go/types: differently named types are not assignable

This is a clean port of CL 360274 to go/types.

Change-Id: Idfa584fab95f7226e10b1a7c5b06d56a0bf9d757
Reviewed-on: https://go-review.googlesource.com/c/go/+/360759
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/go/types/operand.go

@@ -302,19 +302,11 @@ func (x *operand) assignableTo(check *Checker, T Type, reason *string) (bool, er
 		}
 	}
 
-	// common case: if we don't have type parameters, we're done
+	// optimization: if we don't have type parameters, we're done
 	if Vp == nil && Tp == nil {
 		return false, _IncompatibleAssign
 	}
 
-	// determine type parameter operands with specific type terms
-	if Vp != nil && !Vp.hasTerms() {
-		Vp = nil
-	}
-	if Tp != nil && !Tp.hasTerms() {
-		Tp = nil
-	}
-
 	errorf := func(format string, args ...interface{}) {
 		if check != nil && reason != nil {
 			msg := check.sprintf(format, args...)
@@ -325,25 +317,36 @@ func (x *operand) assignableTo(check *Checker, T Type, reason *string) (bool, er
 		}
 	}
 
-	ok := false
-	code := _IncompatibleAssign
-	switch {
-	case Vp != nil && Tp != nil:
-		x := *x // don't clobber outer x
-		ok = Vp.is(func(V *term) bool {
-			x.typ = V.typ
-			return Tp.is(func(T *term) bool {
-				ok, code = x.assignableTo(check, T.typ, reason)
-				if !ok {
-					errorf("cannot assign %s (in %s) to %s (in %s)", V.typ, Vp, T.typ, Tp)
-					return false
-				}
-				return true
-			})
+	// x's type V is not a named type and T is a type parameter, and
+	// x is assignable to each specific type in T's type set.
+	if !hasName(V) && Tp != nil {
+		ok := false
+		code := _IncompatibleAssign
+		Tp.is(func(T *term) bool {
+			if T == nil {
+				return false // no specific types
+			}
+			ok, code = x.assignableTo(check, T.typ, reason)
+			if !ok {
+				errorf("cannot assign %s to %s (in %s)", x.typ, T.typ, Tp)
+				return false
+			}
+			return true
 		})
-	case Vp != nil:
+		return ok, code
+	}
+
+	// x's type V is a type parameter and T is not a named type,
+	// and values x' of each specific type in V's type set are
+	// assignable to T.
+	if Vp != nil && !hasName(T) {
 		x := *x // don't clobber outer x
-		ok = Vp.is(func(V *term) bool {
+		ok := false
+		code := _IncompatibleAssign
+		Vp.is(func(V *term) bool {
+			if V == nil {
+				return false // no specific types
+			}
 			x.typ = V.typ
 			ok, code = x.assignableTo(check, T, reason)
 			if !ok {
@@ -352,17 +355,8 @@ func (x *operand) assignableTo(check *Checker, T Type, reason *string) (bool, er
 			}
 			return true
 		})
-	case Tp != nil:
-		x := *x // don't clobber outer x
-		ok = Tp.is(func(T *term) bool {
-			ok, code = x.assignableTo(check, T.typ, reason)
-			if !ok {
-				errorf("cannot assign %s to %s (in %s)", x.typ, T.typ, Tp)
-				return false
-			}
-			return true
-		})
+		return ok, code
 	}
 
-	return ok, code
+	return false, _IncompatibleAssign
 }

src/go/types/testdata/fixedbugs/issue49242.go2

@@ -0,0 +1,27 @@
+// 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 p
+
+func _[P int](x P) int {
+	return x // ERROR cannot use x .* as int value in return statement
+}
+
+func _[P int]() int {
+	return P /* ERROR cannot use P\(1\) .* as int value in return statement */ (1)
+}
+
+func _[P int](x int) P {
+        return x // ERROR cannot use x .* as P value in return statement
+}
+
+func _[P, Q any](x P) Q {
+        return x // ERROR cannot use x .* as Q value in return statement
+}
+
+// test case from issue
+func F[G interface{ uint }]() int {
+	f := func(uint) int { return 0 }
+	return f(G /* ERROR cannot use G\(1\) .* as uint value in argument to f */ (1))
+}

src/go/types/testdata/spec/assignability.go2

@@ -109,20 +109,48 @@ func _[
 	)
 
 	var (
-		_ _CC = C
-		_ _SC = C
-		_ _RC = C
+		_ _CC = C // ERROR cannot use C .* as _CC value
+		_ _SC = C // ERROR cannot use C .* as _SC value
+		_ _RC = C // ERROR cannot use C .* as _RC value
 
-		_ CC = _CC(nil)
-		_ SC = _CC(nil)
-		_ RC = _CC(nil)
+		_ CC = _CC /* ERROR cannot use _CC\(nil\) .* as CC value */ (nil)
+		_ SC = _CC /* ERROR cannot use _CC\(nil\) .* as SC value */ (nil)
+		_ RC = _CC /* ERROR cannot use _CC\(nil\) .* as RC value */ (nil)
 
-		_ CC = C
-		_ SC = C // ERROR cannot use C .* as SC value .* cannot assign Chan to SendChan
-		_ RC = C // ERROR cannot use C .* as RC value .* cannot assign Chan to RecvChan
+		_ CC = C // ERROR cannot use C .* as CC value
+		_ SC = C // ERROR cannot use C .* as SC value
+		_ RC = C // ERROR cannot use C .* as RC value
 	)
 }
 
+// "x's type V is not a named type and T is a type parameter, and x is assignable to each specific type in T's type set."
+func _[
+	TP0 any,
+	TP1 ~_Chan,
+	TP2 ~chan int | ~chan byte,
+]() {
+	var (
+		_ TP0 = c // ERROR cannot use c .* as TP0 value
+		_ TP0 = C // ERROR cannot use C .* as TP0 value
+		_ TP1 = c
+		_ TP1 = C // ERROR cannot use C .* as TP1 value
+		_ TP2 = c // ERROR .* cannot assign chan int to chan byte
+	)
+}
+
+// "x's type V is a type parameter and T is not a named type, and values x' of each specific type in V's type set are assignable to T."
+func _[
+	TP0 Interface,
+	TP1 ~_Chan,
+	TP2 ~chan int | ~chan byte,
+](X0 TP0, X1 TP1, X2 TP2) {
+	i = X0
+	I = X0
+	c = X1
+	C = X1 // ERROR cannot use X1 .* as Chan value
+	c = X2 // ERROR .* cannot assign chan byte \(in TP2\) to chan int
+}
+
 // "x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type"
 // TODO(rfindley) error messages about untyped nil diverge from types2 here.
 // Consider aligning them.