go/types: ignore struct tags when converting structs

Implementation of spec change https://golang.org/cl/24190/.

For #16085.

Change-Id: I17bbbce38d98a169bc64e84983a7ebfe7142f6e9
Reviewed-on: https://go-review.googlesource.com/30190
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/check_test.go

@@ -80,6 +80,7 @@ var tests = [][]string{
 	{"testdata/shifts.src"},
 	{"testdata/builtins.src"},
 	{"testdata/conversions.src"},
+	{"testdata/conversions2.src"},
 	{"testdata/stmt0.src"},
 	{"testdata/stmt1.src"},
 	{"testdata/gotos.src"},

src/go/types/conversions.go

@@ -69,18 +69,19 @@ func (x *operand) convertibleTo(conf *Config, T Type) bool {
 		return true
 	}
 
-	// "x's type and T have identical underlying types"
+	// "x's type and T have identical underlying types if tags are ignored"
 	V := x.typ
 	Vu := V.Underlying()
 	Tu := T.Underlying()
-	if Identical(Vu, Tu) {
+	if IdenticalIgnoreTags(Vu, Tu) {
 		return true
 	}
 
-	// "x's type and T are unnamed pointer types and their pointer base types have identical underlying types"
+	// "x's type and T are unnamed pointer types and their pointer base types
+	// have identical underlying types if tags are ignored"
 	if V, ok := V.(*Pointer); ok {
 		if T, ok := T.(*Pointer); ok {
-			if Identical(V.base.Underlying(), T.base.Underlying()) {
+			if IdenticalIgnoreTags(V.base.Underlying(), T.base.Underlying()) {
 				return true
 			}
 		}

src/go/types/predicates.go

@@ -112,7 +112,12 @@ func hasNil(typ Type) bool {
 
 // Identical reports whether x and y are identical.
 func Identical(x, y Type) bool {
-	return identical(x, y, nil)
+	return identical(x, y, true, nil)
+}
+
+// IdenticalIgnoreTags reports whether x and y are identical if tags are ignored.
+func IdenticalIgnoreTags(x, y Type) bool {
+	return identical(x, y, false, nil)
 }
 
 // An ifacePair is a node in a stack of interface type pairs compared for identity.
@@ -125,7 +130,7 @@ func (p *ifacePair) identical(q *ifacePair) bool {
 	return p.x == q.x && p.y == q.y || p.x == q.y && p.y == q.x
 }
 
-func identical(x, y Type, p *ifacePair) bool {
+func identical(x, y Type, cmpTags bool, p *ifacePair) bool {
 	if x == y {
 		return true
 	}
@@ -143,13 +148,13 @@ func identical(x, y Type, p *ifacePair) bool {
 		// Two array types are identical if they have identical element types
 		// and the same array length.
 		if y, ok := y.(*Array); ok {
-			return x.len == y.len && identical(x.elem, y.elem, p)
+			return x.len == y.len && identical(x.elem, y.elem, cmpTags, p)
 		}
 
 	case *Slice:
 		// Two slice types are identical if they have identical element types.
 		if y, ok := y.(*Slice); ok {
-			return identical(x.elem, y.elem, p)
+			return identical(x.elem, y.elem, cmpTags, p)
 		}
 
 	case *Struct:
@@ -162,9 +167,9 @@ func identical(x, y Type, p *ifacePair) bool {
 				for i, f := range x.fields {
 					g := y.fields[i]
 					if f.anonymous != g.anonymous ||
-						x.Tag(i) != y.Tag(i) ||
+						cmpTags && x.Tag(i) != y.Tag(i) ||
 						!f.sameId(g.pkg, g.name) ||
-						!identical(f.typ, g.typ, p) {
+						!identical(f.typ, g.typ, cmpTags, p) {
 						return false
 					}
 				}
@@ -175,7 +180,7 @@ func identical(x, y Type, p *ifacePair) bool {
 	case *Pointer:
 		// Two pointer types are identical if they have identical base types.
 		if y, ok := y.(*Pointer); ok {
-			return identical(x.base, y.base, p)
+			return identical(x.base, y.base, cmpTags, p)
 		}
 
 	case *Tuple:
@@ -186,7 +191,7 @@ func identical(x, y Type, p *ifacePair) bool {
 				if x != nil {
 					for i, v := range x.vars {
 						w := y.vars[i]
-						if !identical(v.typ, w.typ, p) {
+						if !identical(v.typ, w.typ, cmpTags, p) {
 							return false
 						}
 					}
@@ -202,8 +207,8 @@ func identical(x, y Type, p *ifacePair) bool {
 		// names are not required to match.
 		if y, ok := y.(*Signature); ok {
 			return x.variadic == y.variadic &&
-				identical(x.params, y.params, p) &&
-				identical(x.results, y.results, p)
+				identical(x.params, y.params, cmpTags, p) &&
+				identical(x.results, y.results, cmpTags, p)
 		}
 
 	case *Interface:
@@ -249,7 +254,7 @@ func identical(x, y Type, p *ifacePair) bool {
 				}
 				for i, f := range a {
 					g := b[i]
-					if f.Id() != g.Id() || !identical(f.typ, g.typ, q) {
+					if f.Id() != g.Id() || !identical(f.typ, g.typ, cmpTags, q) {
 						return false
 					}
 				}
@@ -260,14 +265,14 @@ func identical(x, y Type, p *ifacePair) bool {
 	case *Map:
 		// Two map types are identical if they have identical key and value types.
 		if y, ok := y.(*Map); ok {
-			return identical(x.key, y.key, p) && identical(x.elem, y.elem, p)
+			return identical(x.key, y.key, cmpTags, p) && identical(x.elem, y.elem, cmpTags, p)
 		}
 
 	case *Chan:
 		// Two channel types are identical if they have identical value types
 		// and the same direction.
 		if y, ok := y.(*Chan); ok {
-			return x.dir == y.dir && identical(x.elem, y.elem, p)
+			return x.dir == y.dir && identical(x.elem, y.elem, cmpTags, p)
 		}
 
 	case *Named: