Pin the export/override prefix and ?= first-wins with tests

test/packaging/lint-once.test.ts

@@ -826,6 +826,81 @@ describe("the resolver reads what the shell would run", () => {
         expect(recipes.get("check")?.recipe).toEqual(["script/fmt-check"]);
     });
 
+    // `export FMT := script/fmt-check` is an assignment make honours and the
+    // pattern anchored at the name, so the whole line read as neither an
+    // assignment nor a target and `@$(FMT)` stood unresolved: `make -n check`
+    // printed `script/check` and `script/fmt-check` while this file scored
+    // one. `override` reaches the same place by the same route.
+    it("expands a variable assigned through an export or override prefix", () => {
+        expect(
+            parseMakefile(
+                [
+                    "export FMT := script/fmt-check",
+                    "check:",
+                    "\t@$(FMT)",
+                    "\t@script/check",
+                ].join("\n"),
+            ).get("check")?.recipe,
+        ).toEqual(["script/fmt-check", "script/check"]);
+        expect(
+            parseMakefile(
+                ["override FMT = script/fmt-check", "check:", "\t@$(FMT)"].join(
+                    "\n",
+                ),
+            ).get("check")?.recipe,
+        ).toEqual(["script/fmt-check"]);
+        // `export` on its own line names a variable without assigning one, and
+        // is not read as an assignment of the empty string.
+        expect(
+            parseMakefile(
+                ["FMT := script/fmt-check", "export FMT", "check:", "\t@$(FMT)"].join(
+                    "\n",
+                ),
+            ).get("check")?.recipe,
+        ).toEqual(["script/fmt-check"]);
+    });
+
+    // `?=` assigns only when the name has no value yet, so among assignments
+    // to one name the first `?=` wins where `:=` and `=` let the last one win.
+    // Reading every operator as last-wins resolved `@$(FMT)` to the value make
+    // discards: the file counted an invocation that never happens and missed
+    // the one that does, with the suite green either way.
+    it("keeps the first value when a later assignment is conditional", () => {
+        expect(
+            parseMakefile(
+                [
+                    "FMT := script/fmt-check",
+                    "FMT ?= script/build",
+                    "check:",
+                    "\t@$(FMT)",
+                ].join("\n"),
+            ).get("check")?.recipe,
+        ).toEqual(["script/fmt-check"]);
+        // A `?=` that is itself the first assignment does assign, and a `?=`
+        // after it does not.
+        expect(
+            parseMakefile(
+                [
+                    "FMT ?= script/fmt-check",
+                    "FMT ?= script/build",
+                    "check:",
+                    "\t@$(FMT)",
+                ].join("\n"),
+            ).get("check")?.recipe,
+        ).toEqual(["script/fmt-check"]);
+        // `:=` and `=` stay last-wins, which is what make does.
+        expect(
+            parseMakefile(
+                [
+                    "FMT := script/build",
+                    "FMT := script/fmt-check",
+                    "check:",
+                    "\t@$(FMT)",
+                ].join("\n"),
+            ).get("check")?.recipe,
+        ).toEqual(["script/fmt-check"]);
+    });
+
     // The edges of the expansion, asserted so the header's not-followed list
     // is the code's behaviour rather than a claim about it. Each of these
     // leaves the reference standing verbatim, which reaches nothing — the same