cmd/compile/internal/syntax: better error message when using = instead of ==

When = is used instead of == as part of a conditional expression,
the parser message emphasizes the LHS and RHS of = by always
parenthesizing the two sides. For example, for:

	if x = y {}

the error is:

        cannot use assignment (x) = (y) as value

This is done to highlight the LHS and RHS in case of more complex
cases such as

        if x || y = z {}

which one may incorrectly read as (x) || (y == z) rather than the
correct (x || y) = z.

This CL fine-tunes the error message a bit by only adding the
parentheses if the LHS and RHS are binary expressions.

Fixes #60599.
For #23385.

Change-Id: Ida4c8d12464cc2ac15c934f24858eb6f43cf9950
Reviewed-on: https://go-review.googlesource.com/c/go/+/500975
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

2 files changed, 24 insertions, 4 deletions

diff --git a/src/cmd/compile/internal/syntax/parser.go b/src/cmd/compile/internal/syntax/parser.go
index c8b8ab0601..b5602fcff7 100644
--- a/src/cmd/compile/internal/syntax/parser.go
+++ b/src/cmd/compile/internal/syntax/parser.go
@@ -2369,10 +2369,8 @@ done:
 		// further confusion.
 		var str string
 		if as, ok := s.(*AssignStmt); ok && as.Op == 0 {
-			// Emphasize Lhs and Rhs of assignment with parentheses to highlight '='.
-			// Do it always - it's not worth going through the trouble of doing it
-			// only for "complex" left and right sides.
-			str = "assignment (" + String(as.Lhs) + ") = (" + String(as.Rhs) + ")"
+			// Emphasize complex Lhs and Rhs of assignment with parentheses to highlight '='.
+			str = "assignment " + emphasize(as.Lhs) + " = " + emphasize(as.Rhs)
 		} else {
 			str = String(s)
 		}
@@ -2383,6 +2381,17 @@ done:
 	return
 }
 
+// emphasize returns a string representation of x, with (top-level)
+// binary expressions emphasized by enclosing them in parentheses.
+func emphasize(x Expr) string {
+	s := String(x)
+	if op, _ := x.(*Operation); op != nil && op.Y != nil {
+		// binary expression
+		return "(" + s + ")"
+	}
+	return s
+}
+
 func (p *parser) ifStmt() *IfStmt {
 	if trace {
 		defer p.trace("ifStmt")()
diff --git a/src/cmd/compile/internal/syntax/testdata/issue60599.go b/src/cmd/compile/internal/syntax/testdata/issue60599.go
new file mode 100644
index 0000000000..711d97bde0
--- /dev/null
+++ b/src/cmd/compile/internal/syntax/testdata/issue60599.go
@@ -0,0 +1,11 @@
+// Copyright 2023 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 _(x, y, z int) {
+	if x /* ERROR cannot use assignment x = y as value */ = y {}
+	if x || y /* ERROR cannot use assignment \(x || y\) = z as value */ = z {}
+	if x /* ERROR cannot use assignment x = \(y || z\) as value */ = y || z {}
+}