Fix usage of clocking arguments in sampled value system functions inside of always_comb blocks

Author: MikePopoloski

source/binding/MiscExpressions.cpp

@@ -484,7 +484,14 @@ ConstantValue LValueReferenceExpression::evalImpl(EvalContext& context) const {
 }
 
 Expression& ClockingEventExpression::fromSyntax(const ClockingPropertyExprSyntax& syntax,
-                                                const BindContext& context) {
+                                                const BindContext& argContext) {
+    // Clocking event expressions are only used in special system function calls,
+    // where they don't actually pass any time but instead tell the function which
+    // clock to use. We don't want usage inside of an always_comb to report an error
+    // about passing time, so clear out the context's procedure to avoid that.
+    BindContext context(argContext);
+    context.clearInstanceAndProc();
+
     auto& comp = context.getCompilation();
     auto& timing = TimingControl::bind(*syntax.event, context);
 

tests/unittests/SystemFuncTests.cpp

@@ -1074,3 +1074,19 @@ endmodule
     REQUIRE(diags.size() == 1);
     CHECK(diags[0].code == diag::ExpressionNotAssignable);
 }
+
+TEST_CASE("Sampled value functions with clocking in always_comb") {
+    auto tree = SyntaxTree::fromText(R"(
+module top;
+    logic clk;
+    logic a, b;
+    always_comb begin
+        a = $past(b,,,@(posedge clk));
+    end
+endmodule
+)");
+
+    Compilation compilation;
+    compilation.addSyntaxTree(tree);
+    NO_COMPILATION_ERRORS;
+}