cmd/compile: ignore struct tags when converting structs

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

For #16085.

Change-Id: Id71ef29af5031b073e8be163f578d1bb768ff97a
Reviewed-on: https://go-review.googlesource.com/30169
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/subr.go

@@ -644,7 +644,12 @@ func cplxsubtype(et EType) EType {
 // pointer (t1 == t2), so there's no chance of chasing cycles
 // ad infinitum, so no need for a depth counter.
 func eqtype(t1, t2 *Type) bool {
-	return eqtype1(t1, t2, nil)
+	return eqtype1(t1, t2, true, nil)
+}
+
+// eqtypeIgnoreTags is like eqtype but it ignores struct tags for struct identity.
+func eqtypeIgnoreTags(t1, t2 *Type) bool {
+	return eqtype1(t1, t2, false, nil)
 }
 
 type typePair struct {
@@ -652,7 +657,7 @@ type typePair struct {
 	t2 *Type
 }
 
-func eqtype1(t1, t2 *Type, assumedEqual map[typePair]struct{}) bool {
+func eqtype1(t1, t2 *Type, cmpTags bool, assumedEqual map[typePair]struct{}) bool {
 	if t1 == t2 {
 		return true
 	}
@@ -684,7 +689,7 @@ func eqtype1(t1, t2 *Type, assumedEqual map[typePair]struct{}) bool {
 		t1, i1 := iterFields(t1)
 		t2, i2 := iterFields(t2)
 		for ; t1 != nil && t2 != nil; t1, t2 = i1.Next(), i2.Next() {
-			if t1.Sym != t2.Sym || t1.Embedded != t2.Embedded || !eqtype1(t1.Type, t2.Type, assumedEqual) || t1.Note != t2.Note {
+			if t1.Sym != t2.Sym || t1.Embedded != t2.Embedded || !eqtype1(t1.Type, t2.Type, cmpTags, assumedEqual) || cmpTags && t1.Note != t2.Note {
 				return false
 			}
 		}
@@ -703,7 +708,7 @@ func eqtype1(t1, t2 *Type, assumedEqual map[typePair]struct{}) bool {
 			ta, ia := iterFields(f(t1))
 			tb, ib := iterFields(f(t2))
 			for ; ta != nil && tb != nil; ta, tb = ia.Next(), ib.Next() {
-				if ta.Isddd != tb.Isddd || !eqtype1(ta.Type, tb.Type, assumedEqual) {
+				if ta.Isddd != tb.Isddd || !eqtype1(ta.Type, tb.Type, cmpTags, assumedEqual) {
 					return false
 				}
 			}
@@ -724,13 +729,13 @@ func eqtype1(t1, t2 *Type, assumedEqual map[typePair]struct{}) bool {
 		}
 
 	case TMAP:
-		if !eqtype1(t1.Key(), t2.Key(), assumedEqual) {
+		if !eqtype1(t1.Key(), t2.Key(), cmpTags, assumedEqual) {
 			return false
 		}
-		return eqtype1(t1.Val(), t2.Val(), assumedEqual)
+		return eqtype1(t1.Val(), t2.Val(), cmpTags, assumedEqual)
 	}
 
-	return eqtype1(t1.Elem(), t2.Elem(), assumedEqual)
+	return eqtype1(t1.Elem(), t2.Elem(), cmpTags, assumedEqual)
 }
 
 // Are t1 and t2 equal struct types when field names are ignored?
@@ -906,15 +911,15 @@ func convertop(src *Type, dst *Type, why *string) Op {
 		*why = ""
 	}
 
-	// 2. src and dst have identical underlying types.
+	// 2. Ignoring struct tags, src and dst have identical underlying types.
-	if eqtype(src.Orig, dst.Orig) {
+	if eqtypeIgnoreTags(src.Orig, dst.Orig) {
 		return OCONVNOP
 	}
 
-	// 3. src and dst are unnamed pointer types
+	// 3. src and dst are unnamed pointer types and, ignoring struct tags,
-	// and their base types have identical underlying types.
+	// their base types have identical underlying types.
 	if src.IsPtr() && dst.IsPtr() && src.Sym == nil && dst.Sym == nil {
-		if eqtype(src.Elem().Orig, dst.Elem().Orig) {
+		if eqtypeIgnoreTags(src.Elem().Orig, dst.Elem().Orig) {
 			return OCONVNOP
 		}
 	}

test/convert2.go

@@ -0,0 +1,315 @@
+// errorcheck
+
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test various valid and invalid struct assignments and conversions.
+// Does not compile.
+
+package main
+
+type I interface {
+	m()
+}
+
+// conversions between structs
+
+func _() {
+	type S struct{}
+	type T struct{}
+	var s S
+	var t T
+	var u struct{}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u
+	s = S(s)
+	s = S(t)
+	s = S(u)
+	t = u
+	t = T(u)
+}
+
+func _() {
+	type S struct{ x int }
+	type T struct {
+		x int "foo"
+	}
+	var s S
+	var t T
+	var u struct {
+		x int "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t)
+	s = S(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = T(u)
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct{ x E }
+	type T struct {
+		x E "foo"
+	}
+	var s S
+	var t T
+	var u struct {
+		x E "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t)
+	s = S(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = T(u)
+}
+
+func _() {
+	type S struct {
+		x struct {
+			x int "foo"
+		}
+	}
+	type T struct {
+		x struct {
+			x int "bar"
+		} "foo"
+	}
+	var s S
+	var t T
+	var u struct {
+		x struct {
+			x int "bar"
+		} "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t)
+	s = S(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = T(u)
+}
+
+func _() {
+	type E1 struct {
+		x int "foo"
+	}
+	type E2 struct {
+		x int "bar"
+	}
+	type S struct{ x E1 }
+	type T struct {
+		x E2 "foo"
+	}
+	var s S
+	var t T
+	var u struct {
+		x E2 "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t) // ERROR "cannot convert"
+	s = S(u) // ERROR "cannot convert"
+	t = u    // ERROR "cannot use .* in assignment"
+	t = T(u)
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct {
+		f func(struct {
+			x int "foo"
+		})
+	}
+	type T struct {
+		f func(struct {
+			x int "bar"
+		})
+	}
+	var s S
+	var t T
+	var u struct{ f func(E) }
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t)
+	s = S(u) // ERROR "cannot convert"
+	t = u    // ERROR "cannot use .* in assignment"
+	t = T(u) // ERROR "cannot convert"
+}
+
+// conversions between pointers to structs
+
+func _() {
+	type S struct{}
+	type T struct{}
+	var s *S
+	var t *T
+	var u *struct{}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type S struct{ x int }
+	type T struct {
+		x int "foo"
+	}
+	var s *S
+	var t *T
+	var u *struct {
+		x int "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct{ x E }
+	type T struct {
+		x E "foo"
+	}
+	var s *S
+	var t *T
+	var u *struct {
+		x E "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type S struct {
+		x struct {
+			x int "foo"
+		}
+	}
+	type T struct {
+		x struct {
+			x int "bar"
+		} "foo"
+	}
+	var s *S
+	var t *T
+	var u *struct {
+		x struct {
+			x int "bar"
+		} "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type E1 struct {
+		x int "foo"
+	}
+	type E2 struct {
+		x int "bar"
+	}
+	type S struct{ x E1 }
+	type T struct {
+		x E2 "foo"
+	}
+	var s *S
+	var t *T
+	var u *struct {
+		x E2 "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t) // ERROR "cannot convert"
+	s = (*S)(u) // ERROR "cannot convert"
+	t = u       // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct {
+		f func(struct {
+			x int "foo"
+		})
+	}
+	type T struct {
+		f func(struct {
+			x int "bar"
+		})
+	}
+	var s *S
+	var t *T
+	var u *struct{ f func(E) }
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u) // ERROR "cannot convert"
+	t = u       // ERROR "cannot use .* in assignment"
+	t = (*T)(u) // ERROR "cannot convert"
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct {
+		f func(*struct {
+			x int "foo"
+		})
+	}
+	type T struct {
+		f func(*struct {
+			x int "bar"
+		})
+	}
+	var s *S
+	var t *T
+	var u *struct{ f func(E) }
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u) // ERROR "cannot convert"
+	t = u       // ERROR "cannot use .* in assignment"
+	t = (*T)(u) // ERROR "cannot convert"
+}