fixes #6980

compiler/semexprs.nim

@@ -780,6 +780,19 @@ proc buildEchoStmt(c: PContext, n: PNode): PNode =
 
 proc semExprNoType(c: PContext, n: PNode): PNode =
   result = semExpr(c, n, {efWantStmt})
+  # make an 'if' expression an 'if' statement again for backwards
+  # compatibility (.discardable was a bad idea!); bug #6980
+  var isStmt = false
+  if result.kind == nkIfExpr:
+    isStmt = true
+    for condActionPair in result:
+      let action = condActionPair.lastSon
+      if not implicitlyDiscardable(action) and not
+          endsInNoReturn(action):
+        isStmt = false
+    if isStmt:
+      result.kind = nkIfStmt
+      result.typ = nil
   discardCheck(c, result)
 
 proc isTypeExpr(n: PNode): bool =

tests/exprs/tstmtexprs.nim

@@ -141,3 +141,13 @@ echo(
      quo do (a: int) -> bool:
         a mod 3 != 0
 )
+
+# bug #6980
+
+proc fooBool: bool {.discardable.} =
+  true
+
+if true:
+  fooBool()
+else:
+  raise newException(ValueError, "argh")