fix nim-lang#14667 (nim-lang#21190)

compiler/ccgexprs.nim

@@ -3200,6 +3200,8 @@ proc getDefaultValue(p: BProc; typ: PType; info: TLineInfo; result: var Rope) =
     result.add "}"
   of tyTuple:
     result.add "{"
+    if p.vccAndC and t.len == 0:
+      result.add "0"
     for i in 0..<t.len:
       if i > 0: result.add ", "
       getDefaultValue(p, t[i], info, result)
@@ -3338,6 +3340,8 @@ proc genConstSimpleList(p: BProc, n: PNode; isConst: bool; result: var Rope) =
 
 proc genConstTuple(p: BProc, n: PNode; isConst: bool; tup: PType; result: var Rope) =
   result.add "{"
+  if p.vccAndC and n.len == 0:
+    result.add "0"
   for i in 0..<n.len:
     let it = n[i]
     if i > 0: result.add ",\n"

compiler/cgendata.nim

@@ -172,6 +172,7 @@ type
 
 template config*(m: BModule): ConfigRef = m.g.config
 template config*(p: BProc): ConfigRef = p.module.g.config
+template vccAndC*(p: BProc): bool = p.module.config.cCompiler == ccVcc and p.module.config.backend == backendC
 
 proc includeHeader*(this: BModule; header: string) =
   if not this.headerFiles.contains header:

tests/misc/t14667.nim

@@ -0,0 +1,12 @@
+discard """
+  matrix: "--cc:vcc"
+  disabled: "linux"
+  disabled: "bsd"
+  disabled: "osx"
+  disabled: "unix"
+  disabled: "posix"
+"""
+
+type A = tuple
+discard ()
+discard default(A)