Fix SPM consumer build: qualify cross-directory header imports (#215) (#217)

Public headers in render/ imported sibling headers from lib/ by bare
filename (e.g. `#import "MTMathList.h"`). That only resolves via the
target-internal `lib` header search path, which is applied when building
iosMath's own sources but NOT when a downstream SPM consumer builds the
`iosMath` Clang module from module.modulemap. Consumers therefore failed
with "'MTMathList.h' file not found" / "could not build module 'iosMath'",
while the bundled demo and in-package tests worked.

Qualify the render -> lib imports as `lib/MTMathList.h` so they resolve
from the package's public-headers root (exposed to consumers), and add
that root to each target's header search paths so internal builds keep
resolving the qualified path.

Add `iosMathConsumerTests`, a test target that imports iosMath purely as
a Clang module with no header search paths, reproducing external SPM
consumption. It fails to compile if a bare cross-directory import returns.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: kostub

Package.swift

@@ -22,6 +22,7 @@ let package = Package(
             ],
             publicHeadersPath: ".",
             cSettings: [
+                .headerSearchPath("."),
                 .headerSearchPath("lib"),
                 .headerSearchPath("render"),
                 .headerSearchPath("render/internal"),
@@ -33,6 +34,7 @@ let package = Package(
             path: "iosMathTests",
             exclude: ["en.lproj"],
             cSettings: [
+                .headerSearchPath("../iosMath"),
                 .headerSearchPath("../iosMath/lib"),
                 .headerSearchPath("../iosMath/render"),
                 .headerSearchPath("../iosMath/render/internal"),
@@ -43,6 +45,7 @@ let package = Package(
             dependencies: ["iosMath"],
             path: "iosMathSwiftTests",
             cSettings: [
+                .headerSearchPath("../iosMath"),
                 .headerSearchPath("../iosMath/lib"),
                 .headerSearchPath("../iosMath/render"),
                 .headerSearchPath("../iosMath/render/internal"),
@@ -51,5 +54,14 @@ let package = Package(
                 .swiftLanguageMode(.v5),
             ]
         ),
+        // Regression guard for issue #215. Imports `iosMath` purely as a Clang
+        // module with NO header search paths, reproducing how an external SPM
+        // consumer builds the module. If a public header reintroduces a bare
+        // cross-directory `#import`, this target fails to compile.
+        .testTarget(
+            name: "iosMathConsumerTests",
+            dependencies: ["iosMath"],
+            path: "iosMathConsumerTests"
+        ),
     ]
 )

iosMath/render/MTFontManager.h

@@ -13,7 +13,7 @@
 @import CoreText;
 
 #import "MTFont.h"
-#import "MTMathList.h"
+#import "lib/MTMathList.h"
 
 NS_ASSUME_NONNULL_BEGIN
 

iosMath/render/MTMathListDisplay.h

@@ -18,7 +18,7 @@
 #import "MTConfig.h"
 
 #import "MTFont.h"
-#import "MTMathList.h"
+#import "lib/MTMathList.h"
 
 NS_ASSUME_NONNULL_BEGIN
 

iosMath/render/MTMathUILabel.h

@@ -15,7 +15,7 @@
 #import "MTConfig.h"
 
 #import "MTFont.h"
-#import "MTMathList.h"
+#import "lib/MTMathList.h"
 #import "MTMathListDisplay.h"
 
 /**

iosMathConsumerTests/iosMathModuleConsumerTests.swift

@@ -0,0 +1,41 @@
+// iosMathModuleConsumerTests.swift
+// Regression test for issue #215: "Clang dependency scanning failure,
+// fatal error: MTMathList.h not found."
+//
+// This target deliberately depends on `iosMath` and imports it *purely as a
+// Clang module*, with NO header search paths configured in Package.swift. That
+// reproduces exactly how a downstream Swift Package Manager consumer builds the
+// module: from `module.modulemap` against the package's public-headers root,
+// without iosMath's target-internal `lib`/`render` search paths.
+//
+// The bug was that public headers in `render/` imported sibling headers in
+// `lib/` by bare filename (e.g. `#import "MTMathList.h"`), which only resolves
+// when the internal `lib` search path is present. External consumers (and this
+// target) lack that path, so building the module failed with
+// "'MTMathList.h' file not found".
+//
+// If that regression returns, this target FAILS TO COMPILE — `import iosMath`
+// forces the module (all module-map headers) to build. The assertions below are
+// secondary; the real guard is that this file compiles at all. Referencing a
+// `render/` type (MTMathUILabel) and a `lib/` type (MTMathList) together keeps
+// the cross-directory include in the compiled surface.
+
+import XCTest
+import iosMath
+
+final class iosMathModuleConsumerTests: XCTestCase {
+
+    // Exercises the render -> lib cross-directory include path. MTMathUILabel
+    // lives in render/ and pulls in MTMathList from lib/.
+    @MainActor
+    func testModuleBuildsAndRendersCrossDirectoryTypes() {
+        let label = MTMathUILabel()
+        label.latex = #"\frac{1}{2}"#
+        XCTAssertNotNil(label.mathList)
+        XCTAssertNil(label.error)
+
+        // Use the lib/ type directly as well so it stays in the consumed surface.
+        let list: MTMathList? = label.mathList
+        XCTAssertNotNil(list)
+    }
+}