cmd/vet: don't treat fields like variables in rangeloop check

Fixes #13236

Change-Id: If902ac66718e0a0790fab9835921ce4ef980965b
Reviewed-on: https://go-review.googlesource.com/21183
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

Author: dominikh

src/cmd/vet/rangeloop.go

@@ -62,6 +62,10 @@ func checkRangeLoop(f *File, node ast.Node) {
 		if !ok || id.Obj == nil {
 			return true
 		}
+		if f.pkg.types[id].Type == nil {
+			// Not referring to a variable
+			return true
+		}
 		if key != nil && id.Obj == key.Obj || val != nil && id.Obj == val.Obj {
 			f.Bad(id.Pos(), "range variable", id.Name, "captured by func literal")
 		}

src/cmd/vet/testdata/rangeloop.go

@@ -56,4 +56,13 @@ func RangeLoopTests() {
 			_ = f // ERROR "range variable f captured by func literal"
 		}()
 	}
+	type T struct {
+		v int
+	}
+	for _, v := range s {
+		go func() {
+			_ = T{v: 1}
+			_ = []int{v: 1} // ERROR "range variable v captured by func literal"
+		}()
+	}
 }