Add failing test for #20674

testsuite/tests/th/T20674/Main.hs

+{-# LANGUAGE TemplateHaskell #-}
+module Main where
+
+import P
+
+main = $([| return () |])

testsuite/tests/th/T20674/P.hs

+module P where
+
+foreign export ccall foo :: Int -> IO Int
+
+foo :: Int -> IO Int
+foo n = return (length (f n))
+
+f :: Int -> [Int]
+f 0 = []
+f n = n:(f (n-1))
+

testsuite/tests/th/T20674/all.T

+setTestOpts(req_interp)
+# TH should work with -fexternal-interpreter too
+if config.have_ext_interp :
+     setTestOpts(extra_ways(['ext-interp']))
+     setTestOpts(only_ways(['normal','ghci','ext-interp']))
+
+     if llvm_build():
+         setTestOpts(fragile_for(16087, ['ext-interp']))
+
+test('T20674', [expect_broken(20674), extra_files(['Main.hs', 'P.hs', 'foo.c', 'header.h'])], multimod_compile, ['Main', 'P foo.c'])

testsuite/tests/th/T20674/foo.c

+#include "header.h"
+
+int q() {
+  int e = foo(0);
+  return(0);
+}

testsuite/tests/th/T20674/header.h

+#include <HsFFI.h>
+
+int foo(int);