cmd/compile/internal/types2: report argument type for unsafe.OffsetOf

Before parameterized types, unsafe.OffsetOf was always evaluating to
a constant. With parameterized types, the result may be a run-time
value, and unsafe.OffsetOf(x.f) is a call that is recorded. Also
record the argument x.f.

Fixes #47895.

Change-Id: Ia3da25028d4865d7295ce7990c7216bffe9e7c72
Reviewed-on: https://go-review.googlesource.com/c/go/+/344252
Trust: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

src/cmd/compile/internal/types2/api_test.go

@@ -346,6 +346,9 @@ func TestTypesInfo(t *testing.T) {
 
 		// issue 45096
 		{genericPkg + `issue45096; func _[T interface{ ~int8 | ~int16 | ~int32 }](x T) { _ = x < 0 }`, `0`, `generic_issue45096.T₁`},
+
+		// issue 47895
+		{`package p; import "unsafe"; type S struct { f int }; var s S; var _ = unsafe.Offsetof(s.f)`, `s.f`, `int`},
 	}
 
 	for _, test := range tests {

src/cmd/compile/internal/types2/builtins.go

@@ -662,6 +662,15 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 		// 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)
 
+		// record the selector expression (was bug - issue #47895)
+		{
+			mode := value
+			if x.mode == variable || indirect {
+				mode = variable
+			}
+			check.record(&operand{mode, selx, obj.Type(), nil, 0})
+		}
+
 		// 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-