[dev.typealias] cmd/compile: improved error message for cyles involving type aliases

Known issue: #18640 (requires a bit more work, I believe).

For #18130.

Change-Id: I53dc26012070e0c79f63b7c76266732190a83d47
Reviewed-on: https://go-review.googlesource.com/35129Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/fmt.go

@@ -1077,6 +1077,7 @@ var opprec = []int{
 	OSEND:         3,
 	OANDAND:       2,
 	OOROR:         1,
+
 	// Statements handled by stmtfmt
 	OAS:         -1,
 	OAS2:        -1,
@@ -1104,7 +1105,8 @@ var opprec = []int{
 	OSWITCH:     -1,
 	OXCASE:      -1,
 	OXFALL:      -1,
-	OEND:        0,
+
+	OEND: 0,
 }
 
 func (n *Node) exprfmt(s fmt.State, prec int) {

src/cmd/compile/internal/gc/typecheck.go

@@ -96,16 +96,16 @@ func typekind(t *Type) string {
 	return fmt.Sprintf("etype=%d", et)
 }
 
-// sprint_depchain prints a dependency chain of nodes into fmt.
+// sprint_depchain prints a dependency chain of nodes into trace.
 // It is used by typecheck in the case of OLITERAL nodes
 // to print constant definition loops.
-func sprint_depchain(fmt_ *string, stack []*Node, cur *Node, first *Node) {
+func sprint_depchain(trace *string, stack []*Node, cur *Node, first *Node) {
 	for i := len(stack) - 1; i >= 0; i-- {
 		if n := stack[i]; n.Op == cur.Op {
 			if n != first {
-				sprint_depchain(fmt_, stack[:i], n, first)
+				sprint_depchain(trace, stack[:i], n, first)
 			}
-			*fmt_ += fmt.Sprintf("\n\t%v: %v uses %v", n.Line(), n, cur)
+			*trace += fmt.Sprintf("\n\t%v: %v uses %v", n.Line(), n, cur)
 			return
 		}
 	}
@@ -152,7 +152,6 @@ func typecheck(n *Node, top int) *Node {
 	if n.Typecheck == 2 {
 		// Typechecking loop. Trying printing a meaningful message,
 		// otherwise a stack trace of typechecking.
-		var fmt_ string
 		switch n.Op {
 		// We can already diagnose variables used as types.
 		case ONAME:
@@ -160,22 +159,30 @@ func typecheck(n *Node, top int) *Node {
 				yyerror("%v is not a type", n)
 			}
 
+		case OTYPE:
+			if top&Etype == Etype {
+				var trace string
+				sprint_depchain(&trace, typecheck_tcstack, n, n)
+				yyerrorl(n.Lineno, "invalid recursive type alias %v%s", n, trace)
+			}
+
 		case OLITERAL:
 			if top&(Erv|Etype) == Etype {
 				yyerror("%v is not a type", n)
 				break
 			}
-			sprint_depchain(&fmt_, typecheck_tcstack, n, n)
-			yyerrorl(n.Lineno, "constant definition loop%s", fmt_)
+			var trace string
+			sprint_depchain(&trace, typecheck_tcstack, n, n)
+			yyerrorl(n.Lineno, "constant definition loop%s", trace)
 		}
 
 		if nsavederrors+nerrors == 0 {
-			fmt_ = ""
+			var trace string
 			for i := len(typecheck_tcstack) - 1; i >= 0; i-- {
 				x := typecheck_tcstack[i]
-				fmt_ += fmt.Sprintf("\n\t%v %v", x.Line(), x)
+				trace += fmt.Sprintf("\n\t%v %v", x.Line(), x)
 			}
-			yyerror("typechecking loop involving %v%s", n, fmt_)
+			yyerror("typechecking loop involving %v%s", n, trace)
 		}
 
 		lineno = lno