cmd/compile/internal/syntax: correct follow token for type parameter lists

When parsing a type parameter declaration, parts of the code still
expected a ) as closing token. Use the correct follow token ) or ]
depending on parameter list kind.

Also, consistently use tokstring (not tok.String()) for user-facing
(error) messages.

Follow-up on comment in CL 348730.

For #43527.

Change-Id: Ib1d4feb526771a1668a54c3bb7a671f6c8a65940
Reviewed-on: https://go-review.googlesource.com/c/go/+/348742
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

2 files changed, 10 insertions, 6 deletions

diff --git a/src/cmd/compile/internal/syntax/parser.go b/src/cmd/compile/internal/syntax/parser.go
index c836a21c2f..82cb06b180 100644
--- a/src/cmd/compile/internal/syntax/parser.go
+++ b/src/cmd/compile/internal/syntax/parser.go
@@ -276,7 +276,9 @@ func (p *parser) syntaxErrorAt(pos Pos, msg string) {
 }
 
 // tokstring returns the English word for selected punctuation tokens
-// for more readable error messages.
+// for more readable error messages. Use tokstring (not tok.String())
+// for user-facing (error) messages; use tok.String() for debugging
+// output.
 func tokstring(tok token) string {
 	switch tok {
 	case _Comma:
@@ -1839,7 +1841,7 @@ func (p *parser) embeddedTerm() Expr {
 }
 
 // ParameterDecl = [ IdentifierList ] [ "..." ] Type .
-func (p *parser) paramDeclOrNil(name *Name) *Field {
+func (p *parser) paramDeclOrNil(name *Name, follow token) *Field {
 	if trace {
 		defer p.trace("paramDecl")()
 	}
@@ -1893,8 +1895,8 @@ func (p *parser) paramDeclOrNil(name *Name) *Field {
 		return f
 	}
 
-	p.syntaxError("expecting )")
-	p.advance(_Comma, _Rparen)
+	p.syntaxError("expecting " + tokstring(follow))
+	p.advance(_Comma, follow)
 	return nil
 }
 
@@ -1911,7 +1913,7 @@ func (p *parser) paramList(name *Name, close token, requireNames bool) (list []*
 	var named int // number of parameters that have an explicit name and type
 	var typed int // number of parameters that have an explicit type
 	end := p.list(_Comma, close, func() bool {
-		par := p.paramDeclOrNil(name)
+		par := p.paramDeclOrNil(name, close)
 		name = nil // 1st name was consumed if present
 		if par != nil {
 			if debug && par.Name == nil && par.Type == nil {
@@ -2211,7 +2213,7 @@ func (p *parser) header(keyword token) (init SimpleStmt, cond Expr, post SimpleS
 	if p.tok != _Semi {
 		// accept potential varDecl but complain
 		if p.got(_Var) {
-			p.syntaxError(fmt.Sprintf("var declaration not allowed in %s initializer", keyword.String()))
+			p.syntaxError(fmt.Sprintf("var declaration not allowed in %s initializer", tokstring(keyword)))
 		}
 		init = p.simpleStmt(nil, keyword)
 		// If we have a range clause, we are done (can only happen for keyword == _For).
diff --git a/src/cmd/compile/internal/syntax/testdata/tparams.go2 b/src/cmd/compile/internal/syntax/testdata/tparams.go2
index 8e47ff5ed8..80e155bfe0 100644
--- a/src/cmd/compile/internal/syntax/testdata/tparams.go2
+++ b/src/cmd/compile/internal/syntax/testdata/tparams.go2
@@ -20,3 +20,5 @@ type t interface {
 func f[ /* ERROR empty type parameter list */ ]()
 func f[a, b /* ERROR missing type constraint */ ]()
 func f[a t, b t, c /* ERROR missing type constraint */ ]()
+
+func f[a b,  /* ERROR expecting ] */ 0] ()