[PATCH] D80603: add isAtPosition narrowing matcher for parmVarDecl

Wed May 27 22:52:51 PDT 2020

gribozavr2 added a comment.

In D80603#2058019 <https://reviews.llvm.org/D80603#2058019>, @aaron.ballman wrote:

> I'm sorry if I'm being dense, but `hasParameter`  traverses to the `ParmVarDecl`, so I'm not certain I understand why this new matcher is needed as a public matcher. It seems like you can already accomplish this without adding a second API to do effectively the same thing: `functionDecl(hasParameter(0, parmVarDecl().bind("param")))`, can't you?

It is syntax sugar, true. Note that the proposed matcher is a narrowing matcher for `parmVarDecl()`, while your suggestion is a narrowing matcher for `functionDecl()`, so it is not an entirely apples-to-apples comparison. Think about use cases like: `declRefExpr(to(parmVarDecl(at(...))))`. To rewrite that with `hasParameter()`, we have to use `hasAncestor` to find the `functionDecl` first, and then compare the AST node pointers. So while it is possible to express this proposed operation today, it requires a complex matcher for such a conceptually simple operation.

================
Comment at: clang/include/clang/ASTMatchers/ASTMatchers.h:4621
+template <typename DeclT>
+static bool __isParamAt(const clang::DeclContext *Context,
+                        const clang::ParmVarDecl &Node, unsigned N) {
----------------
Identifiers with double underscores are reserved. Also, this file typically does not define helpers.

My best suggestion is to define a local lambda in the AST_MATCHER_P, or just copy the code three times -- there is not that much duplication.

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https://reviews.llvm.org/D80603