From: Robert Griesemer <gri@golang.org>
Date: Fri, 12 Nov 2021 23:08:00 +0000 (-0800)
Subject: cmd/compile/internal/types2: allow slicing for operands with []byte|string type sets
X-Git-Tag: go1.18beta1~317
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=cfcd71790f0ca8c2ec1ae5989cd60ad1e83ee40c;p=gostls13.git

cmd/compile/internal/types2: allow slicing for operands with []byte|string type sets

Fixes #49566.

Change-Id: I80ff4ca661f82b0981d51e0997d5988a9b82f508
Reviewed-on: https://go-review.googlesource.com/c/go/+/363662
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
---

diff --git a/src/cmd/compile/internal/types2/builtins.go b/src/cmd/compile/internal/types2/builtins.go
index 5b4ffd0dad..53d834507a 100644
--- a/src/cmd/compile/internal/types2/builtins.go
+++ b/src/cmd/compile/internal/types2/builtins.go
@@ -337,26 +337,11 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 		if y.mode == invalid {
 			return
 		}
-		// src, _ := structuralType(y.typ).(*Slice); but also accepts strings
-		var src *Slice
-		var elem Type // == src.elem if valid
-		if underIs(y.typ, func(u Type) bool {
-			switch u := u.(type) {
-			case *Basic:
-				if isString(u) && (elem == nil || Identical(elem, universeByte)) {
-					elem = universeByte
-					return true
-				}
-			case *Slice:
-				if elem == nil || Identical(elem, u.elem) {
-					elem = u.elem
-					return true
-				}
-			}
-			return false
-		}) {
-			src = NewSlice(elem)
+		src0 := structuralString(y.typ)
+		if src0 != nil && isString(src0) {
+			src0 = NewSlice(universeByte)
 		}
+		src, _ := src0.(*Slice)
 
 		if dst == nil || src == nil {
 			check.errorf(x, invalidArg+"copy expects slice arguments; found %s and %s", x, &y)
diff --git a/src/cmd/compile/internal/types2/index.go b/src/cmd/compile/internal/types2/index.go
index 648c7abe6f..524d1957b5 100644
--- a/src/cmd/compile/internal/types2/index.go
+++ b/src/cmd/compile/internal/types2/index.go
@@ -213,7 +213,7 @@ func (check *Checker) sliceExpr(x *operand, e *syntax.SliceExpr) {
 
 	valid := false
 	length := int64(-1) // valid if >= 0
-	switch u := structuralType(x.typ).(type) {
+	switch u := structuralString(x.typ).(type) {
 	case nil:
 		check.errorf(x, invalidOp+"cannot slice %s: %s has no structural type", x, x.typ)
 		x.mode = invalid
@@ -232,7 +232,7 @@ func (check *Checker) sliceExpr(x *operand, e *syntax.SliceExpr) {
 			}
 			// spec: "For untyped string operands the result
 			// is a non-constant value of type string."
-			if u.kind == UntypedString {
+			if isUntyped(x.typ) {
 				x.typ = Typ[String]
 			}
 		}
diff --git a/src/cmd/compile/internal/types2/testdata/check/typeparams.go2 b/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
index 03c3f9a0b5..f77d09391b 100644
--- a/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
@@ -136,6 +136,10 @@ type myByte2 []byte
 func _[T interface{ []byte | myByte1 | myByte2 }] (x T, i, j, k int) { var _ T = x[i:j:k] }
 func _[T interface{ []byte | myByte1 | []int }] (x T, i, j, k int) { var _ T = x[ /* ERROR no structural type */ i:j:k] }
 
+func _[T interface{ []byte | myByte1 | myByte2 | string }] (x T, i, j, k int) { var _ T = x[i:j] }
+func _[T interface{ []byte | myByte1 | myByte2 | string }] (x T, i, j, k int) { var _ T = x /* ERROR 3-index slice of string */ [i:j:k] }
+func _[T interface{ []byte | myByte1 | []int | string }] (x T, i, j, k int) { var _ T = x[ /* ERROR no structural type */ i:j] }
+
 // len/cap built-ins
 
 func _[T any](x T) { _ = len(x /* ERROR invalid argument */ ) }
diff --git a/src/cmd/compile/internal/types2/type.go b/src/cmd/compile/internal/types2/type.go
index 39737d47a7..3ab738eb19 100644
--- a/src/cmd/compile/internal/types2/type.go
+++ b/src/cmd/compile/internal/types2/type.go
@@ -87,3 +87,40 @@ func structuralType(t Type) Type {
 	}
 	return nil
 }
+
+// structuralString is like structuralType but also considers []byte
+// and strings as identical. In this case, if successful and we saw
+// a string, the result is of type (possibly untyped) string.
+func structuralString(t Type) Type {
+	tpar, _ := t.(*TypeParam)
+	if tpar == nil {
+		return under(t) // string or untyped string
+	}
+
+	var su Type
+	hasString := false
+	if tpar.underIs(func(u Type) bool {
+		if u == nil {
+			return false
+		}
+		if isString(u) {
+			u = NewSlice(universeByte)
+			hasString = true
+		}
+		if su != nil {
+			u = match(su, u)
+			if u == nil {
+				return false
+			}
+		}
+		// su == nil || match(su, u) != nil
+		su = u
+		return true
+	}) {
+		if hasString {
+			return Typ[String]
+		}
+		return su
+	}
+	return nil
+}