Report error on invalid syntax for version constraint in opam file

[mjambon]

I mistakenly specified "lsp" {"1.1.0"} instead of "lsp" {= "1.1.0"} in an opam file for a project. No error was reported by either opam lint or opam install. It would be great if an error was reported, rather than the dependency being ignored silently.

The session below shows that lsp is not installed and that having "lsp" {"1.1.0"} as a dependency is like having nothing instead.

$ opam config report
# opam config report
# opam-version      2.0.5 
# self-upgrade      no
# system            arch=x86_64 os=linux os-distribution=ubuntu os-version=20.04
# solver            builtin-mccs+glpk
# install-criteria  -removed,-count[version-lag,request],-count[version-lag,changed],-changed
# upgrade-criteria  -removed,-count[version-lag,solution],-new
# jobs              3
# repositories      1 (http) (default repo at ad125b9b)
# pinned            2 (git)
# current-switch    4.10.0

$ opam list | grep lsp

$ opam lint opam
/home/martin/tmp/report/opam: Passed.

$ opam install --deps-only .
Nothing to do.

$ cat opam
opam-version: "2.0"
name: "thing"
version: "0.0.0"
synopsis: "A a"
description: "A a"

maintainer: "x <x@example.com>"
authors: [ "x <x@example.com>" ]
license: "LGPL-2.1"
homepage: "https://example.com"
dev-repo: "git+https://github.com/example/foo"
bug-reports: "https://github.com/example/foo/issues"

depends: [
  "lsp" {"1.1.0"}
]

build: [make]

[rjbou]

This syntax is allowed as the constrained filter evaluates correctly {"true"}, or {"%{a-var}%"} when a-var is a boolean, it is the default otherwise (here false). But yes, opam should at least warn on this kind of syntax when it is not a bool nor a variable.

[rjbou]

Added a lint warning #66. Thanks for the report

[mjambon]

Awesome, thank you!