Do not constant fold mux before it has been typechecked (#3633)

Signed-off-by: Mihai Budiu <[email protected]>

frontends/common/constantFolding.cpp

@@ -751,6 +751,9 @@ const IR::Node* DoConstantFolding::postorder(IR::LNot* e) {
 }
 
 const IR::Node* DoConstantFolding::postorder(IR::Mux* e) {
+    if (!typesKnown)
+        // We want the typechecker to look at the expression first
+        return e;
     auto cond = getConstant(e->e0);
     if (cond == nullptr)
         return e;

testdata/p4_16_errors/issue3622.p4

@@ -0,0 +1,18 @@
+action a(){}
+action b(){}
+action NoAction(){}
+control c() {
+    table t  {
+        actions = { a ; b;  }
+    }
+    apply {
+        switch(t.apply().action_run) {
+            true ? a : b : {}
+        }
+    }
+}
+
+control C();
+package top(C c);
+
+top(c()) main;

testdata/p4_16_errors_outputs/issue3622.p4

@@ -0,0 +1,24 @@
+action a() {
+}
+action b() {
+}
+action NoAction() {
+}
+control c() {
+    table t {
+        actions = {
+            a;
+            b;
+        }
+    }
+    apply {
+        switch (t.apply().action_run) {
+            (true ? a : b): {
+            }
+        }
+    }
+}
+
+control C();
+package top(C c);
+top(c()) main;

testdata/p4_16_errors_outputs/issue3622.p4-stderr

@@ -0,0 +1,3 @@
+issue3622.p4(10): [--Werror=type-error] error: ?:: 'switch' label must be an action name or 'default'
+            true ? a : b : {}
+            ^^^^^^^^^^^^