go/types: implement generic conversions

This is a port of 4 CLs from types2: CL 356010, CL 357333, CL 357410,
and CL 357249. These 4 CLs are all related to implementing conversions,
and porting them together saved time (particularly because go/types was
already threading a *reason argument in some places).

Change-Id: Ic89b608d7096b61bfb9f7d71fdae2cc50b0ed70e
Reviewed-on: https://go-review.googlesource.com/c/go/+/359137
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -438,7 +438,7 @@ func AssignableTo(V, T Type) bool {
 // ConvertibleTo reports whether a value of type V is convertible to a value of type T.
 func ConvertibleTo(V, T Type) bool {
 	x := operand{mode: value, typ: V}
-	return x.convertibleTo(nil, T, nil) // check not needed for non-constant x; if check == nil, cause can be nil
+	return x.convertibleTo(nil, T, nil) // check not needed for non-constant x
 }
 
 // Implements reports whether type V implements interface T.

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

@@ -178,12 +178,12 @@ func convertibleToImpl(check *Checker, V, T Type, cause *string) bool {
 		return true
 	}
 
-	// "V an integer or a slice of bytes or runes and T is a string type"
+	// "V is an integer or a slice of bytes or runes and T is a string type"
 	if (isInteger(V) || isBytesOrRunes(Vu)) && isString(T) {
 		return true
 	}
 
-	// "V a string and T is a slice of bytes or runes"
+	// "V is a string and T is a slice of bytes or runes"
 	if isString(V) && isBytesOrRunes(Tu) {
 		return true
 	}

src/go/types/conversions.go

@@ -17,7 +17,7 @@ func (check *Checker) conversion(x *operand, T Type) {
 	constArg := x.mode == constant_
 
 	var ok bool
-	var reason string
+	var cause string
 	switch {
 	case constArg && isConstType(T):
 		// constant conversion
@@ -32,15 +32,16 @@ func (check *Checker) conversion(x *operand, T Type) {
 			x.val = constant.MakeString(string(codepoint))
 			ok = true
 		}
-	case x.convertibleTo(check, T, &reason):
+	case x.convertibleTo(check, T, &cause):
 		// non-constant conversion
 		x.mode = value
 		ok = true
 	}
 
 	if !ok {
-		if reason != "" {
+		// TODO(rfindley): use types2-style error reporting here.
-			check.errorf(x, _InvalidConversion, "cannot convert %s to %s (%s)", x, T, reason)
+		if cause != "" {
+			check.errorf(x, _InvalidConversion, "cannot convert %s to %s (%s)", x, T, cause)
 		} else {
 			check.errorf(x, _InvalidConversion, "cannot convert %s to %s", x, T)
 		}
@@ -81,24 +82,75 @@ func (check *Checker) conversion(x *operand, T Type) {
 // is tricky because we'd have to run updateExprType on the argument first.
 // (Issue #21982.)
 
-// convertibleTo reports whether T(x) is valid.
+// convertibleTo reports whether T(x) is valid. In the failure case, *cause
+// may be set to the cause for the failure.
 // The check parameter may be nil if convertibleTo is invoked through an
 // exported API call, i.e., when all methods have been type-checked.
-func (x *operand) convertibleTo(check *Checker, T Type, reason *string) bool {
+func (x *operand) convertibleTo(check *Checker, T Type, cause *string) bool {
 	// "x is assignable to T"
-	if ok, _ := x.assignableTo(check, T, nil); ok {
+	if ok, _ := x.assignableTo(check, T, cause); ok {
 		return true
 	}
 
-	// "x's type and T have identical underlying types if tags are ignored"
+	errorf := func(format string, args ...interface{}) {
-	V := x.typ
+		if check != nil && cause != nil {
+			msg := check.sprintf(format, args...)
+			if *cause != "" {
+				msg += "\n\t" + *cause
+			}
+			*cause = msg
+		}
+	}
+
+	// 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:
+		return Vp.is(func(V *term) bool {
+			return Tp.is(func(T *term) bool {
+				if !convertibleToImpl(check, V.typ, T.typ, cause) {
+					errorf("cannot convert %s (in %s) to %s (in %s)", V.typ, Vp, T.typ, Tp)
+					return false
+				}
+				return true
+			})
+		})
+	case Vp != nil:
+		return Vp.is(func(V *term) bool {
+			if !convertibleToImpl(check, V.typ, T, cause) {
+				errorf("cannot convert %s (in %s) to %s", V.typ, Vp, T)
+				return false
+			}
+			return true
+		})
+	case Tp != nil:
+		return Tp.is(func(T *term) bool {
+			if !convertibleToImpl(check, x.typ, T.typ, cause) {
+				errorf("cannot convert %s to %s (in %s)", x.typ, T.typ, Tp)
+				return false
+			}
+			return true
+		})
+	}
+
+	// non-generic case
+	return convertibleToImpl(check, x.typ, T, cause)
+}
+
+// convertibleToImpl should only be called by convertibleTo
+func convertibleToImpl(check *Checker, V, T Type, cause *string) bool {
+	// "V and T have identical underlying types if tags are ignored"
 	Vu := under(V)
 	Tu := under(T)
 	if IdenticalIgnoreTags(Vu, Tu) {
 		return true
 	}
 
-	// "x's type and T are unnamed pointer types and their pointer base types
+	// "V 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 {
@@ -108,22 +160,22 @@ func (x *operand) convertibleTo(check *Checker, T Type, reason *string) bool {
 		}
 	}
 
-	// "x's type and T are both integer or floating point types"
+	// "V and T are both integer or floating point types"
 	if isIntegerOrFloat(V) && isIntegerOrFloat(T) {
 		return true
 	}
 
-	// "x's type and T are both complex types"
+	// "V and T are both complex types"
 	if isComplex(V) && isComplex(T) {
 		return true
 	}
 
-	// "x is an integer or a slice of bytes or runes and T is a string type"
+	// "V is an integer or a slice of bytes or runes and T is a string type"
 	if (isInteger(V) || isBytesOrRunes(Vu)) && isString(T) {
 		return true
 	}
 
-	// "x is a string and T is a slice of bytes or runes"
+	// "V is a string and T is a slice of bytes or runes"
 	if isString(V) && isBytesOrRunes(Tu) {
 		return true
 	}
@@ -138,7 +190,7 @@ func (x *operand) convertibleTo(check *Checker, T Type, reason *string) bool {
 		return true
 	}
 
-	// "x is a slice, T is a pointer-to-array type,
+	// "V is a slice, T is a pointer-to-array type,
 	// and the slice and array types have identical element types."
 	if s := asSlice(V); s != nil {
 		if p := asPointer(T); p != nil {
@@ -147,8 +199,8 @@ func (x *operand) convertibleTo(check *Checker, T Type, reason *string) bool {
 					if check == nil || check.allowVersion(check.pkg, 1, 17) {
 						return true
 					}
-					if reason != nil {
+					if cause != nil {
-						*reason = "conversion of slices to array pointers requires go1.17 or later"
+						*cause = "conversion of slices to array pointers requires go1.17 or later"
 					}
 				}
 			}

src/go/types/operand.go

@@ -159,17 +159,14 @@ func operandString(x *operand, qf Qualifier) string {
 	if hasType {
 		if x.typ != Typ[Invalid] {
 			var intro string
-			var tpar *TypeParam
 			if isGeneric(x.typ) {
 				intro = " of parameterized type "
-			} else if tpar = asTypeParam(x.typ); tpar != nil {
-				intro = " of type parameter "
 			} else {
 				intro = " of type "
 			}
 			buf.WriteString(intro)
 			WriteType(&buf, x.typ, qf)
-			if tpar != nil {
+			if tpar := asTypeParam(x.typ); tpar != nil {
 				buf.WriteString(" constrained by ")
 				WriteType(&buf, tpar.bound, qf) // do not compute interface type sets here
 			}
@@ -284,6 +281,7 @@ func (x *operand) assignableTo(check *Checker, T Type, reason *string) (bool, er
 	if Ti, ok := Tu.(*Interface); ok {
 		if m, wrongType := check.missingMethod(V, Ti, true); m != nil /* Implements(V, Ti) */ {
 			if reason != nil {
+				// TODO(gri) the error messages here should follow the style in Checker.typeAssertion (factor!)
 				if wrongType != nil {
 					if Identical(m.typ, wrongType.typ) {
 						*reason = fmt.Sprintf("missing method %s (%s has pointer receiver)", m.name, m.name)

src/go/types/testdata/examples/conversions.go2

@@ -0,0 +1,144 @@
+// 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 \(variable of type X constrained by Foo\) to T.*cannot convert Foo \(in X\) to Far \(in T\) */ ) }
+
+// "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 \(variable of type X constrained by ~\*Foo\) to T.*cannot convert \*Foo \(in X\) to \*Far \(in T\) */ ) }
+
+// Verify that the defined types in constraints are considered for the rule above.
+
+type (
+	B int
+	C int
+	X0 *B
+	T0 *C
+)
+
+func _(x X0) T0 { return T0(x /* ERROR cannot convert */ ) } // non-generic reference
+func _[X X0, T T0](x X) T { return T(x /* ERROR cannot convert */ ) }
+func _[T T0](x X0) T { return T(x /* ERROR cannot convert */ ) }
+func _[X X0](x X) T0 { return T0(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 \(variable of type X constrained by Integer\|~string\) to T.*cannot convert string \(in X\) to int \(in T\) */ ) }
+
+// "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 \(variable of type X constrained by Float\|Complex\) to T.*cannot convert float32 \(in X\) to complex64 \(in T\) */ ) }
+
+// "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 x \(variable of type X constrained by Integer\) to \*string.*cannot convert int \(in X\) to \*string */ ) }
+
+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 \(variable of type X constrained by Integer\|~\[\]byte\|~\[\]rune\) to T.*cannot convert int \(in X\) to \*string \(in T\) */ ) }
+
+// "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 x \(variable of type \*string\) to T.*cannot convert \*string to \[\]rune \(in T\) */ ) }
+
+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 x \(variable of type X constrained by ~\*string\) to T.*cannot convert \*string \(in X\) to \[\]byte \(in T\) */ ) }
+
+// 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 x \(variable of type int64\) to T.*cannot convert int64 to unsafe\.Pointer \(in T\) */ ) }
+
+// "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 \(variable of type X constrained by unsafe\.Pointer\) to int64.*cannot convert unsafe\.Pointer \(in X\) to int64 */ ) }
+
+// "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 x \(variable of type X constrained by ~\[\]E\) to T.*cannot convert \[\]E \(in X\) to \[10\]E \(in T\) */ ) }
+
+// ----------------------------------------------------------------------------
+// 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
+}