go/types, types2: better error message for some inference failures

For a unification failure involving a constraint, rather than just
reporting (for instance)

        S does not match []E

now report the inferred type for the type parameter, use spec
terminology when referring to the constraint, and print the
constraint in full:

        S (type func()) does not satisfy ~[]E

There's more we can do, but this is better than what we had.

For #60542.

Change-Id: I033369fa0dfc475f0ec0da0582e8cbefb109f3cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/499639
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -231,7 +231,10 @@ func (check *Checker) infer(pos syntax.Pos, tparams []*TypeParam, targs []Type,
 					// 2) If the core type doesn't have a tilde, we also must unify tx
 					//    with the core type.
 					if !u.unify(tx, core.typ, 0) {
-						check.errorf(pos, CannotInferTypeArgs, "%s does not match %s", tpar, core.typ)
+						// TODO(gri) Type parameters that appear in the constraint and
+						//           for which we have type arguments inferred should
+						//           use those type arguments for a better error message.
+						check.errorf(pos, CannotInferTypeArgs, "%s (type %s) does not satisfy %s", tpar, tx, tpar.Constraint())
 						return nil
 					}
 				case single && !core.tilde:
@@ -249,7 +252,8 @@ func (check *Checker) infer(pos syntax.Pos, tparams []*TypeParam, targs []Type,
 					var cause string
 					constraint := tpar.iface()
 					if m, _ := check.missingMethod(tx, constraint, true, func(x, y Type) bool { return u.unify(x, y, 0) }, &cause); m != nil {
-						check.errorf(pos, CannotInferTypeArgs, "%s does not satisfy %s %s", tx, constraint, cause)
+						// TODO(gri) better error message (see TODO above)
+						check.errorf(pos, CannotInferTypeArgs, "%s (type %s) does not satisfy %s %s", tpar, tx, tpar.Constraint(), cause)
 						return nil
 					}
 				}

src/go/types/infer.go

@@ -233,7 +233,10 @@ func (check *Checker) infer(posn positioner, tparams []*TypeParam, targs []Type,
 					// 2) If the core type doesn't have a tilde, we also must unify tx
 					//    with the core type.
 					if !u.unify(tx, core.typ, 0) {
-						check.errorf(posn, CannotInferTypeArgs, "%s does not match %s", tpar, core.typ)
+						// TODO(gri) Type parameters that appear in the constraint and
+						//           for which we have type arguments inferred should
+						//           use those type arguments for a better error message.
+						check.errorf(posn, CannotInferTypeArgs, "%s (type %s) does not satisfy %s", tpar, tx, tpar.Constraint())
 						return nil
 					}
 				case single && !core.tilde:
@@ -251,7 +254,8 @@ func (check *Checker) infer(posn positioner, tparams []*TypeParam, targs []Type,
 					var cause string
 					constraint := tpar.iface()
 					if m, _ := check.missingMethod(tx, constraint, true, func(x, y Type) bool { return u.unify(x, y, 0) }, &cause); m != nil {
-						check.errorf(posn, CannotInferTypeArgs, "%s does not satisfy %s %s", tx, constraint, cause)
+						// TODO(gri) better error message (see TODO above)
+						check.errorf(posn, CannotInferTypeArgs, "%s (type %s) does not satisfy %s %s", tpar, tx, tpar.Constraint(), cause)
 						return nil
 					}
 				}

src/internal/types/testdata/examples/inference.go

@@ -99,7 +99,7 @@ func _() {
 	// last.
 	related2(1.2, []float64{})
 	related2(1.0, []int{})
-	related2 /* ERROR "Slice does not match []Elem" */ (float64(1.0), []int{}) // TODO(gri) better error message
+	related2 /* ERROR "Slice (type []int) does not satisfy interface{[]Elem}" */ (float64(1.0), []int{}) // TODO(gri) better error message
 }
 
 type List[P any] []P
@@ -117,7 +117,11 @@ func _() {
 	related3 /* ERROR "cannot infer Slice" */ [int]()
 }
 
-func wantsMethods[P interface{ m1(Q); m2() R }, Q, R any](P) {}
+func wantsMethods[P interface {
+	m1(Q)
+	m2() R
+}, Q, R any](P) {
+}
 
 type hasMethods1 struct{}
 
@@ -129,12 +133,12 @@ type hasMethods2 struct{}
 func (*hasMethods2) m1(int)
 func (*hasMethods2) m2() string
 
-type hasMethods3 interface{
+type hasMethods3 interface {
 	m1(float64)
 	m2() complex128
 }
 
-type hasMethods4 interface{
+type hasMethods4 interface {
 	m1()
 }
 
@@ -144,11 +148,11 @@ func _() {
 	// signatures.
 	wantsMethods(hasMethods1{})
 	wantsMethods(&hasMethods1{})
-	wantsMethods /* ERROR "hasMethods2 does not satisfy interface{m1(Q); m2() R} (method m1 has pointer receiver)" */ (hasMethods2{})
+	wantsMethods /* ERROR "P (type hasMethods2) does not satisfy interface{m1(Q); m2() R} (method m1 has pointer receiver)" */ (hasMethods2{})
 	wantsMethods(&hasMethods2{})
 	wantsMethods(hasMethods3(nil))
-	wantsMethods /* ERROR "any does not satisfy interface{m1(Q); m2() R} (missing method m1)" */ (any(nil))
-	wantsMethods /* ERROR "hasMethods4 does not satisfy interface{m1(Q); m2() R} (wrong type for method m1)" */ (hasMethods4(nil))
+	wantsMethods /* ERROR "P (type any) does not satisfy interface{m1(Q); m2() R} (missing method m1)" */ (any(nil))
+	wantsMethods /* ERROR "P (type hasMethods4) does not satisfy interface{m1(Q); m2() R} (wrong type for method m1)" */ (hasMethods4(nil))
 }
 
 // "Reverse" type inference is not yet permitted.

src/internal/types/testdata/fixedbugs/issue45985.go

@@ -9,5 +9,5 @@ func app[S interface{ ~[]T }, T any](s S, e T) S {
 }
 
 func _() {
-	_ = app /* ERROR "S does not match []T" */ [int] // TODO(gri) better error message
+	_ = app /* ERROR "S (type int) does not satisfy interface{~[]T}" */ [int] // TODO(gri) better error message
 }

src/internal/types/testdata/fixedbugs/issue49112.go

@@ -7,9 +7,9 @@ package p
 func f[P int](P) {}
 
 func _() {
-        _ = f[int]
-        _ = f[[ /* ERROR "[]int does not satisfy int" */ ]int]
+	_ = f[int]
+	_ = f[[ /* ERROR "[]int does not satisfy int ([]int missing in int)" */ ]int]
 
-        f(0)
-        f /* ERROR "P does not match int" */ ([]int{}) // TODO(gri) better error message
+	f(0)
+	f /* ERROR "P (type []int) does not satisfy int" */ ([]int{}) // TODO(gri) better error message
 }

src/internal/types/testdata/fixedbugs/issue51472.go

@@ -49,6 +49,6 @@ func f[T interface{comparable; []byte|string}](x T) {
 }
 
 func _(s []byte) {
-	f /* ERROR "T does not match string" */ (s) // TODO(gri) better error message (T's type set only contains string!)
+	f /* ERROR "T (type []byte) does not satisfy interface{comparable; []byte | string}" */ (s) // TODO(gri) better error message (T's type set only contains string!)
         _ = f[[ /* ERROR "does not satisfy" */ ]byte]
 }

src/internal/types/testdata/fixedbugs/issue60542.go

@@ -0,0 +1,12 @@
+// Copyright 2023 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 Clip[S ~[]E, E any](s S) S {
+	return s
+}
+
+var versions func()
+var _ = Clip /* ERROR "S (type func()) does not satisfy ~[]E" */ (versions)