go/parser: allow trailing commas in embedded instantiated types

go/parser can correctly parse interfaces that instantiate and embed
generic interfaces, but not structs. This is because in the case of
structs, it does not expect RBRACK as a token trailing COMMA in the type
argument, even though it is allowed by the spec.

For example, go/parser produces an error for the type declaration below:

type A struct {
    B[byte, []byte,]
}

Fixes #56748

Change-Id: Ibb2addd6cf9b381d8470a6d20eedb93f13f93cd6
Reviewed-on: https://go-review.googlesource.com/c/go/+/450175
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

src/go/parser/parser.go

@@ -587,15 +587,19 @@ func (p *parser) parseArrayFieldOrTypeInstance(x *ast.Ident) (*ast.Ident, ast.Ex
 		defer un(trace(p, "ArrayFieldOrTypeInstance"))
 	}
 
-	// TODO(gri) Should we allow a trailing comma in a type argument
-	//           list such as T[P,]? (We do in parseTypeInstance).
 	lbrack := p.expect(token.LBRACK)
+	trailingComma := token.NoPos // if valid, the position of a trailing comma preceding the ']'
 	var args []ast.Expr
 	if p.tok != token.RBRACK {
 		p.exprLev++
 		args = append(args, p.parseRhs())
 		for p.tok == token.COMMA {
+			comma := p.pos
 			p.next()
+			if p.tok == token.RBRACK {
+				trailingComma = comma
+				break
+			}
 			args = append(args, p.parseRhs())
 		}
 		p.exprLev--
@@ -613,6 +617,10 @@ func (p *parser) parseArrayFieldOrTypeInstance(x *ast.Ident) (*ast.Ident, ast.Ex
 		elt := p.tryIdentOrType()
 		if elt != nil {
 			// x [P]E
+			if trailingComma.IsValid() {
+				// Trailing commas are invalid in array type fields.
+				p.error(trailingComma, "unexpected comma; expecting ]")
+			}
 			return x, &ast.ArrayType{Lbrack: lbrack, Len: args[0], Elt: elt}
 		}
 	}

src/go/parser/testdata/tparams.go2

@@ -9,12 +9,19 @@ type _[a t, b t, c /* ERROR "type parameters must be named" */ ] struct{}
 type _ struct {
 	t [n]byte
 	t[a]
+	t[a,]
 	t[a, b]
+	t[a, b,]
+}
+type _ struct {
+	t [n, /* ERROR "unexpected comma; expecting ]" */ ]byte
 }
 type _ interface {
 	t[a]
+	t[a,]
 	m[ /* ERROR "method must have no type parameters" */ _ _, /* ERROR mixed */ _]()
 	t[a, b]
+	t[a, b,]
 }
 
 func _[] /* ERROR "empty type parameter list" */ ()