[dev.typeparams] go/types: use a new ast.ListExpr for multi-type instances

Modify go/parser to consistently represent type instantiation as an
ast.IndexExpr, rather than use an ast.CallExpr (with Brackets:true) for
instantiations with multiple type parameters. To enable this, introduce
a new ast expr type: ListExpr.

This brings go/types in line with types2, with the exception of a small
change to funcInst to eliminate redundant errors if values are
erroneously used as types. In a subsequent CL, call.go and expr.go will
be marked as reviewed.

This also catches some type instance syntax using '()' that was
previously accepted incorrectly. Tests are updated accordingly.

Change-Id: I30cd0181c7608f1be7486a9a8b63df993b412e85
Reviewed-on: https://go-review.googlesource.com/c/go/+/293010
Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 5 insertions, 10 deletions

diff --git a/src/go/parser/parser.go b/src/go/parser/parser.go
index ccbcef8f26..e12eee79bf 100644
--- a/src/go/parser/parser.go
+++ b/src/go/parser/parser.go
@@ -754,7 +754,7 @@ func (p *parser) parseArrayFieldOrTypeInstance(x *ast.Ident) (*ast.Ident, ast.Ex
 	}
 
 	// x[P], x[P1, P2], ...
-	return nil, &ast.CallExpr{Fun: x, Lparen: lbrack, Args: args, Rparen: rbrack, Brackets: true}
+	return nil, &ast.IndexExpr{X: x, Lbrack: lbrack, Index: &ast.ListExpr{ElemList: args}, Rbrack: rbrack}
 }
 
 func (p *parser) parseFieldDecl(scope *ast.Scope) *ast.Field {
@@ -1153,7 +1153,7 @@ func (p *parser) parseMethodSpec(scope *ast.Scope) *ast.Field {
 					p.exprLev--
 				}
 				rbrack := p.expectClosing(token.RBRACK, "type argument list")
-				typ = &ast.CallExpr{Fun: ident, Lparen: lbrack, Args: list, Rparen: rbrack, Brackets: true}
+				typ = &ast.IndexExpr{X: ident, Lbrack: lbrack, Index: &ast.ListExpr{ElemList: list}, Rbrack: rbrack}
 			}
 		case p.tok == token.LPAREN:
 			// ordinary method
@@ -1281,7 +1281,7 @@ func (p *parser) parseTypeInstance(typ ast.Expr) ast.Expr {
 
 	closing := p.expectClosing(token.RBRACK, "type argument list")
 
-	return &ast.CallExpr{Fun: typ, Lparen: opening, Args: list, Rparen: closing, Brackets: true}
+	return &ast.IndexExpr{X: typ, Lbrack: opening, Index: &ast.ListExpr{ElemList: list}, Rbrack: closing}
 }
 
 // If the result is an identifier, it is not resolved.
@@ -1557,7 +1557,7 @@ func (p *parser) parseIndexOrSliceOrInstance(x ast.Expr) ast.Expr {
 	}
 
 	// instance expression
-	return &ast.CallExpr{Fun: x, Lparen: lbrack, Args: args, Rparen: rbrack, Brackets: true}
+	return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: &ast.ListExpr{ElemList: args}, Rbrack: rbrack}
 }
 
 func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
@@ -1773,17 +1773,12 @@ func (p *parser) parsePrimaryExpr(lhs bool) (x ast.Expr) {
 			// type; accept it but complain if we have a complit
 			t := unparen(x)
 			// determine if '{' belongs to a composite literal or a block statement
-			switch t := t.(type) {
+			switch t.(type) {
 			case *ast.BadExpr, *ast.Ident, *ast.SelectorExpr:
 				if p.exprLev < 0 {
 					return
 				}
 				// x is possibly a composite literal type
-			case *ast.CallExpr:
-				if !t.Brackets || p.exprLev < 0 {
-					return
-				}
-				// x is possibly a composite literal type
 			case *ast.IndexExpr:
 				if p.exprLev < 0 {
 					return