Re #6342 Add an integration test

tests/integration/tests/6342-say-ghc-version-in-build/.gitignore

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+myPackage.cabal

tests/integration/tests/6342-say-ghc-version-in-build/Main.hs

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+-- Stack outputs the GHC version that it is using during a build.
+--
+-- See: https://github.com/commercialhaskell/stack/issues/6342
+
+import           Control.Monad ( unless )
+import           Data.Char ( isSpace )
+import           Data.List ( dropWhileEnd, isInfixOf )
+import           StackTest
+
+main :: IO ()
+main =
+  -- Query the actual compiler
+  stackCheckStdout ["query", "compiler", "actual"] $ \compiler -> do
+    stackCheckStderr ["build"] (expectMessage $ buildWith (trimEnd compiler))
+
+buildWith :: String -> String
+buildWith compiler = "build (lib) with " <> compiler
+
+expectMessage :: String -> String -> IO ()
+expectMessage msg stderr = do
+  unless (words msg `isInfixOf` words stderr)
+         (error $ "Expected message: \n" ++ show msg)
+
+trimEnd :: String -> String
+trimEnd = dropWhileEnd isSpace

tests/integration/tests/6342-say-ghc-version-in-build/files/package.yaml

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+spec-version: 0.36.0
+
+name: myPackage
+
+dependencies:
+- base < 5
+
+library:
+  source-dirs: src

tests/integration/tests/6342-say-ghc-version-in-build/files/src/Lib.hs

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+module Lib where

tests/integration/tests/6342-say-ghc-version-in-build/files/stack.yaml

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+snapshot: ghc-9.10.3