[dev.go2go] go/types: avoid endless recursion in self-recursive newtype declarations.

Updates #39634.
Fixes #40037.

Author: tdakkota

src/go/types/check_test.go

@@ -140,6 +140,7 @@ var tests = [][]string{
 	{"fixedbugs/issue39948.go2"},
 	{"fixedbugs/issue39976.go2"},
 	{"fixedbugs/issue39982.go2"},
+	{"fixedbugs/issue40037.go2"},
 }
 
 var fset = token.NewFileSet()

src/go/types/fixedbugs/issue40037.go2

@@ -0,0 +1,10 @@
+package main
+
+// from issue #40037
+type comp(type T comparable) T
+type K /* ERROR illegal cycle */ comp(K) // becomes 'type K K', where K is comparable
+
+// from issue #39634 crash 22
+type T(type _ interface{ a() }) int
+type A(type P) T(A(int))
+

src/go/types/subst.go

@@ -142,6 +142,14 @@ func (check *Checker) instantiate(pos token.Pos, typ Type, targs []Type, poslist
 		// - check only if we have methods
 		check.completeInterface(token.NoPos, iface)
 		if len(iface.allMethods) > 0 {
+			// Break cycle in cases like
+			//
+			//		type constr(type T comparable) T
+			//		type K constr(K)
+			if t1 := typ.Named(); t1 != nil && t1.underlying == t1.orig {
+				break
+			}
+
 			// If the type argument is a type parameter itself, its pointer designation
 			// must match the pointer designation of the callee's type parameter.
 			// If the type argument is a pointer to a type parameter, the type argument's