Bad parse error for malformed `--constraint` argument

[sjakobi]

~/s/tomland|main⚡*
➤ cabal test --constraint 'mtl >= 2.3' --constraint=ghc:installed
Error: cabal: invalid argument to option `--constraint': expected a (possibly
qualified) package name followed by a constraint, which is either a version
range, 'installed', 'source', 'test', 'bench', or flags. "<eitherParsec>"
(line 1, column 5):
unexpected "i"
expecting "setup."
~/s/tomland|main⚡*
➤ cabal test --constraint 'mtl >= 2.3' --constraint=ghc:'installed'
Error: cabal: invalid argument to option `--constraint': expected a (possibly
qualified) package name followed by a constraint, which is either a version
range, 'installed', 'source', 'test', 'bench', or flags. "<eitherParsec>"
(line 1, column 5):
unexpected "i"
expecting "setup."
➤ cabal --version
cabal-install version 3.8.1.0
compiled using version 3.8.1.0 of the Cabal library 

The error message suggests that installed (or 'installed'?!) is a valid constraint, but the parser doesn't accept it.

BTW, I think that the "followed by" bit could be clearer. Maybe an example could point out that a colon is required.

[ulysses4ever]

Any chance we could check what 3.6 has to say?

[sjakobi]

Any chance we could check what 3.6 has to say?

➤ cabal-3.6.2.0 test --constraint 'mtl >= 2.3' --constraint=ghc:installed
expected a (possibly qualified) package name followed by a constraint, which is either a version range, 'installed', 'source', 'test', 'bench', or flags. "<eitherParsec>" (line 1, column 5):
unexpected "i"
expecting "setup."
CallStack (from HasCallStack):
  error, called at src/Distribution/ReadE.hs:42:24 in Cabal-3.6.2.0-inplace:Distribution.ReadE
➤ cabal-3.6.2.0 test --constraint 'mtl >= 2.3' --constraint=ghc:'installed'
expected a (possibly qualified) package name followed by a constraint, which is either a version range, 'installed', 'source', 'test', 'bench', or flags. "<eitherParsec>" (line 1, column 5):
unexpected "i"
expecting "setup."
CallStack (from HasCallStack):
  error, called at src/Distribution/ReadE.hs:42:24 in Cabal-3.6.2.0-inplace:Distribution.ReadE

[ulysses4ever]

I think it should be --constraint 'ghc installed'. Colon is used for another thing.

Error message should have an example, indeed.

[Mikolaj]

@sjakobi: thank you for editing the title. Yes, we'd welcome a PR.