reflect: StructOf interpretation of input StructFields is confusing

[rsc]

Normally, if you have

type T struct {
	A1
	*A2
	a1
	*a2
	F int
	f int
}

then there are six reflect.StructFields:

reflect.StructField{Name: "A1", Anonymous:true, Type:TypeOf(A1{}), PkgPath: ""}
reflect.StructField{Name: "A2", Anonymous:true, Type:TypeOf(&A2{}), PkgPath: ""}
reflect.StructField{Name: "a1", Anonymous:true, Type:TypeOf(a1{}), PkgPath: "mypkg"}
reflect.StructField{Name: "a2", Anonymous:true, Type:TypeOf(&a2{}), PkgPath: "mypkg"}
reflect.StructField{Name: "F", Anonymous:false, Type:TypeOf(int(0)), PkgPath: ""}
reflect.StructField{Name: "f", Anonymous:false, Type:TypeOf(int(0)), PkgPath: "mypkg"}

However, these appear not to be the StructFields expected by reflect.StructOf.

1. PkgPath != "" panics unconditionally. That appears to be a limitation, which I understand.
2. Anonymous is completely ignored.
3. Name == "" appears to set Anonymous==true.
4. Name == "" with an unexported type appears to set PkgPath incorrectly (and not panic).

This does not match the usual meaning of reflect.StructField, and it's undocumented.

I changed TestStructOfExportRules to print the input and output StructFields:

#0: {Name: PkgPath: Type:reflect_test.S1 Tag: Offset:0 Index:[] Anonymous:false} =>
	{Name:S1 PkgPath: Type:reflect_test.S1 Tag: Offset:0 Index:[0] Anonymous:true}
#1: {Name: PkgPath: Type:*reflect_test.S1 Tag: Offset:0 Index:[] Anonymous:false} =>
	{Name:S1 PkgPath: Type:*reflect_test.S1 Tag: Offset:0 Index:[0] Anonymous:true}
#2: {Name: PkgPath: Type:reflect_test.s2 Tag: Offset:0 Index:[] Anonymous:false} =>
	{Name:s2 PkgPath:runtime Type:reflect_test.s2 Tag: Offset:0 Index:[0] Anonymous:true}
#3: {Name: PkgPath: Type:*reflect_test.s2 Tag: Offset:0 Index:[] Anonymous:false} =>
	{Name:s2 PkgPath:runtime Type:*reflect_test.s2 Tag: Offset:0 Index:[0] Anonymous:true}
#8: {Name:S PkgPath: Type:reflect_test.S1 Tag: Offset:0 Index:[] Anonymous:false} =>
	{Name:S PkgPath: Type:reflect_test.S1 Tag: Offset:0 Index:[0] Anonymous:false}
#9: {Name:S PkgPath: Type:*reflect_test.S1 Tag: Offset:0 Index:[] Anonymous:false} =>
	{Name:S PkgPath: Type:*reflect_test.S1 Tag: Offset:0 Index:[0] Anonymous:false}
#10: {Name:S PkgPath: Type:reflect_test.s2 Tag: Offset:0 Index:[] Anonymous:false} =>
	{Name:S PkgPath: Type:reflect_test.s2 Tag: Offset:0 Index:[0] Anonymous:false}
#11: {Name:S PkgPath: Type:*reflect_test.s2 Tag: Offset:0 Index:[] Anonymous:false} =>
	{Name:S PkgPath: Type:*reflect_test.s2 Tag: Offset:0 Index:[0] Anonymous:false}
#20: {Name: PkgPath: Type:reflect_test.ΦType Tag: Offset:0 Index:[] Anonymous:false} =>
	{Name:ΦType PkgPath: Type:reflect_test.ΦType Tag: Offset:0 Index:[0] Anonymous:true}
#21: {Name: PkgPath: Type:reflect_test.φType Tag: Offset:0 Index:[] Anonymous:false} =>
	{Name:φType PkgPath:runtime Type:reflect_test.φType Tag: Offset:0 Index:[0] Anonymous:true}
#22: {Name:Φ PkgPath: Type:int Tag: Offset:0 Index:[] Anonymous:false} =>
	{Name:Φ PkgPath: Type:int Tag: Offset:0 Index:[0] Anonymous:false}
#23: {Name:φ PkgPath: Type:int Tag: Offset:0 Index:[] Anonymous:false} =>
	{Name:φ PkgPath: Type:int Tag: Offset:0 Index:[0] Anonymous:false}

Note that in case 2, the input struct field says Anonymous false but the missing name triggers Anonymous true.

Note that in case 2, the input struct field has an empty PkgPath, but the missing name triggers setting PkgPath="runtime" (incorrectly, since the type is from package reflect_test).

I believe we should change StructOf so that if you have a struct type t, either StructOf([]StructField{t.Field(i)}) works and produces exactly the same field meaning or else it panics (if it's not implementable). If an input StructField is invalid (for example, it has no name), StructOf should panic.

I'm not sure what will break as a result of this; I hope not much, but the current situation is undocumented and contradicts the rest of the package, so a little breakage seems justifiable, and I can't - even with backwards compatibility hacks - make StructOf([]StructField{t.Field(i)}) work properly for embedded alias fields the way things are now.

For #18130.

/cc @sbinet

[gopherbot]

CL https://golang.org/cl/35731 mentions this issue.