fixes #5136

compiler/cgen.nim

@@ -732,10 +732,12 @@ proc genProcNoForward(m: BModule, prc: PSym) =
     # dependency to a compilerproc:
     discard cgsym(m, prc.name.s)
     return
-  if lfNoDecl in prc.loc.flags:
+  if sfImportc in prc.flags:
     fillProcLoc(m, prc)
-    useHeader(m, prc)
-    genProcPrototype(m, prc)
+    if lfHeader in prc.loc.flags:
+      useHeader(m, prc)
+    elif lfNoDecl notin prc.loc.flags:
+      genProcPrototype(m, prc)
   elif prc.typ.callConv == ccInline:
     # We add inline procs to the calling module to enable C based inlining.
     # This also means that a check with ``q.declaredThings`` is wrong, we need
@@ -760,15 +762,13 @@ proc genProcNoForward(m: BModule, prc: PSym) =
       symInDynamicLib(q, prc)
     else:
       symInDynamicLibPartial(m, prc)
-  elif sfImportc notin prc.flags:
+  else:
     var q = findPendingModule(m, prc)
     fillProcLoc(q, prc)
     useHeader(m, prc)
     genProcPrototype(m, prc)
     if q != nil and not containsOrIncl(q.declaredThings, prc.id):
       genProcAux(q, prc)
-  else:
-    fillProcLoc(m, prc)
 
 proc requestConstImpl(p: BProc, sym: PSym) =
   var m = p.module

tests/cpp/tgen_prototype_for_importc.nim

@@ -0,0 +1,10 @@
+discard """
+  cmd: "nim cpp $file"
+  output: '''Hello world'''
+"""
+
+# bug #5136
+
+{.compile: "foo.c".}
+proc myFunc(): cstring {.importc.}
+echo myFunc()