
+++ b/internal/gcimporter/ureader_yes.go
@@ -524,7 +524,7 @@
 		case pkgbits.ObjAlias:
 			pos := r.pos()
 			typ := r.typ()
-			declare(types.NewTypeName(pos, objPkg, objName, typ))
+			declare(aliases.NewAlias(pos, objPkg, objName, typ))
 
 		case pkgbits.ObjConst:
 			pos := r.pos()