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cmd/compile/internal/syntax: better error msg for some 'if' statements

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R=go1.11

A common error is to write '=' instead of '==' inside the condition
of a simple 'if' statement:

	if x = 0 { ... }

Highlight the fact that we have an assignment in the error message
to prevent further confusion.

Fixes #23385.

Change-Id: I1552050fd6da927bd12a1be0977bd2e98eca5885
Reviewed-on: https://go-review.googlesource.com/87316
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
This commit is contained in:
Robert Griesemer 2018-01-10 16:05:12 -08:00
parent ac45cb9aa0
commit be9a1774f2
1 changed files with 11 additions and 3 deletions
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14
src/cmd/compile/internal/syntax/parser.go
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@ -170,9 +170,9 @@ func (p *parser) syntax_error_at(pos src.Pos, msg string) {
switch {
case msg == "":
// nothing to do
case strings.HasPrefix(msg, "in"), strings.HasPrefix(msg, "at"), strings.HasPrefix(msg, "after"):
case strings.HasPrefix(msg, "in "), strings.HasPrefix(msg, "at "), strings.HasPrefix(msg, "after "):
msg = " " + msg
case strings.HasPrefix(msg, "expecting"):
case strings.HasPrefix(msg, "expecting "):
msg = ", " + msg
default:
// plain error - we don't care about current token
@ -1844,7 +1844,15 @@ done:
case *ExprStmt:
cond = s.X
default:
p.syntax_error(fmt.Sprintf("%s used as value", String(s)))
// A common syntax error is to write '=' instead of '==',
// which turns an expression into an assignment. Provide
// a more explicit error message in that case to prevent
// further confusion.
str := String(s)
if as, ok := s.(*AssignStmt); ok && as.Op == 0 {
str = "assignment " + str
}
p.syntax_error(fmt.Sprintf("%s used as value", str))
}
p.xnest = outer