diff --git a/src/cmd/compile/internal/noder/irgen.go b/src/cmd/compile/internal/noder/irgen.go
index 34201545b5..281f619f6f 100644
--- a/src/cmd/compile/internal/noder/irgen.go
+++ b/src/cmd/compile/internal/noder/irgen.go
@@ -49,9 +49,7 @@ func checkFiles(m posMap, noders []*noder) (*types2.Package, *types2.Info) {
 		IgnoreBranchErrors: true, // parser already checked via syntax.CheckBranches mode
 		Importer:           &importer,
 		Sizes:              types2.SizesFor("gc", buildcfg.GOARCH),
-		// Currently, the compiler panics when using Alias types.
-		// TODO(gri) set to true once this is fixed (issue #66873)
-		EnableAlias: false,
+		EnableAlias:        true,
 	}
 	if base.Flag.ErrorURL {
 		conf.ErrorURL = " [go.dev/e/%s]"
diff --git a/src/cmd/compile/internal/noder/reader.go b/src/cmd/compile/internal/noder/reader.go
index abd07ebb62..a7feadaf6e 100644
--- a/src/cmd/compile/internal/noder/reader.go
+++ b/src/cmd/compile/internal/noder/reader.go
@@ -427,7 +427,9 @@ func (pr *pkgReader) typIdx(info typeInfo, dict *readerDict, wrapped bool) *type
 	r.dict = dict
 
 	typ := r.doTyp()
-	assert(typ != nil)
+	if typ == nil {
+		base.Fatalf("doTyp returned nil for info=%v", info)
+	}
 
 	// For recursive type declarations involving interfaces and aliases,
 	// above r.doTyp() call may have already set pr.typs[idx], so just
@@ -741,7 +743,26 @@ func (pr *pkgReader) objIdxMayFail(idx pkgbits.Index, implicits, explicits []*ty
 
 	case pkgbits.ObjAlias:
 		name := do(ir.OTYPE, false)
-		setType(name, r.typ())
+
+		// Clumsy dance: the r.typ() call here might recursively find this
+		// type alias name, before we've set its type (#66873). So we
+		// temporarily clear sym.Def and then restore it later, if still
+		// unset.
+		hack := sym.Def == name
+		if hack {
+			sym.Def = nil
+		}
+		typ := r.typ()
+		if hack {
+			if sym.Def != nil {
+				name = sym.Def.(*ir.Name)
+				assert(name.Type() == typ)
+				return name, nil
+			}
+			sym.Def = name
+		}
+
+		setType(name, typ)
 		name.SetAlias(true)
 		return name, nil
 
diff --git a/src/cmd/compile/internal/noder/writer.go b/src/cmd/compile/internal/noder/writer.go
index 453b08dbf9..a48e193c5f 100644
--- a/src/cmd/compile/internal/noder/writer.go
+++ b/src/cmd/compile/internal/noder/writer.go
@@ -488,6 +488,18 @@ func (w *writer) typInfo(info typeInfo) {
 // typIdx also reports whether typ is a derived type; that is, whether
 // its identity depends on type parameters.
 func (pw *pkgWriter) typIdx(typ types2.Type, dict *writerDict) typeInfo {
+	// Strip non-global aliases, because they only appear in inline
+	// bodies anyway. Otherwise, they can cause types.Sym collisions
+	// (e.g., "main.C" for both of the local type aliases in
+	// test/fixedbugs/issue50190.go).
+	for {
+		if alias, ok := typ.(*types2.Alias); ok && !isGlobal(alias.Obj()) {
+			typ = alias.Rhs()
+		} else {
+			break
+		}
+	}
+
 	if idx, ok := pw.typsIdx[typ]; ok {
 		return typeInfo{idx: idx, derived: false}
 	}
diff --git a/test/alias2.go b/test/alias2.go
index 2846e5dc31..95eb25a94b 100644
--- a/test/alias2.go
+++ b/test/alias2.go
@@ -47,7 +47,7 @@ var _ T0 = A0{}
 
 // But aliases and original types cannot be used with new types based on them.
 var _ N0 = T0{} // ERROR "cannot use T0{} \(value of type T0\) as N0 value in variable declaration"
-var _ N0 = A0{} // ERROR "cannot use A0{} \(value of type T0\) as N0 value in variable declaration"
+var _ N0 = A0{} // ERROR "cannot use A0{} \(value of type A0\) as N0 value in variable declaration"
 
 var _ A5 = Value{}
 
@@ -83,7 +83,7 @@ func _() {
 	var _ T0 = A0{}
 
 	var _ N0 = T0{} // ERROR "cannot use T0{} \(value of type T0\) as N0 value in variable declaration"
-	var _ N0 = A0{} // ERROR "cannot use A0{} \(value of type T0\) as N0 value in variable declaration"
+	var _ N0 = A0{} // ERROR "cannot use A0{} \(value of type A0\) as N0 value in variable declaration"
 
 	var _ A5 = Value{} // ERROR "cannot use Value{} \(value of type reflect\.Value\) as A5 value in variable declaration"
 }
@@ -92,10 +92,10 @@ func _() {
 
 type _ = reflect.ValueOf // ERROR "reflect.ValueOf .*is not a type|expected type"
 
-func (A1) m() {} // ERROR "cannot define new methods on non-local type int|may not define methods on non-local type"
+func (A1) m() {} // ERROR "cannot define new methods on non-local type|may not define methods on non-local type"
 func (A2) m() {} // ERROR "invalid receiver type"
-func (A3) m() {} // ERROR "cannot define new methods on non-local type reflect.Value|may not define methods on non-local type"
-func (A4) m() {} // ERROR "cannot define new methods on non-local type reflect.Value|may not define methods on non-local type"
+func (A3) m() {} // ERROR "cannot define new methods on non-local type|may not define methods on non-local type"
+func (A4) m() {} // ERROR "cannot define new methods on non-local type|may not define methods on non-local type"
 
 type B1 = struct{}
 
diff --git a/test/fixedbugs/issue66873.go b/test/fixedbugs/issue66873.go
new file mode 100644
index 0000000000..2f49cc933d
--- /dev/null
+++ b/test/fixedbugs/issue66873.go
@@ -0,0 +1,15 @@
+// compile
+
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f(A) {}
+
+type T int
+
+type A = T
+
+func (A) m() {}