cmd/compile/internal/types2: implement generic conversions

Fixes #47150.

Change-Id: I7531ca5917d4e52ca0b9211d6f2114495b19ba09
Reviewed-on: https://go-review.googlesource.com/c/go/+/356010
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/conversions.go

@@ -89,6 +89,39 @@ func (x *operand) convertibleTo(check *Checker, T Type) bool {
 		return true
 	}
 
+	// TODO(gri) consider passing under(x.typ), under(T) into convertibleToImpl (optimization)
+	Vp, _ := under(x.typ).(*TypeParam)
+	Tp, _ := under(T).(*TypeParam)
+
+	// generic cases
+	// (generic operands cannot be constants, so we can ignore x.val)
+	switch {
+	case Vp != nil && Tp != nil:
+		x := *x // don't modify outer x
+		return Vp.underIs(func(V Type) bool {
+			x.typ = V
+			return Tp.underIs(func(T Type) bool {
+				return x.convertibleToImpl(check, T)
+			})
+		})
+	case Vp != nil:
+		x := *x // don't modify outer x
+		return Vp.underIs(func(V Type) bool {
+			x.typ = V
+			return x.convertibleToImpl(check, T)
+		})
+	case Tp != nil:
+		return Tp.underIs(func(T Type) bool {
+			return x.convertibleToImpl(check, T)
+		})
+	}
+
+	// non-generic case
+	return x.convertibleToImpl(check, T)
+}
+
+// convertibleToImpl should only be called by convertibleTo
+func (x *operand) convertibleToImpl(check *Checker, T Type) bool {
 	// "x's type and T have identical underlying types if tags are ignored"
 	V := x.typ
 	Vu := under(V)

src/cmd/compile/internal/types2/testdata/examples/conversions.go2

@@ -0,0 +1,130 @@
+// Copyright 2021 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.
+
+package conversions
+
+import "unsafe"
+
+// "x is assignable to T"
+// - tested via assignability tests
+
+// "x's type and T have identical underlying types if tags are ignored"
+
+func _[X ~int, T ~int](x X) T { return T(x) }
+func _[X struct{f int "foo"}, T struct{f int "bar"}](x X) T { return T(x) }
+
+type Foo struct{f int "foo"}
+type Bar struct{f int "bar"}
+type Far struct{f float64 }
+
+func _[X Foo, T Bar](x X) T { return T(x) }
+func _[X Foo|Bar, T Bar](x X) T { return T(x) }
+func _[X Foo, T Foo|Bar](x X) T { return T(x) }
+func _[X Foo, T Far](x X) T { return T(x /* ERROR cannot convert */ ) }
+
+// "x's type and T are unnamed pointer types and their pointer base types
+// have identical underlying types if tags are ignored"
+
+func _[X ~*Foo, T ~*Bar](x X) T { return T(x) }
+func _[X ~*Foo|~*Bar, T ~*Bar](x X) T { return T(x) }
+func _[X ~*Foo, T ~*Foo|~*Bar](x X) T { return T(x) }
+func _[X ~*Foo, T ~*Far](x X) T { return T(x /* ERROR cannot convert */ ) }
+
+// "x's type and T are both integer or floating point types"
+
+func _[X Integer, T Integer](x X) T { return T(x) }
+func _[X Unsigned, T Integer](x X) T { return T(x) }
+func _[X Float, T Integer](x X) T { return T(x) }
+
+func _[X Integer, T Unsigned](x X) T { return T(x) }
+func _[X Unsigned, T Unsigned](x X) T { return T(x) }
+func _[X Float, T Unsigned](x X) T { return T(x) }
+
+func _[X Integer, T Float](x X) T { return T(x) }
+func _[X Unsigned, T Float](x X) T { return T(x) }
+func _[X Float, T Float](x X) T { return T(x) }
+
+func _[X, T Integer|Unsigned|Float](x X) T { return T(x) }
+func _[X, T Integer|~string](x X) T { return T(x /* ERROR cannot convert */ ) }
+
+// "x's type and T are both complex types"
+
+func _[X, T Complex](x X) T { return T(x) }
+func _[X, T Float|Complex](x X) T { return T(x /* ERROR cannot convert */ ) }
+
+// "x is an integer or a slice of bytes or runes and T is a string type"
+
+type myInt int
+type myString string
+
+func _[T ~string](x int) T { return T(x) }
+func _[T ~string](x myInt) T { return T(x) }
+func _[X Integer](x X) string { return string(x) }
+func _[X Integer](x X) myString { return myString(x) }
+func _[X Integer](x X) *string { return (*string)(x /* ERROR cannot convert */ ) }
+
+func _[T ~string](x []byte) T { return T(x) }
+func _[T ~string](x []rune) T { return T(x) }
+func _[X ~[]byte, T ~string](x X) T { return T(x) }
+func _[X ~[]rune, T ~string](x X) T { return T(x) }
+func _[X Integer|~[]byte|~[]rune, T ~string](x X) T { return T(x) }
+func _[X Integer|~[]byte|~[]rune, T ~*string](x X) T { return T(x /* ERROR cannot convert */ ) }
+
+// "x is a string and T is a slice of bytes or runes"
+
+func _[T ~[]byte](x string) T { return T(x) }
+func _[T ~[]rune](x string) T { return T(x) }
+func _[T ~[]rune](x *string) T { return T(x /* ERROR cannot convert */ ) }
+
+func _[X ~string, T ~[]byte](x X) T { return T(x) }
+func _[X ~string, T ~[]rune](x X) T { return T(x) }
+func _[X ~string, T ~[]byte|~[]rune](x X) T { return T(x) }
+func _[X ~*string, T ~[]byte|~[]rune](x X) T { return T(x /* ERROR cannot convert */ ) }
+
+// package unsafe:
+// "any pointer or value of underlying type uintptr can be converted into a unsafe.Pointer"
+
+type myUintptr uintptr
+
+func _[X ~uintptr](x X) unsafe.Pointer { return unsafe.Pointer(x) }
+func _[T unsafe.Pointer](x myUintptr) T { return T(x) }
+func _[T unsafe.Pointer](x int64) T { return T(x /* ERROR cannot convert */ ) }
+
+// "and vice versa"
+
+func _[T ~uintptr](x unsafe.Pointer) T { return T(x) }
+func _[X unsafe.Pointer](x X) uintptr { return uintptr(x) }
+func _[X unsafe.Pointer](x X) myUintptr { return myUintptr(x) }
+func _[X unsafe.Pointer](x X) int64 { return int64(x /* ERROR cannot convert */ ) }
+
+// "x is a slice, T is a pointer-to-array type,
+// and the slice and array types have identical element types."
+
+func _[X ~[]E, T ~*[10]E, E any](x X) T { return T(x) }
+func _[X ~[]E, T ~[10]E, E any](x X) T { return T(x /* ERROR cannot convert */ ) }
+
+// ----------------------------------------------------------------------------
+// The following declarations can be replaced by the exported types of the
+// constraints package once all builders support importing interfaces with
+// type constraints.
+
+type Signed interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64
+}
+
+type Unsigned interface {
+	~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr
+}
+
+type Integer interface {
+	Signed | Unsigned
+}
+
+type Float interface {
+	~float32 | ~float64
+}
+
+type Complex interface {
+	~complex64 | ~complex128
+}