diff --git a/src/cmd/compile/internal/noder/linker.go b/src/cmd/compile/internal/noder/linker.go
index ed47a355d8..eefb5083e5 100644
--- a/src/cmd/compile/internal/noder/linker.go
+++ b/src/cmd/compile/internal/noder/linker.go
@@ -149,9 +149,11 @@ func (l *linker) relocObj(pr *pkgReader, idx int) int {
 		var ok bool
 		obj, ok = sym.Def.(*ir.Name)
 
-		// Generic types and functions won't have definitions.
+		// Generic types and functions and declared constraint types won't
+		// have definitions.
 		// For now, just generically copy their extension data.
-		if !ok && base.Flag.G == 0 {
+		// TODO(mdempsky): Restore assertion.
+		if !ok && false {
 			base.Fatalf("missing definition for %v", sym)
 		}
 	}
diff --git a/src/cmd/compile/internal/noder/noder.go b/src/cmd/compile/internal/noder/noder.go
index 3d83129aea..6a2aacd3fe 100644
--- a/src/cmd/compile/internal/noder/noder.go
+++ b/src/cmd/compile/internal/noder/noder.go
@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"go/constant"
 	"go/token"
+	"internal/buildcfg"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -30,8 +31,11 @@ import (
 func LoadPackage(filenames []string) {
 	base.Timer.Start("fe", "parse")
 
+	// -G=3 and unified expect generics syntax, but -G=0 does not.
+	supportsGenerics := base.Flag.G != 0 || buildcfg.Experiment.Unified
+
 	mode := syntax.CheckBranches
-	if base.Flag.G != 0 {
+	if supportsGenerics && types.AllowsGoVersion(types.LocalPkg, 1, 18) {
 		mode |= syntax.AllowGenerics
 	}