dajiaohuang · Oct 1, 2026 · Oct 1, 2026 · Oct 1, 2026 · Oct 1, 2026 · Oct 2, 2026
diff --git a/go/types/typeutil/map_test.go b/go/types/typeutil/map_test.go
@@ -0,0 +1,67 @@
+package typeutil
+
+import (
+	"go/types"
+	"testing"
+)
+
+func TestMapNilValue(t *testing.T) {
+	key := types.Typ[types.Int]
+	m := new(Map[*int])
+
+	m.Set(key, nil)
+	if got, ok := m.At(key); !ok || got != nil {
+		t.Fatalf("At(%v) = (%v, %t), want (nil, true)", key, got, ok)
+	}
+
+	called := false
+	m.Iterate(func(gotKey types.Type, gotValue *int) {
+		called = true
+		if gotKey != key || gotValue != nil {
+			t.Errorf("Iterate got (%v, %v), want (%v, nil)", gotKey, gotValue, key)
+		}
+	})
+	if !called {
+		t.Fatal("Iterate did not visit the nil-valued entry")
+	}
+}
+
+func TestMapStringUnwrapsValues(t *testing.T) {
+	key := types.Typ[types.Int]
+	m := new(Map[string])
+	m.Set(key, "value")
+
+	if got, want := m.String(), `{int: "value"}`; got != want {
+		t.Fatalf("String() = %q, want %q", got, want)
+	}
+}
+
+func TestNilMapReadOperations(t *testing.T) {
+	var m *Map[int]
+	key := types.Typ[types.Int]
+
+	if got, ok := m.At(key); ok || got != 0 {
+		t.Fatalf("At(%v) = (%d, %t), want (0, false)", key, got, ok)
+	}
+	if m.Delete(key) {
+		t.Fatal("Delete on nil map returned true")
+	}
+	if m.Len() != 0 {
+		t.Fatalf("Len() = %d, want 0", m.Len())
+	}
+	called := false
+	m.Iterate(func(types.Type, int) { called = true })
+	if called {
+		t.Fatal("Iterate called its callback on a nil map")
+	}
+	if keys := m.Keys(); keys == nil || len(keys) != 0 {
+		t.Fatalf("Keys() = %v, want a non-nil empty slice", keys)
+	}
+	if got := m.String(); got != "{}" {
+		t.Fatalf("String() = %q, want {}", got)
+	}
+	if got := m.KeysString(); got != "{}" {
+		t.Fatalf("KeysString() = %q, want {}", got)
+	}
+	m.SetHasher(MakeHasher())
+}
diff --git a/go/types/typeutil/upstream.go b/go/types/typeutil/upstream.go
@@ -1,8 +1,10 @@
 package typeutil
 
 import (
+	"fmt"
 	"go/ast"
 	"go/types"
+	"strings"
 	_ "unsafe"
 
 	"golang.org/x/tools/go/types/typeutil"
@@ -27,26 +29,68 @@ type Map[V any] struct {
 	m typeutil.Map
 }
 
-func (m *Map[V]) Delete(key types.Type) bool { return m.m.Delete(key) }
+type mapValue[V any] struct{ value V }
+
+func (m *Map[V]) Delete(key types.Type) bool {
+	return m != nil && m.m.Delete(key)
+}
 func (m *Map[V]) At(key types.Type) (V, bool) {
+	if m == nil {
+		var zero V
+		return zero, false
+	}
 	v := m.m.At(key)
 	if v == nil {
 		var zero V
 		return zero, false
-	} else {
-		return v.(V), true
 	}
+	return v.(mapValue[V]).value, true
+}
+func (m *Map[V]) Set(key types.Type, value V) { m.m.Set(key, mapValue[V]{value}) }
+func (m *Map[V]) Len() int {
+	if m == nil {
+		return 0
+	}
+	return m.m.Len()
 }
-func (m *Map[V]) Set(key types.Type, value V) { m.m.Set(key, value) }
-func (m *Map[V]) Len() int                    { return m.m.Len() }
 func (m *Map[V]) Iterate(f func(key types.Type, value V)) {
+	if m == nil {
+		return
+	}
 	ff := func(key types.Type, value any) {
-		f(key, value.(V))
+		f(key, value.(mapValue[V]).value)
 	}
 	m.m.Iterate(ff)
 
 }
-func (m *Map[V]) Keys() []types.Type          { return m.m.Keys() }
-func (m *Map[V]) String() string              { return m.m.String() }
-func (m *Map[V]) KeysString() string          { return m.m.KeysString() }
-func (m *Map[V]) SetHasher(h typeutil.Hasher) { m.m.SetHasher(h) }
+func (m *Map[V]) Keys() []types.Type {
+	if m == nil {
+		return []types.Type{}
+	}
+	return m.m.Keys()
+}
+func (m *Map[V]) String() string {
+	if m == nil {
+		return "{}"
+	}
+	var b strings.Builder
+	b.WriteByte('{')
+	sep := ""
+	m.Iterate(func(key types.Type, value V) {
+		fmt.Fprintf(&b, "%s%v: %q", sep, key, any(value))
+		sep = ", "
+	})
+	b.WriteByte('}')
+	return b.String()
+}
+func (m *Map[V]) KeysString() string {
+	if m == nil {
+		return "{}"
+	}
+	return m.m.KeysString()
+}
+func (m *Map[V]) SetHasher(h typeutil.Hasher) {
+	if m != nil {
+		m.m.SetHasher(h)
+	}
+}