cmd/compile: fix conv of slice of user-define byte type to string

types2 allows the conversion of a slice of a user-defined byte type B
(not builtin uint8 or byte) to string. But runtime.slicebytetostring
requires a []byte argument, so add in a CONVNOP from []B to []byte if
needed. Same for the conversion of a slice of user-defined rune types to
string.

I made the same change in the transformations of the old typechecker, so
as to keep tcConv() and transformConv() in sync. That fixes the bug for
-G=0 mode as well.

Fixes #23536

Change-Id: Ic79364427f27489187f3f8015bdfbf0769a70d69
Reviewed-on: https://go-review.googlesource.com/c/go/+/376056
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/internal/noder/transform.go

@@ -115,6 +115,31 @@ func transformConv(n *ir.ConvExpr) ir.Node {
 		if n.X.Op() == ir.OLITERAL {
 			return stringtoruneslit(n)
 		}
+
+	case ir.OBYTES2STR:
+		assert(t.IsSlice())
+		assert(t.Elem().Kind() == types.TUINT8)
+		if t.Elem() != types.ByteType && t.Elem() != types.Types[types.TUINT8] {
+			// If t is a slice of a user-defined byte type B (not uint8
+			// or byte), then add an extra CONVNOP from []B to []byte, so
+			// that the call to slicebytetostring() added in walk will
+			// typecheck correctly.
+			n.X = ir.NewConvExpr(n.X.Pos(), ir.OCONVNOP, types.NewSlice(types.ByteType), n.X)
+			n.X.SetTypecheck(1)
+		}
+
+	case ir.ORUNES2STR:
+		assert(t.IsSlice())
+		assert(t.Elem().Kind() == types.TINT32)
+		if t.Elem() != types.RuneType && t.Elem() != types.Types[types.TINT32] {
+			// If t is a slice of a user-defined rune type B (not uint32
+			// or rune), then add an extra CONVNOP from []B to []rune, so
+			// that the call to slicerunetostring() added in walk will
+			// typecheck correctly.
+			n.X = ir.NewConvExpr(n.X.Pos(), ir.OCONVNOP, types.NewSlice(types.RuneType), n.X)
+			n.X.SetTypecheck(1)
+		}
+
 	}
 	return n
 }

src/cmd/compile/internal/typecheck/expr.go

@@ -466,6 +466,27 @@ func tcConv(n *ir.ConvExpr) ir.Node {
 		if n.X.Op() == ir.OLITERAL {
 			return stringtoruneslit(n)
 		}
+
+	case ir.OBYTES2STR:
+		if t.Elem() != types.ByteType && t.Elem() != types.Types[types.TUINT8] {
+			// If t is a slice of a user-defined byte type B (not uint8
+			// or byte), then add an extra CONVNOP from []B to []byte, so
+			// that the call to slicebytetostring() added in walk will
+			// typecheck correctly.
+			n.X = ir.NewConvExpr(n.X.Pos(), ir.OCONVNOP, types.NewSlice(types.ByteType), n.X)
+			n.X.SetTypecheck(1)
+		}
+
+	case ir.ORUNES2STR:
+		if t.Elem() != types.RuneType && t.Elem() != types.Types[types.TINT32] {
+			// If t is a slice of a user-defined rune type B (not uint32
+			// or rune), then add an extra CONVNOP from []B to []rune, so
+			// that the call to slicerunetostring() added in walk will
+			// typecheck correctly.
+			n.X = ir.NewConvExpr(n.X.Pos(), ir.OCONVNOP, types.NewSlice(types.RuneType), n.X)
+			n.X.SetTypecheck(1)
+		}
+
 	}
 	return n
 }

test/fixedbugs/issue23536.go

@@ -0,0 +1,22 @@
+// run
+
+// Copyright 2022 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 case where a slice of a user-defined byte type (not uint8 or byte) is
+// converted to a string.  Same for slice of runes.
+
+package main
+
+type MyByte byte
+
+type MyRune rune
+
+func main() {
+	var y []MyByte
+	_ = string(y)
+
+	var z []MyRune
+	_ = string(z)
+}

test/typeparam/issue23536.go

@@ -0,0 +1,32 @@
+// run -gcflags=-G=3
+
+// Copyright 2022 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 case where a slice of a user-defined byte type (not uint8 or byte) is
+// converted to a string.  Same for slice of runes.
+
+package main
+
+type MyByte byte
+
+type MyRune rune
+
+func f[T []MyByte](x T) string {
+	return string(x)
+}
+
+func g[T []MyRune](x T) string {
+	return string(x)
+}
+
+func main() {
+	var y []MyByte
+	_ = f(y)
+	_ = string(y)
+
+	var z []MyRune
+	_ = g(z)
+	_ = string(z)
+}