go/parser: complain if type parameters are not permitted

Always accept them but complain if they are not permitted.
The analogous parser check was lost in a recent change.

Also: Fix index computation in collectTypeParams.

Change-Id: I7cb89944984df6dadd7f603b56f0c3941d60f7c5

src/go/parser/parser.go

@@ -1116,11 +1116,14 @@ func (p *parser) parseTypeParams(scope *ast.Scope) *ast.FieldList {
 	return &ast.FieldList{Opening: lbrack, List: fields, Closing: rbrack}
 }
 
-func (p *parser) parseParameters(scope *ast.Scope, typeParamsOk, ellipsisOk bool) (tparams, params *ast.FieldList) {
+func (p *parser) parseParameters(scope *ast.Scope, typeParamsOk, ellipsisOk bool, context string) (tparams, params *ast.FieldList) {
 	if p.trace {
 		defer un(trace(p, "Parameters"))
 	}
 
+	// We always accept type parameters for robustness
+	// and complain later if they are not permitted.
+
 	var lparen token.Pos
 	if useBrackets && p.tok == token.LBRACK {
 		// assume [type T](params) syntax
@@ -1142,6 +1145,11 @@ func (p *parser) parseParameters(scope *ast.Scope, typeParamsOk, ellipsisOk bool
 		}
 	}
 
+	if tparams != nil && !typeParamsOk {
+		p.error(tparams.Opening, context+" must have no type parameters")
+		tparams = nil
+	}
+
 	var fields []*ast.Field
 	if p.tok != token.RPAREN {
 		fields = p.parseParameterList(scope, ellipsisOk)
@@ -1159,7 +1167,7 @@ func (p *parser) parseResult(scope *ast.Scope, typeContext bool) *ast.FieldList
 	}
 
 	if p.tok == token.LPAREN {
-		_, results := p.parseParameters(scope, false, false)
+		_, results := p.parseParameters(scope, false, false, "result")
 		return results
 	}
 
@@ -1180,7 +1188,7 @@ func (p *parser) parseFuncType(typeContext bool) (*ast.FuncType, *ast.Scope) {
 
 	pos := p.expect(token.FUNC)
 	scope := ast.NewScope(p.topScope) // function scope
-	_, params := p.parseParameters(scope, false, true)
+	_, params := p.parseParameters(scope, false, true, "function type")
 	results := p.parseResult(scope, typeContext)
 
 	return &ast.FuncType{Func: pos, Params: params, Results: results}, scope
@@ -1199,7 +1207,7 @@ func (p *parser) parseMethodSpec(scope *ast.Scope) *ast.Field {
 		// method
 		idents = []*ast.Ident{ident}
 		scope := ast.NewScope(nil) // method scope
-		_, params := p.parseParameters(scope, false, true)
+		_, params := p.parseParameters(scope, false, true, "method")
 		results := p.parseResult(scope, true)
 		typ = &ast.FuncType{Func: token.NoPos, Params: params, Results: results}
 	} else {
@@ -1340,7 +1348,7 @@ func (p *parser) parseConstraint() *ast.Constraint {
 			// method
 			mname = ident
 			scope := ast.NewScope(nil) // method scope
-			_, params := p.parseParameters(scope, false, true)
+			_, params := p.parseParameters(scope, false, true, "method")
 			results := p.parseResult(scope, true)
 			typ = &ast.FuncType{Func: token.NoPos, Params: params, Results: results}
 		}
@@ -2886,12 +2894,12 @@ func (p *parser) parseFuncDecl() *ast.FuncDecl {
 
 	var recv *ast.FieldList
 	if p.tok == token.LPAREN {
-		_, recv = p.parseParameters(scope, false, false)
+		_, recv = p.parseParameters(scope, false, false, "receiver")
 	}
 
 	ident := p.parseIdent()
 
-	tparams, params := p.parseParameters(scope, recv == nil, true)
+	tparams, params := p.parseParameters(scope, recv == nil, true, "method") // context string only used in methods
 	results := p.parseResult(scope, true)
 
 	var body *ast.BlockStmt

src/go/types/decl.go

@@ -635,13 +635,12 @@ func (check *Checker) collectTypeParams(list *ast.FieldList) (tparams []*TypeNam
 		}
 
 		// set the type parameter's bound
-		if bound != nil {
-			for _, name := range f.Names {
-				tname := tparams[index]
-				assert(name.Name == tname.name)
-				tname.typ.(*TypeParam).bound = bound
-				index++
-			}
+		// (do this even if bound == nil to make sure index is correctly increasing)
+		for _, name := range f.Names {
+			tname := tparams[index]
+			assert(name.Name == tname.name) // keep - this assertion caught index errors
+			tname.typ.(*TypeParam).bound = bound
+			index++
 		}
 	}