go/parser, syntax: better error message for parameter missing type

Fixes #69506.

Change-Id: I18215e11f214b12d5f65be1d1740181e427f8817
Reviewed-on: https://go-review.googlesource.com/c/go/+/617015
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -2075,26 +2075,31 @@ func (p *parser) paramList(name *Name, typ Expr, close token, requireNames bool)
 			}
 		}
 		if errPos.IsKnown() {
-			var msg string
-			if requireNames {
 			// Not all parameters are named because named != len(list).
-				// If named == typed we must have parameters that have no types,
+			// If named == typed, there must be parameters that have no types.
-				// and they must be at the end of the parameter list, otherwise
+			// They must be at the end of the parameter list, otherwise types
-				// the types would have been filled in by the right-to-left sweep
+			// would have been filled in by the right-to-left sweep above and
-				// above and we wouldn't have an error. Since we are in a type
+			// there would be no error.
-				// parameter list, the missing types are constraints.
+			// If requireNames is set, the parameter list is a type parameter
+			// list.
+			var msg string
 			if named == typed {
-					errPos = end // position error at closing ]
+				errPos = end // position error at closing token ) or ]
+				if requireNames {
 					msg = "missing type constraint"
 				} else {
+					msg = "missing parameter type"
+				}
+			} else {
+				if requireNames {
 					msg = "missing type parameter name"
 					// go.dev/issue/60812
 					if len(list) == 1 {
 						msg += " or invalid array length"
 					}
-				}
 				} else {
-				msg = "mixed named and unnamed parameters"
+					msg = "missing parameter name"
+				}
 			}
 			p.syntaxErrorAt(errPos, msg)
 		}

src/cmd/compile/internal/syntax/testdata/issue69506.go

@@ -0,0 +1,9 @@
+// Copyright 2024 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
+
+func _(a int, b /* ERROR missing parameter type */ )
+func _(a int, /* ERROR missing parameter name */ []int)
+func _(a int, /* ERROR missing parameter name */ []int, c int)

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

@@ -13,7 +13,7 @@ type t struct {
 }
 type t interface {
 	t[a]
-	m /* ERROR method must have no type parameters */ [_ _, /* ERROR mixed */ _]()
+	m /* ERROR method must have no type parameters */ [_ _, _ /* ERROR missing parameter type */ ]()
 	t[a, b]
 }
 

src/go/parser/parser.go

@@ -978,26 +978,30 @@ func (p *parser) parseParameterList(name0 *ast.Ident, typ0 ast.Expr, closing tok
 			}
 		}
 		if errPos.IsValid() {
-			var msg string
-			if tparams {
 			// Not all parameters are named because named != len(list).
-				// If named == typed we must have parameters that have no types,
+			// If named == typed, there must be parameters that have no types.
-				// and they must be at the end of the parameter list, otherwise
+			// They must be at the end of the parameter list, otherwise types
-				// the types would have been filled in by the right-to-left sweep
+			// would have been filled in by the right-to-left sweep above and
-				// above and we wouldn't have an error. Since we are in a type
+			// there would be no error.
-				// parameter list, the missing types are constraints.
+			// If tparams is set, the parameter list is a type parameter list.
+			var msg string
 			if named == typed {
-					errPos = p.pos // position error at closing ]
+				errPos = p.pos // position error at closing token ) or ]
+				if tparams {
 					msg = "missing type constraint"
 				} else {
+					msg = "missing parameter type"
+				}
+			} else {
+				if tparams {
 					msg = "missing type parameter name"
 					// go.dev/issue/60812
 					if len(list) == 1 {
 						msg += " or invalid array length"
 					}
-				}
 				} else {
-				msg = "mixed named and unnamed parameters"
+					msg = "missing parameter name"
+				}
 			}
 			p.error(errPos, msg)
 		}

src/go/parser/testdata/issue69506.go2

@@ -0,0 +1,9 @@
+// Copyright 2024 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
+
+func _(a int, b) /* ERROR "missing parameter type" */
+func _(a int, [ /* ERROR "missing parameter name" */ ]int)
+func _(a int, [ /* ERROR "missing parameter name" */ ]int, c int)

test/fixedbugs/bug388.go

@@ -9,7 +9,7 @@
 package main
 import "runtime"
 
-func foo(runtime.UintType, i int) {  // ERROR "cannot declare name runtime.UintType|mixed named and unnamed|undefined identifier"
+func foo(runtime.UintType, i int) {  // ERROR "cannot declare name runtime.UintType|missing parameter name|undefined identifier"
 	println(i, runtime.UintType) // GCCGO_ERROR "undefined identifier"
 }
 

test/func3.go

@@ -13,8 +13,8 @@ type t1 int
 type t2 int
 type t3 int
 
-func f1(*t2, x t3)	// ERROR "named"
+func f1(*t2, x t3)	// ERROR "missing parameter name"
-func f2(t1, *t2, x t3)	// ERROR "named"
+func f2(t1, *t2, x t3)	// ERROR "missing parameter name"
-func f3() (x int, *string)	// ERROR "named"
+func f3() (x int, *string)	// ERROR "missing parameter name"
 
 func f4() (t1 t1)	// legal - scope of parameter named t1 starts in body of f4.