fix #17351; switch to c++17 and remove hacks (#20407)

* fix #17351; switch to c++17

* remove workaround

compiler/cgen.nim

@@ -376,8 +376,7 @@ proc genObjectInit(p: BProc, section: TCProcSection, t: PType, a: var TLoc,
     else:
       linefmt(p, section, "$1.m_type = $2;$n", [r, genTypeInfoV1(p.module, t, a.lode.info)])
   of frEmbedded:
-    # inheritance in C++ does not allow struct initialization: bug #18410
+    if optTinyRtti in p.config.globalOptions:
-    if not p.module.compileToCpp and optTinyRtti in p.config.globalOptions:
       var tmp: TLoc
       if mode == constructRefObj:
         let objType = t.skipTypes(abstractInst+{tyRef})

compiler/extccomp.nim

@@ -89,7 +89,7 @@ compiler gcc:
     asmStmtFrmt: "__asm__($1);$n",
     structStmtFmt: "$1 $3 $2 ", # struct|union [packed] $name
     produceAsm: gnuAsmListing,
-    cppXsupport: "-std=gnu++14 -funsigned-char",
+    cppXsupport: "-std=gnu++17 -funsigned-char",
     props: {hasSwitchRange, hasComputedGoto, hasCpp, hasGcGuard, hasGnuAsm,
             hasAttribute})
 
@@ -116,7 +116,7 @@ compiler nintendoSwitchGCC:
     asmStmtFrmt: "asm($1);$n",
     structStmtFmt: "$1 $3 $2 ", # struct|union [packed] $name
     produceAsm: gnuAsmListing,
-    cppXsupport: "-std=gnu++14 -funsigned-char",
+    cppXsupport: "-std=gnu++17 -funsigned-char",
     props: {hasSwitchRange, hasComputedGoto, hasCpp, hasGcGuard, hasGnuAsm,
             hasAttribute})
 

tests/cpp/torc.nim

@@ -13,3 +13,13 @@ type
 proc p(): Option[O] = none(O)
 
 doAssert $p() == "none(O)"
+
+# bug #17351
+type
+  Foo = object of RootObj
+  Foo2 = object of Foo
+  Bar = object
+    x: Foo2
+
+var b = Bar()
+discard b