fixes #8781 by appending "_U" instead of 'U' (#8787)

compiler/ccgtypes.nim

@@ -449,7 +449,9 @@ proc genRecordFieldsAux(m: BModule, n: PNode,
   of nkRecCase:
     if n.sons[0].kind != nkSym: internalError(m.config, n.info, "genRecordFieldsAux")
     add(result, genRecordFieldsAux(m, n.sons[0], accessExpr, rectype, check))
-    let uname = rope(mangle(n.sons[0].sym.name.s) & 'U')
+    # prefix mangled name with "_U" to avoid clashes with other field names,
+    # since identifiers are not allowed to start with '_'
+    let uname = rope("_U" & mangle(n.sons[0].sym.name.s))
     let ae = if accessExpr != nil: "$1.$2" % [accessExpr, uname]
              else: uname
     var unionBody: Rope = nil

tests/ccgbugs/t8781.nim

@@ -0,0 +1,25 @@
+discard """
+output: ""
+"""
+
+type
+  Drawable = object of RootObj
+    discard
+
+  # issue #8781, following type was broken due to 'U' suffix
+  # on `animatedU`. U also added as union identifier for C.
+  # replaced by "_U" prefix, which is not allowed as an
+  # identifier
+  TypeOne = ref object of Drawable
+    animatedU: bool
+    case animated: bool
+    of true:
+        frames: seq[int]
+    of false:
+        region: float
+
+when isMainModule:
+  let r = 1.5
+  let a = TypeOne(animatedU: true,
+                  animated: false,
+                  region: r)