cmd/compile: better error message for when a type is in a constraint but not the type set

While at it, also remove the word "constraint" in the detail explanation
of an unsatisfied constraint.

Fixes #57500.

Change-Id: I55dae1694de2cfdb434aeba9d4a3530af7aca8f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/460455
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>

6 files changed, 88 insertions, 14 deletions

diff --git a/src/cmd/compile/internal/types2/instantiate.go b/src/cmd/compile/internal/types2/instantiate.go
index f028161118..8193682993 100644
--- a/src/cmd/compile/internal/types2/instantiate.go
+++ b/src/cmd/compile/internal/types2/instantiate.go
@@ -324,14 +324,43 @@ func (check *Checker) implements(V, T Type, constraint bool, cause *string) bool
 		return false
 	}) {
 		if cause != nil {
-			if alt != nil {
-				*cause = check.sprintf("%s does not %s %s (possibly missing ~ for %s in constraint %s)", V, verb, T, alt, T)
-			} else {
-				*cause = check.sprintf("%s does not %s %s (%s missing in %s)", V, verb, T, V, Ti.typeSet().terms)
+			var detail string
+			switch {
+			case alt != nil:
+				detail = check.sprintf("possibly missing ~ for %s in %s", alt, T)
+			case mentions(Ti, V):
+				detail = check.sprintf("%s mentions %s, but %s is not in the type set of %s", T, V, V, T)
+			default:
+				detail = check.sprintf("%s missing in %s", V, Ti.typeSet().terms)
 			}
+			*cause = check.sprintf("%s does not %s %s (%s)", V, verb, T, detail)
 		}
 		return false
 	}
 
 	return checkComparability()
 }
+
+// mentions reports whether type T "mentions" typ in an (embedded) element or term
+// of T (whether typ is in the type set of T or not). For better error messages.
+func mentions(T, typ Type) bool {
+	switch T := T.(type) {
+	case *Interface:
+		for _, e := range T.embeddeds {
+			if mentions(e, typ) {
+				return true
+			}
+		}
+	case *Union:
+		for _, t := range T.terms {
+			if mentions(t.typ, typ) {
+				return true
+			}
+		}
+	default:
+		if Identical(T, typ) {
+			return true
+		}
+	}
+	return false
+}
diff --git a/src/go/types/instantiate.go b/src/go/types/instantiate.go
index 9f565c326b..2cf48c17d2 100644
--- a/src/go/types/instantiate.go
+++ b/src/go/types/instantiate.go
@@ -324,14 +324,43 @@ func (check *Checker) implements(V, T Type, constraint bool, cause *string) bool
 		return false
 	}) {
 		if cause != nil {
-			if alt != nil {
-				*cause = check.sprintf("%s does not %s %s (possibly missing ~ for %s in constraint %s)", V, verb, T, alt, T)
-			} else {
-				*cause = check.sprintf("%s does not %s %s (%s missing in %s)", V, verb, T, V, Ti.typeSet().terms)
+			var detail string
+			switch {
+			case alt != nil:
+				detail = check.sprintf("possibly missing ~ for %s in %s", alt, T)
+			case mentions(Ti, V):
+				detail = check.sprintf("%s mentions %s, but %s is not in the type set of %s", T, V, V, T)
+			default:
+				detail = check.sprintf("%s missing in %s", V, Ti.typeSet().terms)
 			}
+			*cause = check.sprintf("%s does not %s %s (%s)", V, verb, T, detail)
 		}
 		return false
 	}
 
 	return checkComparability()
 }
+
+// mentions reports whether type T "mentions" typ in an (embedded) element or term
+// of T (whether typ is in the type set of T or not). For better error messages.
+func mentions(T, typ Type) bool {
+	switch T := T.(type) {
+	case *Interface:
+		for _, e := range T.embeddeds {
+			if mentions(e, typ) {
+				return true
+			}
+		}
+	case *Union:
+		for _, t := range T.terms {
+			if mentions(t.typ, typ) {
+				return true
+			}
+		}
+	default:
+		if Identical(T, typ) {
+			return true
+		}
+	}
+	return false
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue40350.go b/src/internal/types/testdata/fixedbugs/issue40350.go
index 96ad1678d4..08eb426410 100644
--- a/src/internal/types/testdata/fixedbugs/issue40350.go
+++ b/src/internal/types/testdata/fixedbugs/issue40350.go
@@ -12,5 +12,5 @@ type number interface {
 func f[T number]() {}
 
 func _() {
-	_ = f[int /* ERROR int does not satisfy number \(int missing in float64 \| ~int32\)*/]
+	_ = f[int /* ERROR int does not satisfy number \(number mentions int, but int is not in the type set of number\)*/]
 }
diff --git a/src/internal/types/testdata/fixedbugs/issue49179.go b/src/internal/types/testdata/fixedbugs/issue49179.go
index 468d83edbe..2ddfa3312d 100644
--- a/src/internal/types/testdata/fixedbugs/issue49179.go
+++ b/src/internal/types/testdata/fixedbugs/issue49179.go
@@ -13,11 +13,11 @@ type myFloat float64
 
 func _() {
 	_ = f1[int]
-	_ = f1[myInt /* ERROR possibly missing ~ for int in constraint int \| string */]
+	_ = f1[myInt /* ERROR possibly missing ~ for int in int \| string */]
 	_ = f2[myInt]
-	_ = f2[myFloat /* ERROR possibly missing ~ for float64 in constraint ~int \| string \| float64 */]
+	_ = f2[myFloat /* ERROR possibly missing ~ for float64 in ~int \| string \| float64 */]
 	var x myInt
-	f3 /* ERROR myInt does not satisfy int \(possibly missing ~ for int in constraint int\) */ (x)
+	f3 /* ERROR myInt does not satisfy int \(possibly missing ~ for int in int\) */ (x)
 }
 
 // test case from the issue
@@ -33,5 +33,5 @@ func Map[S SliceConstraint[E], E any](s S, f func(E) E) S {
 type MySlice []int
 
 func f(s MySlice) {
-	Map[MySlice /* ERROR MySlice does not satisfy SliceConstraint\[int\] \(possibly missing ~ for \[\]int in constraint SliceConstraint\[int\]\) */, int](s, nil)
+	Map[MySlice /* ERROR MySlice does not satisfy SliceConstraint\[int\] \(possibly missing ~ for \[\]int in SliceConstraint\[int\]\) */, int](s, nil)
 }
diff --git a/src/internal/types/testdata/fixedbugs/issue57486.go b/src/internal/types/testdata/fixedbugs/issue57486.go
index f6ba1b60b8..43ba1b0440 100644
--- a/src/internal/types/testdata/fixedbugs/issue57486.go
+++ b/src/internal/types/testdata/fixedbugs/issue57486.go
@@ -24,6 +24,6 @@ func F1[V [2]any](v V) {
 
 func F2[V [2]any](v V) {
 	_ = G2[V /* ERROR "V does not satisfy C2" */]
-	_ = G2[[ /* ERROR "\[2\]any does not satisfy C2 \(\[2\]any missing in int\)" */ 2]any]
+	_ = G2[[ /* ERROR "\[2\]any does not satisfy C2 \(C2 mentions \[2\]any, but \[2\]any is not in the type set of C2\)" */ 2]any]
 	_ = G2[int]
 }
diff --git a/src/internal/types/testdata/fixedbugs/issue57500.go b/src/internal/types/testdata/fixedbugs/issue57500.go
new file mode 100644
index 0000000000..abdcb5ea35
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue57500.go
@@ -0,0 +1,16 @@
+// 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
+
+type C interface {
+	comparable
+	[2]any | int
+}
+
+func f[T C]() {}
+
+func _() {
+	_ = f[[ /* ERROR \[2\]any does not satisfy C \(C mentions \[2\]any, but \[2\]any is not in the type set of C\) */ 2]any]
+}