go/types, types2: report empty type sets in operand descriptions

This leads to better error messages where operations are not
permitted because of empty type sets.

Fixes #51525.

Change-Id: I8d15645e2aff5145e458bdf9aaa4d2bee28d37fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/452535
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

5 files changed, 36 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/types2/operand.go b/src/cmd/compile/internal/types2/operand.go
index bdbbfc1ecb..e49afee987 100644
--- a/src/cmd/compile/internal/types2/operand.go
+++ b/src/cmd/compile/internal/types2/operand.go
@@ -184,6 +184,10 @@ func operandString(x *operand, qf Qualifier) string {
 			if tpar, _ := x.typ.(*TypeParam); tpar != nil {
 				buf.WriteString(" constrained by ")
 				WriteType(&buf, tpar.bound, qf) // do not compute interface type sets here
+				// If we have the type set and it's empty, say so for better error messages.
+				if hasEmptyTypeset(tpar) {
+					buf.WriteString(" with empty type set")
+				}
 			}
 		} else {
 			buf.WriteString(" with invalid type")
diff --git a/src/cmd/compile/internal/types2/predicates.go b/src/cmd/compile/internal/types2/predicates.go
index c4d11dcac4..acc1549084 100644
--- a/src/cmd/compile/internal/types2/predicates.go
+++ b/src/cmd/compile/internal/types2/predicates.go
@@ -96,6 +96,18 @@ func isTypeParam(t Type) bool {
 	return ok
 }
 
+// hasEmptyTypeset reports whether t is a type parameter with an empty type set.
+// The function does not force the computation of the type set and so is safe to
+// use anywhere, but it may report a false negative if the type set has not been
+// computed yet.
+func hasEmptyTypeset(t Type) bool {
+	if tpar, _ := t.(*TypeParam); tpar != nil && tpar.bound != nil {
+		iface, _ := safeUnderlying(tpar.bound).(*Interface)
+		return iface != nil && iface.tset != nil && iface.tset.IsEmpty()
+	}
+	return false
+}
+
 // isGeneric reports whether a type is a generic, uninstantiated type
 // (generic signatures are not included).
 // TODO(gri) should we include signatures or assert that they are not present?
diff --git a/src/go/types/operand.go b/src/go/types/operand.go
index e21a51a77b..819c99e684 100644
--- a/src/go/types/operand.go
+++ b/src/go/types/operand.go
@@ -171,6 +171,10 @@ func operandString(x *operand, qf Qualifier) string {
 			if tpar, _ := x.typ.(*TypeParam); tpar != nil {
 				buf.WriteString(" constrained by ")
 				WriteType(&buf, tpar.bound, qf) // do not compute interface type sets here
+				// If we have the type set and it's empty, say so for better error messages.
+				if hasEmptyTypeset(tpar) {
+					buf.WriteString(" with empty type set")
+				}
 			}
 		} else {
 			buf.WriteString(" with invalid type")
diff --git a/src/go/types/predicates.go b/src/go/types/predicates.go
index aaf4dd52fc..e9a0e438d8 100644
--- a/src/go/types/predicates.go
+++ b/src/go/types/predicates.go
@@ -98,6 +98,18 @@ func isTypeParam(t Type) bool {
 	return ok
 }
 
+// hasEmptyTypeset reports whether t is a type parameter with an empty type set.
+// The function does not force the computation of the type set and so is safe to
+// use anywhere, but it may report a false negative if the type set has not been
+// computed yet.
+func hasEmptyTypeset(t Type) bool {
+	if tpar, _ := t.(*TypeParam); tpar != nil && tpar.bound != nil {
+		iface, _ := safeUnderlying(tpar.bound).(*Interface)
+		return iface != nil && iface.tset != nil && iface.tset.IsEmpty()
+	}
+	return false
+}
+
 // isGeneric reports whether a type is a generic, uninstantiated type
 // (generic signatures are not included).
 // TODO(gri) should we include signatures or assert that they are not present?
diff --git a/src/internal/types/testdata/fixedbugs/issue51525.go b/src/internal/types/testdata/fixedbugs/issue51525.go
index af1d1e6063..58569056f0 100644
--- a/src/internal/types/testdata/fixedbugs/issue51525.go
+++ b/src/internal/types/testdata/fixedbugs/issue51525.go
@@ -9,6 +9,10 @@ func _[T interface {
 	string
 }](x T) {
 	_ = x /* ERROR empty type set */ == x
+	_ = x /* ERROR empty type set */ + x
+	<-x /* ERROR empty type set */
+	x <- /* ERROR empty type set */ 0
+	close(x /* ERROR empty type set */)
 }
 
 func _[T interface{ int | []byte }](x T) {