Make import VAR ?= default revert to default when env-var is unset

src/graph/builder.cpp

@@ -1192,7 +1192,18 @@ auto process_import(
     if (auto const* env_val = std::getenv(name_buf.c_str())) {
         value_id = intern(env_val);
     }
-    // 2. Fall back to cached value from previous build (passed via options)
+    // 2. If the author wrote `?= default`, that default is the explicit
+    //    revert-target for the env-unset case and wins over the cache.
+    else if (imp.default_value) {
+        auto expanded = parser::expand(*ctx.eval, *imp.default_value);
+        if (!expanded) {
+            return pup::unexpected<Error>(expanded.error());
+        }
+        value_id = *expanded;
+    }
+    // 3. No env, no default — fall back to the cached value from the previous
+    //    build so plain `import VAR` stays stable across shell sessions that
+    //    forget to re-export the variable.
     else if (auto it = std::lower_bound(
                  state.options.cached_env_vars.begin(),
                  state.options.cached_env_vars.end(),
@@ -1202,14 +1213,6 @@ auto process_import(
              it != state.options.cached_env_vars.end() && str(it->first) == var_name_sv) {
         value_id = it->second;
     }
-    // 3. Fall back to default value
-    else if (imp.default_value) {
-        auto expanded = parser::expand(*ctx.eval, *imp.default_value);
-        if (!expanded) {
-            return pup::unexpected<Error>(expanded.error());
-        }
-        value_id = *expanded;
-    }
 
     auto value_sv = str(value_id);
 

test/unit/test_e2e.cpp

@@ -4343,6 +4343,39 @@ SCENARIO("import with ?= operator", "[e2e][import]")
     }
 }
 
+SCENARIO("import ?= default reverts when env is unset on warm index", "[e2e][import]")
+{
+    GIVEN("a Tupfile with import VAR ?= default, after a build that captured the env value")
+    {
+        auto f = E2EFixture { "import_soft_set" };
+
+        {
+            auto env = EnvGuard { "MY_VAR", "from_env" };
+            REQUIRE(f.init().success());
+            auto build1 = f.build();
+            REQUIRE(build1.success());
+            REQUIRE(f.read_file("out.txt") == "from_env\n");
+        }
+
+        WHEN("the env var is unset and the project is rebuilt")
+        {
+            auto build2 = f.build();
+
+            THEN("build succeeds")
+            {
+                INFO("stdout: " << build2.stdout_output);
+                INFO("stderr: " << build2.stderr_output);
+                REQUIRE(build2.success());
+            }
+
+            THEN("the default value is restored, not the cached env value")
+            {
+                REQUIRE(f.read_file("out.txt") == "default_value\n");
+            }
+        }
+    }
+}
+
 // =============================================================================
 // Error Handling Tests
 // =============================================================================