cmd/dist: inverted precedence for && and || in exprParser

[dmitshur]

cmd/dist has a small //go:build constraint parser and evaluator for the needs of bootstrapping. Back when it was added (CL 359314, CC @rsc), go/build/constraint couldn't be used because cmd/dist needed to be able to be built with much older Go releases, so an implementation was added.

@cuiweixie noticed and reported that the precedence && and || appeared wrong in a golang-dev message at https://groups.google.com/g/golang-dev/c/0k7jlbEV29o/m/3cfUZHrpCAAJ, causing gccgo && gc || gc to evaluate to false (given gc is set, gccgo is not set), though it should be true.

It can be fixed with:

-		{tok: "&&", prec: 1, infix: func(x, y val) val { return x && y }},
-		{tok: "||", prec: 2, infix: func(x, y val) val { return x || y }},
+		{tok: "||", prec: 1, infix: func(x, y val) val { return x || y }},
+		{tok: "&&", prec: 2, infix: func(x, y val) val { return x && y }},

Though it's also possible to start using go/build/constraint now and delete the parser copy. A stack ending with CL 773823 does that.

CC @cuiweixie, @adonovan.

[gopherbot]

Change https://go.dev/cl/773822 mentions this issue: cmd/dist: use go/build/constraint to parse build constraints

[gopherbot]

Change https://go.dev/cl/773823 mentions this issue: cmd/dist: inline matchexpr into its only caller

[gopherbot]

Change https://go.dev/cl/773821 mentions this issue: cmd/dist: add a 'gccgo && gc || gc' build constraint testcase

[gabyhelp]

Related Code Changes

• cmd/dist: use go/build/constraint to parse build constraints

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}