go.tools/go/types: unsafe.Offsetof must deref named pointers to structs

R=adonovan
CC=golang-dev
https://golang.org/cl/11889043

go/types/builtins.go

@@ -340,7 +340,7 @@ func (check *checker) builtin(x *operand, call *ast.CallExpr, bin *Builtin) {
 		if x.mode == invalid {
 			goto Error
 		}
-		base, _ := deref(x.typ)
+		base := derefStructPtr(x.typ)
 		sel := arg.Sel.Name
 		obj, index, indirect := LookupFieldOrMethod(base, check.pkg, arg.Sel.Name)
 		switch obj.(type) {

go/types/lookup.go

@@ -292,7 +292,7 @@ func MissingMethod(typ Type, T *Interface) (method *Func, wrongType bool) {
 	return
 }
 
-// Deref dereferences typ if it is a pointer and returns its base and true.
+// deref dereferences typ if it is a *Pointer and returns its base and true.
 // Otherwise it returns (typ, false).
 func deref(typ Type) (Type, bool) {
 	if p, _ := typ.(*Pointer); p != nil {
@@ -301,6 +301,17 @@ func deref(typ Type) (Type, bool) {
 	return typ, false
 }
 
+// derefStructPtr dereferences typ if it is a (named or unnamed) pointer to a
+// (named or unnamed) struct and returns its base. Otherwise it returns typ.
+func derefStructPtr(typ Type) Type {
+	if p, _ := typ.Underlying().(*Pointer); p != nil {
+		if _, ok := p.base.Underlying().(*Struct); ok {
+			return p.base
+		}
+	}
+	return typ
+}
+
 // concat returns the result of concatenating list and i.
 // The result does not share its underlying array with list.
 func concat(list []int, i int) []int {

go/types/testdata/builtins.src

@@ -373,6 +373,10 @@ func _Offsetof() {
 	var y1p *S1
 	assert(unsafe.Offsetof(y1p.S0) == 32)
 
+	type P *S1
+	var p P = y1p
+	assert(unsafe.Offsetof(p.S0) == 32)
+
 	var y2 S2
 	assert(unsafe.Offsetof(y2.S1) == 0)
 	_ = unsafe.Offsetof(y2 /* ERROR "embedded via a pointer" */ .x)