C++ codegen: emit 'extern C' for importc'ed functions that we declare a prototype for

compiler/ccgthreadvars.nim

@@ -57,8 +57,8 @@ proc generateThreadLocalStorage(m: BModule) =
 
 proc generateThreadVarsSize(m: BModule) =
   if nimtv != nil:
-    let externc = if gCmd != cmdCompileToCpp and
-                       sfCompileToCpp in m.module.flags: "extern \"C\""
+    let externc = if gCmd == cmdCompileToCpp or
+                       sfCompileToCpp in m.module.flags: "extern \"C\" "
                   else: ""
     addf(m.s[cfsProcs],
       "$#NI NimThreadVarsSize(){return (NI)sizeof(NimThreadVars);}$n",

compiler/cgen.nim

@@ -715,10 +715,13 @@ proc genProcAux(m: BModule, prc: PSym) =
     add(generatedProc, ~"}$N")
   add(m.s[cfsProcs], generatedProc)
 
-proc crossesCppBoundary(m: BModule; sym: PSym): bool {.inline.} =
-  result = sfCompileToCpp in m.module.flags and
+proc requiresExternC(m: BModule; sym: PSym): bool {.inline.} =
+  result = (sfCompileToCpp in m.module.flags and
            sfCompileToCpp notin sym.getModule().flags and
-           gCmd != cmdCompileToCpp
+           gCmd != cmdCompileToCpp) or (
+           sym.flags * {sfImportc, sfInfixCall, sfCompilerProc} == {sfImportc} and
+           sym.magic == mNone and
+           gCmd == cmdCompileToCpp)
 
 proc genProcPrototype(m: BModule, sym: PSym) =
   useHeader(m, sym)
@@ -732,7 +735,7 @@ proc genProcPrototype(m: BModule, sym: PSym) =
     var header = genProcHeader(m, sym)
     if sfNoReturn in sym.flags and hasDeclspec in extccomp.CC[cCompiler].props:
       header = "__declspec(noreturn) " & header
-    if sym.typ.callConv != ccInline and crossesCppBoundary(m, sym):
+    if sym.typ.callConv != ccInline and requiresExternC(m, sym):
       header = "extern \"C\" " & header
     if sfPure in sym.flags and hasAttribute in CC[cCompiler].props:
       header.add(" __attribute__((naked))")