From 7a6da0ff7907c08035d42653bb4cc0dabd46269f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=B6nke=20Ludwig?= <sludwig@outerproduct.org>
Date: Sat, 4 Apr 2020 11:19:40 +0200
Subject: [PATCH] Avoid instantiating generic visitor with types it cannot
 handle.

In cases where statically typed visitors need to be defined to avoid compile errors in a generic visitor, the generic one was still instantiated with the types already handled by the statically typed one(s), possibly resulting in a compile error.
---
 source/taggedalgebraic/visit.d | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/source/taggedalgebraic/visit.d b/source/taggedalgebraic/visit.d
index 8aeac99..46e9086 100644
--- a/source/taggedalgebraic/visit.d
+++ b/source/taggedalgebraic/visit.d
@@ -217,7 +217,7 @@ private template matchesType(alias fun) {
 			else static if (Params.length == 1 && is(T == Params[0])) enum matchesType = true;
 			else enum matchesType = false;
 		} else static if (!isUnitType!T) {
-			static if (isSomeFunction!(fun!T)) {
+			static if (__traits(compiles, fun!T) && isSomeFunction!(fun!T)) {
 				alias Params = ParameterTypeTuple!(fun!T);
 				static if (Params.length == 1 && is(T == Params[0])) enum matchesType = true;
 				else enum matchesType = false;
@@ -226,6 +226,16 @@ private template matchesType(alias fun) {
 	}
 }
 
+unittest {
+	class C {}
+	alias mt1 = matchesType!((C c) => true);
+	alias mt2 = matchesType!((c) { static assert(!is(typeof(c) == C)); });
+	static assert(mt1!C);
+	static assert(!mt1!int);
+	static assert(mt2!int);
+	static assert(!mt2!C);
+}
+
 private template selectHandler(T, VISITORS...)
 {
 	import std.traits : ParameterTypeTuple, isSomeFunction;