cmd/compile/internal/syntax: better error message for missing type constraint

For #43527.

Change-Id: I8c706e68572286d5675383eb2dfd75b5618b646b
Reviewed-on: https://go-review.googlesource.com/c/go/+/348730
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>

src/cmd/compile/internal/syntax/parser.go

@@ -1908,8 +1908,9 @@ 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
+	var named int // number of parameters that have an explicit name and type
-	p.list(_Comma, close, func() bool {
+	var typed int // number of parameters that have an explicit type
+	end := p.list(_Comma, close, func() bool {
 		par := p.paramDeclOrNil(name)
 		name = nil // 1st name was consumed if present
 		if par != nil {
@@ -1919,6 +1920,9 @@ 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
@@ -1940,9 +1944,10 @@ 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 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 +1966,12 @@ func (p *parser) paramList(name *Name, close token, requireNames bool) (list []*
 		if pos.IsKnown() {
 			var msg string
 			if requireNames {
+				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"
 			}

src/cmd/compile/internal/syntax/testdata/issue43527.go2

@@ -0,0 +1,23 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type (
+        // 0 and 1-element []-lists are syntactically valid
+        _[A, B /* ERROR missing type constraint */ ] int
+        _[A, /* ERROR type parameters must be named */ interface{}] int
+        _[A, B, C /* ERROR missing type constraint */ ] int
+        _[A B, C /* ERROR missing type constraint */ ] int
+        _[A B, /* ERROR type parameters must be named */ interface{}] int
+        _[A B, /* ERROR type parameters must be named */ interface{}, C D] int
+        _[A B, /* ERROR type parameters must be named */ interface{}, C, D] int
+        _[A B, /* ERROR type parameters must be named */ interface{}, C, interface{}] int
+        _[A B, C interface{}, D, /* ERROR type parameters must be named */ interface{}] int
+)
+
+// function type parameters use the same parsing routine - just have a couple of tests
+
+func _[A, B /* ERROR missing type constraint */ ]() {}
+func _[A, /* ERROR type parameters must be named */ interface{}]() {}

src/cmd/compile/internal/syntax/testdata/tparams.go2

@@ -4,8 +4,8 @@
 
 package p
 
-type t[ /* ERROR type parameters must be named */ a, b] struct{}
+type t[a, b /* ERROR missing type constraint */ ] struct{}
-type t[a t, b t, /* ERROR type parameters must be named */ c] struct{}
+type t[a t, b t, c /* ERROR missing type constraint */ ] struct{}
 type t struct {
 	t [n]byte
 	t[a]
@@ -18,5 +18,5 @@ type t interface {
 }
 
 func f[ /* ERROR empty type parameter list */ ]()
-func f[ /* ERROR type parameters must be named */ a, b]()
+func f[a, b /* ERROR missing type constraint */ ]()
-func f[a t, b t, /* ERROR type parameters must be named */ c]()
+func f[a t, b t, c /* ERROR missing type constraint */ ]()

src/cmd/compile/internal/types2/testdata/fixedbugs/issue47996.go2

@@ -5,4 +5,4 @@
 package p
 
 // don't crash
-func T /* ERROR missing */ [P /* ERROR named */ ] m /* ERROR m */ () /* ERROR \) */ { /* ERROR { */ } /* ERROR } */
+func T /* ERROR missing */ [P] /* ERROR missing */ m /* ERROR unexpected */ () /* ERROR \) */ { /* ERROR { */ } /* ERROR } */