1 files changed, 24 insertions, 12 deletions

diff --git a/src/cmd/compile/internal/syntax/parser.go b/src/cmd/compile/internal/syntax/parser.go
index e89796cb31..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
 }
 
@@ -1908,9 +1910,10 @@ func (p *parser) paramList(name *Name, close token, requireNames bool) (list []*
 		defer p.trace("paramList")()
 	}
 
-	var named int // number of parameters that have an explicit name and type/bound
-	p.list(_Comma, close, func() bool {
-		par := p.paramDeclOrNil(name)
+	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, close)
 		name = nil // 1st name was consumed if present
 		if par != nil {
 			if debug && par.Name == nil && par.Type == nil {
@@ -1919,6 +1922,9 @@ func (p *parser) paramList(name *Name, close token, requireNames bool) (list []*
 			if par.Name != nil && par.Type != nil {
 				named++
 			}
+			if par.Type != nil {
+				typed++
+			}
 			list = append(list, par)
 		}
 		return false
@@ -1939,10 +1945,11 @@ func (p *parser) paramList(name *Name, close token, requireNames bool) (list []*
 		}
 	} else if named != len(list) {
 		// some named => all must have names and types
-		var pos Pos // left-most error position (or unknown)
-		var typ Expr
+		var pos Pos  // left-most error position (or unknown)
+		var typ Expr // current type (from right to left)
 		for i := len(list) - 1; i >= 0; i-- {
-			if par := list[i]; par.Type != nil {
+			par := list[i]
+			if par.Type != nil {
 				typ = par.Type
 				if par.Name == nil {
 					pos = typ.Pos()
@@ -1961,7 +1968,12 @@ func (p *parser) paramList(name *Name, close token, requireNames bool) (list []*
 		if pos.IsKnown() {
 			var msg string
 			if requireNames {
-				msg = "type parameters must be named"
+				if named == typed {
+					pos = end // position error at closing ]
+					msg = "missing type constraint"
+				} else {
+					msg = "type parameters must be named"
+				}
 			} else {
 				msg = "mixed named and unnamed parameters"
 			}
@@ -2201,7 +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).