guide the user towards solving test-plan issues

cabal-install/src/Distribution/Client/CmdErrorMessages.hs

@@ -330,10 +330,13 @@ renderTargetProblemNoneEnabled verb targetSelector targets =
               [ "the " ++ showComponentName availableTargetComponentName
               | AvailableTarget {availableTargetComponentName} <- targets' ]
          ++ plural (listPlural targets') " is " " are "
-         ++ "not available because the solver did not find a plan that "
-         ++ "included the " ++ renderOptionalStanza Plural stanza
-         ++ ". Force the solver to enable this for all packages by adding the "
-         ++ "line 'tests: True' to the 'cabal.project.local' file."
+         ++ "not available because the solver picked a plan that does not "
+         ++ "include the " ++ renderOptionalStanza Plural stanza
+         ++ ", perhaps because no such plan exists. To see the error message "
+         ++ "explaining the problems with such plans, force the solver to "
+         ++ "include the " ++ renderOptionalStanza Plural stanza ++ " for all "
+         ++ "packages, by adding the line 'tests: True' to the "
+         ++ "'cabal.project.local' file."
         (TargetNotBuildable, _) ->
             renderListCommaAnd
               [ "the " ++ showComponentName availableTargetComponentName

changelog.d/pr-7834

@@ -0,0 +1,12 @@
+synopsis: Improve error message when no test/benchmark is found
+packages: cabal-install
+prs: #7834
+issues: #5079 #5255
+
+description: {
+
+When `cabal test` and the error message recommends setting `tests: True`, the next `cabal test` run is likely to fail with a solver error. The user might incorrectly conclude that setting `tests: True` made the problem worse, because the failure now occurs earlier (at solving time rather than at testing time).
+
+By including the fact that a plan failure is expected in the error message, hopefully users will be more confident that setting `tests: True` was the right move, so they will be able to focus on the true cause of the problem: the fact that no plan including the tests exists.
+
+}