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>

3 files changed, 26 insertions, 12 deletions

diff --git a/src/cmd/compile/internal/syntax/parser.go b/src/cmd/compile/internal/syntax/parser.go
index cd6b6696a2..77abdda867 100644
--- a/src/cmd/compile/internal/syntax/parser.go
+++ b/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() {
+			// Not all parameters are named because named != len(list).
+			// If named == typed, there must be parameters that have no types.
+			// They must be at the end of the parameter list, otherwise types
+			// would have been filled in by the right-to-left sweep above and
+			// there would be no error.
+			// If requireNames is set, the parameter list is a type parameter
+			// list.
 			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,
-				// and they must be at the end of the parameter list, otherwise
-				// the types would have been filled in by the right-to-left sweep
-				// above and we wouldn't have an error. Since we are in a type
-				// parameter list, the missing types are constraints.
-				if named == typed {
-					errPos = end // position error at closing ]
+			if named == typed {
+				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 = "missing parameter name"
 				}
-			} else {
-				msg = "mixed named and unnamed parameters"
 			}
 			p.syntaxErrorAt(errPos, msg)
 		}
diff --git a/src/cmd/compile/internal/syntax/testdata/issue69506.go b/src/cmd/compile/internal/syntax/testdata/issue69506.go
new file mode 100644
index 0000000000..36e9da77c1
--- /dev/null
+++ b/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)
diff --git a/src/cmd/compile/internal/syntax/testdata/tparams.go b/src/cmd/compile/internal/syntax/testdata/tparams.go
index 4b68a1585f..a4967bf70f 100644
--- a/src/cmd/compile/internal/syntax/testdata/tparams.go
+++ b/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]
 }