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>

1 files changed, 12 insertions, 0 deletions

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?