cmd/compile/internal/syntax: return correct start pos for KeyValueExprs

Fixes #67866.

Change-Id: Id9d345aab87e493b8ed94319c5acaa1900362648
Reviewed-on: https://go-review.googlesource.com/c/go/+/591695
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -0,0 +1,51 @@
+// 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.
+
+// This file holds test cases for individual issues
+// for which there is (currently) no better location.
+
+package syntax
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestIssue67866(t *testing.T) {
+	var tests = []string{
+		"package p; var _ = T{@0: 0}",
+		"package p; var _ = T{@1 + 2: 0}",
+		"package p; var _ = T{@x[i]: 0}",
+		"package p; var _ = T{@f(1, 2, 3): 0}",
+		"package p; var _ = T{@a + f(b) + <-ch: 0}",
+	}
+
+	for _, src := range tests {
+		// identify column position of @ and remove it from src
+		i := strings.Index(src, "@")
+		if i < 0 {
+			t.Errorf("%s: invalid test case (missing @)", src)
+			continue
+		}
+		src = src[:i] + src[i+1:]
+		want := colbase + uint(i)
+
+		f, err := Parse(nil, strings.NewReader(src), nil, nil, 0)
+		if err != nil {
+			t.Errorf("%s: %v", src, err)
+			continue
+		}
+
+		// locate KeyValueExpr
+		Inspect(f, func(n Node) bool {
+			_, ok := n.(*KeyValueExpr)
+			if ok {
+				if got := StartPos(n).Col(); got != want {
+					t.Errorf("%s: got col = %d, want %d", src, got, want)
+				}
+			}
+			return !ok
+		})
+	}
+}

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

@@ -36,7 +36,8 @@ func StartPos(n Node) Pos {
 				continue
 			}
 			return n.Pos()
-		// case *KeyValueExpr:
+		case *KeyValueExpr:
+			m = n.Key
 		// case *FuncLit:
 		// case *ParenExpr:
 		case *SelectorExpr: