go/types: improve recursive type error message

This change improves error message for recursive types.
Currently, compilation of the [following program](https://go.dev/play/p/3ef84ObpzfG):

package main

type T1[T T2] struct{}
type T2[T T1] struct{}

returns an error:

./prog.go:3:6: invalid recursive type T1
	./prog.go:3:6: T1 refers to
	./prog.go:4:6: T2 refers to
	./prog.go:3:6: T1

With the patch applied the error message looks like:

./prog.go:3:6: invalid recursive type T1
	./prog.go:3:6: T1 refers to T2
	./prog.go:4:6: T2 refers to T1

Change-Id: Ic07cdffcffb1483c672b241fede4e694269b5b79
GitHub-Last-Rev: cd042fdc384cf5591b3258ca80fdc002bb8c5e0d
GitHub-Pull-Request: golang/go#69574
Reviewed-on: https://go-review.googlesource.com/c/go/+/614084
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tim King <taking@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

4 files changed, 23 insertions, 32 deletions

diff --git a/src/cmd/compile/internal/types2/decl.go b/src/cmd/compile/internal/types2/decl.go
index 037155a6ca..b8e43231a1 100644
--- a/src/cmd/compile/internal/types2/decl.go
+++ b/src/cmd/compile/internal/types2/decl.go
@@ -334,17 +334,14 @@ func (check *Checker) cycleError(cycle []Object, start int) {
 	} else {
 		err.addf(obj, "invalid cycle in declaration of %s", objName)
 	}
-	i := start
-	for range cycle {
-		err.addf(obj, "%s refers to", objName)
-		i++
-		if i >= len(cycle) {
-			i = 0
-		}
-		obj = cycle[i]
-		objName = name(obj)
+	// "cycle[i] refers to cycle[j]" for (i,j) = (s, s+1), (s+1, s+2), ..., (n, 0), (0,1), ..., (s-1,s) for len(cycle) = n, s = start.
+	n := len(cycle)
+	rotate := func(i int) int { return (i + start) % n }
+	for i := range n {
+		obj := cycle[rotate(i)]
+		next := cycle[rotate(i+1)]
+		err.addf(obj, "%s refers to %s", name(obj), name(next))
 	}
-	err.addf(obj, "%s", objName)
 	err.report()
 }
 
diff --git a/src/cmd/compile/internal/types2/initorder.go b/src/cmd/compile/internal/types2/initorder.go
index 09a53c98ef..9efbf7f69f 100644
--- a/src/cmd/compile/internal/types2/initorder.go
+++ b/src/cmd/compile/internal/types2/initorder.go
@@ -163,13 +163,11 @@ func (check *Checker) reportCycle(cycle []Object) {
 
 	err := check.newError(InvalidInitCycle)
 	err.addf(obj, "initialization cycle for %s", obj.Name())
-	// subtle loop: print cycle[i] for i = 0, n-1, n-2, ... 1 for len(cycle) = n
-	for i := len(cycle) - 1; i >= 0; i-- {
-		err.addf(obj, "%s refers to", obj.Name())
-		obj = cycle[i]
+	// "cycle[i] refers to cycle[j]" for (i,j) = (0, n-1), (n-1, n-2), ..., (1,0) for len(cycle) = n.
+	for j := len(cycle) - 1; j >= 0; j-- {
+		err.addf(obj, "%s refers to %s", obj.Name(), cycle[j].Name())
+		obj = cycle[j]
 	}
-	// print cycle[0] again to close the cycle
-	err.addf(obj, "%s", obj.Name())
 	err.report()
 }
 
diff --git a/src/go/types/decl.go b/src/go/types/decl.go
index 4fd37df786..498eb16f84 100644
--- a/src/go/types/decl.go
+++ b/src/go/types/decl.go
@@ -335,17 +335,15 @@ func (check *Checker) cycleError(cycle []Object, start int) {
 	} else {
 		err.addf(obj, "invalid cycle in declaration of %s", objName)
 	}
-	i := start
-	for range cycle {
-		err.addf(obj, "%s refers to", objName)
-		i++
-		if i >= len(cycle) {
-			i = 0
-		}
-		obj = cycle[i]
-		objName = name(obj)
+
+	// "cycle[i] refers to cycle[j]" for (i,j) = (s, s+1), (s+1, s+2), ..., (n, 0), (0,1), ..., (s-1,s) for len(cycle) = n, s = start.
+	n := len(cycle)
+	rotate := func(i int) int { return (i + start) % n }
+	for i := range n {
+		obj := cycle[rotate(i)]
+		next := cycle[rotate(i+1)]
+		err.addf(obj, "%s refers to %s", name(obj), name(next))
 	}
-	err.addf(obj, "%s", objName)
 	err.report()
 }
 
diff --git a/src/go/types/initorder.go b/src/go/types/initorder.go
index 077f2eccfe..682a63e2d7 100644
--- a/src/go/types/initorder.go
+++ b/src/go/types/initorder.go
@@ -166,13 +166,11 @@ func (check *Checker) reportCycle(cycle []Object) {
 
 	err := check.newError(InvalidInitCycle)
 	err.addf(obj, "initialization cycle for %s", obj.Name())
-	// subtle loop: print cycle[i] for i = 0, n-1, n-2, ... 1 for len(cycle) = n
-	for i := len(cycle) - 1; i >= 0; i-- {
-		err.addf(obj, "%s refers to", obj.Name())
-		obj = cycle[i]
+	// "cycle[i] refers to cycle[j]" for (i,j) = (0, n-1), (n-1, n-2), ..., (1,0) for len(cycle) = n.
+	for j := len(cycle) - 1; j >= 0; j-- {
+		err.addf(obj, "%s refers to %s", obj.Name(), cycle[j].Name())
+		obj = cycle[j]
 	}
-	// print cycle[0] again to close the cycle
-	err.addf(obj, "%s", obj.Name())
 	err.report()
 }