5 files changed, 19 insertions, 13 deletions
diff --git a/src/cmd/compile/internal/types2/expr.go b/src/cmd/compile/internal/types2/expr.go
index 17096ee418..25e2060100 100644
--- a/src/cmd/compile/internal/types2/expr.go
+++ b/src/cmd/compile/internal/types2/expr.go
@@ -1459,9 +1459,14 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 		if x.mode == invalid {
 			goto Error
 		}
+		// TODO(gri) we may want to permit type assertions on type parameter values at some point
+		if isTypeParam(x.typ) {
+			check.errorf(x, invalidOp+"cannot use type assertion on type parameter value %s", x)
+			goto Error
+		}
 		xtyp, _ := under(x.typ).(*Interface)
 		if xtyp == nil {
-			check.errorf(x, "%s is not an interface type", x)
+			check.errorf(x, invalidOp+"%s is not an interface", x)
 			goto Error
 		}
 		// x.(type) expressions are encoded via TypeSwitchGuards
diff --git a/src/cmd/compile/internal/types2/stmt.go b/src/cmd/compile/internal/types2/stmt.go
index f9c07e38cd..6869c87929 100644
--- a/src/cmd/compile/internal/types2/stmt.go
+++ b/src/cmd/compile/internal/types2/stmt.go
@@ -733,13 +733,14 @@ func (check *Checker) typeSwitchStmt(inner stmtContext, s *syntax.SwitchStmt, gu
 	if x.mode == invalid {
 		return
 	}
-	// Caution: We're not using asInterface here because we don't want
-	//          to switch on a suitably constrained type parameter (for
-	//          now).
-	// TODO(gri) Need to revisit this.
+	// TODO(gri) we may want to permit type switches on type parameter values at some point
+	if isTypeParam(x.typ) {
+		check.errorf(&x, "cannot use type switch on type parameter value %s", &x)
+		return
+	}
 	xtyp, _ := under(x.typ).(*Interface)
 	if xtyp == nil {
-		check.errorf(&x, "%s is not an interface type", &x)
+		check.errorf(&x, "%s is not an interface", &x)
 		return
 	}
 
diff --git a/src/cmd/compile/internal/types2/testdata/check/typeparams.go2 b/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
index b1d02efdb5..03c3f9a0b5 100644
--- a/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
@@ -482,8 +482,8 @@ func (_ R2[X, Y]) m2(X) Y
 // type assertions and type switches over generic types lead to errors for now
 
 func _[T any](x T) {
-	_ = x /* ERROR not an interface */ .(int)
-	switch x /* ERROR not an interface */ .(type) {
+	_ = x /* ERROR cannot use type assertion */ .(int)
+	switch x /* ERROR cannot use type switch */ .(type) {
 	}
 
 	// work-around
@@ -494,8 +494,8 @@ func _[T any](x T) {
 }
 
 func _[T interface{~int}](x T) {
-	_ = x /* ERROR not an interface */ .(int)
-	switch x /* ERROR not an interface */ .(type) {
+	_ = x /* ERROR cannot use type assertion */ .(int)
+	switch x /* ERROR cannot use type switch */ .(type) {
 	}
 
 	// work-around
diff --git a/test/interface/explicit.go b/test/interface/explicit.go
index 1b7af6712b..f769f5878c 100644
--- a/test/interface/explicit.go
+++ b/test/interface/explicit.go
@@ -57,7 +57,7 @@ func main() {
 
 	// cannot type-assert non-interfaces
 	f := 2.0
-	_ = f.(int) // ERROR "non-interface type|only valid for interface types|not an interface type"
+	_ = f.(int) // ERROR "non-interface type|only valid for interface types|not an interface"
 
 }
 
diff --git a/test/typeswitch3.go b/test/typeswitch3.go
index a57889bc1d..2e144d81c0 100644
--- a/test/typeswitch3.go
+++ b/test/typeswitch3.go
@@ -42,7 +42,7 @@ func main() {
 
 func noninterface() {
 	var i int
-	switch i.(type) { // ERROR "cannot type switch on non-interface value|not an interface type"
+	switch i.(type) { // ERROR "cannot type switch on non-interface value|not an interface"
 	case string:
 	case int:
 	}
@@ -51,6 +51,6 @@ func noninterface() {
 		name string
 	}
 	var s S
-	switch s.(type) { // ERROR "cannot type switch on non-interface value|not an interface type"
+	switch s.(type) { // ERROR "cannot type switch on non-interface value|not an interface"
 	}
 }