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>
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