add an impl of Expr::show for ExprInheritFrom that doesn't crash

ExprVar::show() assumes it has a name. dynamic inherits do not
necessarily (ever?) have a name.

Change-Id: If10893188e307431da17f0c1bd0787adc74f7141

src/libexpr/nixexpr.cc

@@ -50,6 +50,16 @@ void ExprVar::show(const SymbolTable & symbols, std::ostream & str) const
     str << symbols[name];
 }
 
+void ExprInheritFrom::show(SymbolTable const & symbols, std::ostream & str) const
+{
+    if (name) {
+        str << symbols[name];
+    } else {
+        // We can't get at the actual dynamic expression from here.
+        str << "(dynamic inherit)";
+    }
+}
+
 void ExprSelect::show(const SymbolTable & symbols, std::ostream & str) const
 {
     str << "(";

src/libexpr/nixexpr.hh

@@ -154,7 +154,8 @@ struct ExprInheritFrom : ExprVar
         this->fromWith = nullptr;
     }
 
-    void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env);
+    void show(SymbolTable const & symbols, std::ostream & str) const override;
+    void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env) override;
 };
 
 struct ExprSelect : Expr

tests/unit/libexpr/expr-print.cc

@@ -0,0 +1,32 @@
+#include <sstream>
+#include <string_view>
+
+#include <gtest/gtest.h>
+
+#include "tests/libexpr.hh"
+
+#include "nixexpr.hh"
+
+namespace nix
+{
+
+using namespace testing;
+struct ExprPrintingTests : LibExprTest
+{
+    void test(Expr const & expr, std::string_view expected)
+    {
+        std::stringstream out;
+        expr.show(state.symbols, out);
+        ASSERT_EQ(out.str(), expected);
+    }
+};
+
+TEST_F(ExprPrintingTests, ExprInheritFrom)
+{
+    // ExprInheritFrom has its own show() impl.
+    // If it uses its parent class's impl it will crash.
+    ExprInheritFrom const eInheritFrom(noPos, 0);
+    test(eInheritFrom, "(dynamic inherit)");
+}
+
+}

tests/unit/meson.build

@@ -185,6 +185,7 @@ libexpr_tests_sources = files(
   'libexpr/primops.cc',
   'libexpr/search-path.cc',
   'libexpr/trivial.cc',
+  'libexpr/expr-print.cc',
   'libexpr/value/context.cc',
   'libexpr/value/print.cc',
 )