diff --git a/src/cmd/compile/internal/types2/builtins.go b/src/cmd/compile/internal/types2/builtins.go
index 14be24e2514..2af2679d5ed 100644
--- a/src/cmd/compile/internal/types2/builtins.go
+++ b/src/cmd/compile/internal/types2/builtins.go
@@ -624,19 +624,22 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 
 	case _Alignof:
 		// unsafe.Alignof(x T) uintptr
-		if asTypeParam(x.typ) != nil {
-			check.errorf(call, invalidOp+"unsafe.Alignof undefined for %s", x)
-			return
-		}
 		check.assignment(x, nil, "argument to unsafe.Alignof")
 		if x.mode == invalid {
 			return
 		}
 
-		x.mode = constant_
-		x.val = constant.MakeInt64(check.conf.alignof(x.typ))
+		if hasVarSize(x.typ) {
+			x.mode = value
+			if check.Types != nil {
+				check.recordBuiltinType(call.Fun, makeSig(Typ[Uintptr], x.typ))
+			}
+		} else {
+			x.mode = constant_
+			x.val = constant.MakeInt64(check.conf.alignof(x.typ))
+			// result is constant - no need to record signature
+		}
 		x.typ = Typ[Uintptr]
-		// result is constant - no need to record signature
 
 	case _Offsetof:
 		// unsafe.Offsetof(x T) uintptr, where x must be a selector
@@ -674,30 +677,43 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 			return
 		}
 
-		// TODO(gri) Should we pass x.typ instead of base (and indirect report if derefStructPtr indirected)?
+		// TODO(gri) Should we pass x.typ instead of base (and have indirect report if derefStructPtr indirected)?
 		check.recordSelection(selx, FieldVal, base, obj, index, false)
 
-		offs := check.conf.offsetof(base, index)
-		x.mode = constant_
-		x.val = constant.MakeInt64(offs)
+		// The field offset is considered a variable even if the field is declared before
+		// the part of the struct which is variable-sized. This makes both the rules
+		// simpler and also permits (or at least doesn't prevent) a compiler from re-
+		// arranging struct fields if it wanted to.
+		if hasVarSize(base) {
+			x.mode = value
+			if check.Types != nil {
+				check.recordBuiltinType(call.Fun, makeSig(Typ[Uintptr], obj.Type()))
+			}
+		} else {
+			x.mode = constant_
+			x.val = constant.MakeInt64(check.conf.offsetof(base, index))
+			// result is constant - no need to record signature
+		}
 		x.typ = Typ[Uintptr]
-		// result is constant - no need to record signature
 
 	case _Sizeof:
 		// unsafe.Sizeof(x T) uintptr
-		if asTypeParam(x.typ) != nil {
-			check.errorf(call, invalidOp+"unsafe.Sizeof undefined for %s", x)
-			return
-		}
 		check.assignment(x, nil, "argument to unsafe.Sizeof")
 		if x.mode == invalid {
 			return
 		}
 
-		x.mode = constant_
-		x.val = constant.MakeInt64(check.conf.sizeof(x.typ))
+		if hasVarSize(x.typ) {
+			x.mode = value
+			if check.Types != nil {
+				check.recordBuiltinType(call.Fun, makeSig(Typ[Uintptr], x.typ))
+			}
+		} else {
+			x.mode = constant_
+			x.val = constant.MakeInt64(check.conf.sizeof(x.typ))
+			// result is constant - no need to record signature
+		}
 		x.typ = Typ[Uintptr]
-		// result is constant - no need to record signature
 
 	case _Slice:
 		// unsafe.Slice(ptr *T, len IntegerType) []T
@@ -769,6 +785,25 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 	return true
 }
 
+// hasVarSize reports if the size of type t is variable due to type parameters.
+func hasVarSize(t Type) bool {
+	switch t := under(t).(type) {
+	case *Array:
+		return hasVarSize(t.elem)
+	case *Struct:
+		for _, f := range t.fields {
+			if hasVarSize(f.typ) {
+				return true
+			}
+		}
+	case *TypeParam:
+		return true
+	case *Named, *Union, *instance, *top:
+		unreachable()
+	}
+	return false
+}
+
 // applyTypeFunc applies f to x. If x is a type parameter,
 // the result is a type parameter constrained by an new
 // interface bound. The type bounds for that interface
diff --git a/src/cmd/compile/internal/types2/builtins_test.go b/src/cmd/compile/internal/types2/builtins_test.go
index 82c786b86ea..52dbba1cb98 100644
--- a/src/cmd/compile/internal/types2/builtins_test.go
+++ b/src/cmd/compile/internal/types2/builtins_test.go
@@ -7,6 +7,7 @@ package types2_test
 import (
 	"cmd/compile/internal/syntax"
 	"fmt"
+	"strings"
 	"testing"
 
 	. "cmd/compile/internal/types2"
@@ -111,12 +112,15 @@ var builtinCalls = []struct {
 
 	{"Alignof", `_ = unsafe.Alignof(0)`, `invalid type`},                 // constant
 	{"Alignof", `var x struct{}; _ = unsafe.Alignof(x)`, `invalid type`}, // constant
+	{"Alignof", `var x P; _ = unsafe.Alignof(x)`, `func(p.P₁) uintptr`},
 
 	{"Offsetof", `var x struct{f bool}; _ = unsafe.Offsetof(x.f)`, `invalid type`},           // constant
 	{"Offsetof", `var x struct{_ int; f bool}; _ = unsafe.Offsetof((&x).f)`, `invalid type`}, // constant
+	{"Offsetof", `var x struct{_ int; f P}; _ = unsafe.Offsetof((&x).f)`, `func(p.P₁) uintptr`},
 
 	{"Sizeof", `_ = unsafe.Sizeof(0)`, `invalid type`},                 // constant
 	{"Sizeof", `var x struct{}; _ = unsafe.Sizeof(x)`, `invalid type`}, // constant
+	{"Sizeof", `var x P; _ = unsafe.Sizeof(x)`, `func(p.P₁) uintptr`},
 
 	{"Slice", `var p *int; _ = unsafe.Slice(p, 1)`, `func(*int, int) []int`},
 	{"Slice", `var p *byte; var n uintptr; _ = unsafe.Slice(p, n)`, `func(*byte, uintptr) []byte`},
@@ -149,9 +153,14 @@ func TestBuiltinSignatures(t *testing.T) {
 	}
 }
 
+func parseGenericSrc(path, src string) (*syntax.File, error) {
+	errh := func(error) {} // dummy error handler so that parsing continues in presence of errors
+	return syntax.Parse(syntax.NewFileBase(path), strings.NewReader(src), errh, nil, syntax.AllowGenerics)
+}
+
 func testBuiltinSignature(t *testing.T, name, src0, want string) {
-	src := fmt.Sprintf(`package p; import "unsafe"; type _ unsafe.Pointer /* use unsafe */; func _() { %s }`, src0)
-	f, err := parseSrc("", src)
+	src := fmt.Sprintf(`package p; import "unsafe"; type _ unsafe.Pointer /* use unsafe */; func _[P any]() { %s }`, src0)
+	f, err := parseGenericSrc("", src)
 	if err != nil {
 		t.Errorf("%s: %s", src0, err)
 		return
diff --git a/src/cmd/compile/internal/types2/infer.go b/src/cmd/compile/internal/types2/infer.go
index e5d94e44d97..b44ff7377a4 100644
--- a/src/cmd/compile/internal/types2/infer.go
+++ b/src/cmd/compile/internal/types2/infer.go
@@ -401,8 +401,8 @@ func (check *Checker) inferB(tparams []*TypeName, targs []Type, report bool) (ty
 
 	// u.x.types() now contains the incoming type arguments plus any additional type
 	// arguments for which there were structural constraints. The newly inferred non-
-	// nil entries may still contain references to other type parameters. For instance,
-	// for [A any, B interface{type []C}, C interface{type *A}], if A == int
+	// nil entries may still contain references to other type parameters.
+	// For instance, for [A any, B interface{ []C }, C interface{ *A }], if A == int
 	// was given, unification produced the type list [int, []C, *A]. We eliminate the
 	// remaining type parameters by substituting the type parameters in this type list
 	// until nothing changes anymore.
diff --git a/src/cmd/compile/internal/types2/sizes.go b/src/cmd/compile/internal/types2/sizes.go
index cb789598e57..6a3d19d8eaf 100644
--- a/src/cmd/compile/internal/types2/sizes.go
+++ b/src/cmd/compile/internal/types2/sizes.go
@@ -48,7 +48,7 @@ type StdSizes struct {
 func (s *StdSizes) Alignof(T Type) int64 {
 	// For arrays and structs, alignment is defined in terms
 	// of alignment of the elements and fields, respectively.
-	switch t := optype(T).(type) {
+	switch t := under(T).(type) {
 	case *Array:
 		// spec: "For a variable x of array type: unsafe.Alignof(x)
 		// is the same as unsafe.Alignof(x[0]), but at least 1."
@@ -73,6 +73,8 @@ func (s *StdSizes) Alignof(T Type) int64 {
 		if t.Info()&IsString != 0 {
 			return s.WordSize
 		}
+	case *TypeParam, *Union:
+		unreachable()
 	}
 	a := s.Sizeof(T) // may be 0
 	// spec: "For a variable x of any type: unsafe.Alignof(x) is at least 1."
@@ -118,7 +120,7 @@ var basicSizes = [...]byte{
 }
 
 func (s *StdSizes) Sizeof(T Type) int64 {
-	switch t := optype(T).(type) {
+	switch t := under(T).(type) {
 	case *Basic:
 		assert(isTyped(T))
 		k := t.kind
@@ -148,10 +150,10 @@ func (s *StdSizes) Sizeof(T Type) int64 {
 		}
 		offsets := s.Offsetsof(t.fields)
 		return offsets[n-1] + s.Sizeof(t.fields[n-1].typ)
-	case *Union:
-		panic("Sizeof unimplemented for union")
 	case *Interface:
 		return s.WordSize * 2
+	case *TypeParam, *Union:
+		unreachable()
 	}
 	return s.WordSize // catch-all
 }
diff --git a/src/cmd/compile/internal/types2/testdata/check/builtins.go2 b/src/cmd/compile/internal/types2/testdata/check/builtins.go2
index 8fe6d7b332e..3881090603e 100644
--- a/src/cmd/compile/internal/types2/testdata/check/builtins.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/builtins.go2
@@ -6,6 +6,8 @@
 
 package builtins
 
+import "unsafe"
+
 // close
 
 type C0 interface{ int }
@@ -127,3 +129,108 @@ func _[T Bss]() {
 	_ = make(T, 10)
 	_ = make(T, 10, 20)
 }
+
+// unsafe.Alignof
+
+func _[T comparable]() {
+	var (
+		b int64
+		a [10]T
+		s struct{ f T }
+		p *T
+		l []T
+		f func(T)
+		i interface{ m() T }
+		c chan T
+		m map[T]T
+		t T
+	)
+
+	const bb = unsafe.Alignof(b)
+	assert(bb == 8)
+	const _ = unsafe /* ERROR not constant */ .Alignof(a)
+	const _ = unsafe /* ERROR not constant */ .Alignof(s)
+	const pp = unsafe.Alignof(p)
+	assert(pp == 8)
+	const ll = unsafe.Alignof(l)
+	assert(ll == 8)
+	const ff = unsafe.Alignof(f)
+	assert(ff == 8)
+	const ii = unsafe.Alignof(i)
+	assert(ii == 8)
+	const cc = unsafe.Alignof(c)
+	assert(cc == 8)
+	const mm = unsafe.Alignof(m)
+	assert(mm == 8)
+	const _ = unsafe /* ERROR not constant */ .Alignof(t)
+}
+
+// unsafe.Offsetof
+
+func _[T comparable]() {
+	var (
+		b struct{ _, f int64 }
+		a struct{ _, f [10]T }
+		s struct{ _, f struct{ f T } }
+		p struct{ _, f *T }
+		l struct{ _, f []T }
+		f struct{ _, f func(T) }
+		i struct{ _, f interface{ m() T } }
+		c struct{ _, f chan T }
+		m struct{ _, f map[T]T }
+		t struct{ _, f T }
+	)
+
+	const bb = unsafe.Offsetof(b.f)
+	assert(bb == 8)
+	const _ = unsafe /* ERROR not constant */ .Alignof(a)
+	const _ = unsafe /* ERROR not constant */ .Alignof(s)
+	const pp = unsafe.Offsetof(p.f)
+	assert(pp == 8)
+	const ll = unsafe.Offsetof(l.f)
+	assert(ll == 24)
+	const ff = unsafe.Offsetof(f.f)
+	assert(ff == 8)
+	const ii = unsafe.Offsetof(i.f)
+	assert(ii == 16)
+	const cc = unsafe.Offsetof(c.f)
+	assert(cc == 8)
+	const mm = unsafe.Offsetof(m.f)
+	assert(mm == 8)
+	const _ = unsafe /* ERROR not constant */ .Alignof(t)
+}
+
+// unsafe.Sizeof
+
+func _[T comparable]() {
+	var (
+		b int64
+		a [10]T
+		s struct{ f T }
+		p *T
+		l []T
+		f func(T)
+		i interface{ m() T }
+		c chan T
+		m map[T]T
+		t T
+	)
+
+	const bb = unsafe.Sizeof(b)
+	assert(bb == 8)
+	const _ = unsafe /* ERROR not constant */ .Alignof(a)
+	const _ = unsafe /* ERROR not constant */ .Alignof(s)
+	const pp = unsafe.Sizeof(p)
+	assert(pp == 8)
+	const ll = unsafe.Sizeof(l)
+	assert(ll == 24)
+	const ff = unsafe.Sizeof(f)
+	assert(ff == 8)
+	const ii = unsafe.Sizeof(i)
+	assert(ii == 16)
+	const cc = unsafe.Sizeof(c)
+	assert(cc == 8)
+	const mm = unsafe.Sizeof(m)
+	assert(mm == 8)
+	const _ = unsafe /* ERROR not constant */ .Alignof(t)
+}
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue40301.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue40301.go2
index 5d97855f8a1..c78f9a1fa04 100644
--- a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue40301.go2
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue40301.go2
@@ -7,6 +7,6 @@ package p
 import "unsafe"
 
 func _[T any](x T) {
-	_ = unsafe /* ERROR undefined */ .Alignof(x)
-	_ = unsafe /* ERROR undefined */ .Sizeof(x)
+	_ = unsafe.Alignof(x)
+	_ = unsafe.Sizeof(x)
 }