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>

0 files changed, 0 insertions, 0 deletions