swift build fails because it can't find cmath

[alexanderuv]

Is it reproducible with SwiftPM command-line tools: swift build, swift test, swift package etc?

• Confirmed reproduction steps with SwiftPM CLI. The description text must include reproduction steps with either of command-line SwiftPM commands, swift build, swift test, swift package etc.

Description

I'm trying to define a target to act as a wrapper to a header-only C++ library (Eigen). I have followed the instructions in here: https://www.swift.org/documentation/cxx-interop/
However, when I run swift build I keep getting an error that complains about the library being unable to find <cmath> which should be available in the SDK include header paths as it's part of StdLib ?.

I tried adding unsafeFlags to include the lib parameter but no change (also I learned this is deprecated? I'm not a C++ developer so not sure here...).

I have attached a sample project with minimal code to reproduce the issue.
BugRepro.zip

Expected behavior

swift build should work without any problems.

Actual behavior

swift build fails with error :

❯ swift build --verbose
warning: 'bugrepro': /Library/Developer/CommandLineTools/usr/bin/swift-frontend -frontend -c -primary-file /Users/alexander/dev/apple/BugRepro/Package.swift -target arm64-apple-macosx13.0 -Xllvm -aarch64-use-tbi -enable-objc-interop -stack-check -sdk /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -I /Library/Developer/CommandLineTools/usr/lib/swift/pm/ManifestAPI -F /Library/Developer/CommandLineTools/Library/Developer/Frameworks -vfsoverlay /var/folders/m5/7nr7qwz951d9mn1l5qnsfncc0000gn/T/TemporaryDirectory.jsoL1M/vfs.yaml -swift-version 6 -package-description-version 6.0.0 -new-driver-path /Library/Developer/CommandLineTools/usr/bin/swift-driver -empty-abi-descriptor -resource-dir /Library/Developer/CommandLineTools/usr/lib/swift -module-name main -disable-clang-spi -target-sdk-version 15.2 -target-sdk-name macosx15.2 -external-plugin-path '/Library/Developer/Developer/usr/lib/swift/host/plugins#/Library/Developer/Developer/usr/bin/swift-plugin-server' -external-plugin-path '/Library/Developer/Developer/usr/local/lib/swift/host/plugins#/Library/Developer/Developer/usr/bin/swift-plugin-server' -plugin-path /Library/Developer/CommandLineTools/usr/lib/swift/host/plugins -plugin-path /Library/Developer/CommandLineTools/usr/local/lib/swift/host/plugins -o /var/folders/m5/7nr7qwz951d9mn1l5qnsfncc0000gn/T/TemporaryDirectory.DFui0Q/Package-1.o
/Library/Developer/CommandLineTools/usr/bin/clang /var/folders/m5/7nr7qwz951d9mn1l5qnsfncc0000gn/T/TemporaryDirectory.DFui0Q/Package-1.o -F /Library/Developer/CommandLineTools/Library/Developer/Frameworks --sysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk --target=arm64-apple-macosx13.0 /Library/Developer/CommandLineTools/usr/lib/swift/macosx/libswiftCompatibilityPacks.a -L /Library/Developer/CommandLineTools/usr/lib/swift/macosx -L /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib/swift -L /Library/Developer/CommandLineTools/usr/lib/swift/pm/ManifestAPI -lPackageDescription -Xlinker -rpath -Xlinker /Library/Developer/CommandLineTools/usr/lib/swift/pm/ManifestAPI -o /var/folders/m5/7nr7qwz951d9mn1l5qnsfncc0000gn/T/TemporaryDirectory.anG2Ng/bugrepro-manifest
Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
Target: arm64-apple-macosx13.0
Planning build
Building for debugging...
Write auxiliary file /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/swift-version--1AB21518FC5DEDBE.txt
/Library/Developer/CommandLineTools/usr/bin/swiftc -module-name BugRepro -emit-dependencies -emit-module -emit-module-path /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/Modules/BugRepro.swiftmodule -output-file-map /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/BugRepro.build/output-file-map.json -parse-as-library -incremental -c @/Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/BugRepro.build/sources -I /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/Modules -target arm64-apple-macosx10.13 -v -enable-batch-mode -index-store-path /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/index/store -Onone -enable-testing -j12 -DSWIFT_PACKAGE -DDEBUG -Xcc -fmodule-map-file=/Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/MyCLibrary.build/module.modulemap -Xcc -I -Xcc /Users/alexander/dev/apple/BugRepro/Sources/MyCLibrary/include -module-cache-path /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/ModuleCache -parseable-output -parse-as-library -emit-objc-header -emit-objc-header-path /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/BugRepro.build/BugRepro-Swift.h -color-diagnostics -swift-version 6 -F /Library/Developer/CommandLineTools/Library/Developer/Frameworks -sdk /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -g -Xcc -isysroot -Xcc /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -Xcc -fPIC -Xcc -g -package-name bugrepro
Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
Target: arm64-apple-macosx10.13
/Library/Developer/CommandLineTools/usr/bin/swift-frontend -frontend -emit-module -experimental-skip-non-inlinable-function-bodies-without-types /Users/alexander/dev/apple/BugRepro/Sources/BugRepro/BugRepro.swift -target arm64-apple-macosx10.13 -Xllvm -aarch64-use-tbi -enable-objc-interop -sdk /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -I /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/Modules -F /Library/Developer/CommandLineTools/Library/Developer/Frameworks -color-diagnostics -enable-testing -g -debug-info-format=dwarf -dwarf-version=4 -module-cache-path /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/ModuleCache -swift-version 6 -Onone -D SWIFT_PACKAGE -D DEBUG -new-driver-path /Library/Developer/CommandLineTools/usr/bin/swift-driver -empty-abi-descriptor -resource-dir /Library/Developer/CommandLineTools/usr/lib/swift -enable-anonymous-context-mangled-names -file-compilation-dir /Users/alexander/dev/apple/BugRepro -Xcc -fmodule-map-file=/Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/MyCLibrary.build/module.modulemap -Xcc -I -Xcc /Users/alexander/dev/apple/BugRepro/Sources/MyCLibrary/include -Xcc -isysroot -Xcc /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -Xcc -fPIC -Xcc -g -module-name BugRepro -package-name bugrepro -disable-clang-spi -target-sdk-version 15.2 -target-sdk-name macosx15.2 -external-plugin-path /Library/Developer/Developer/usr/lib/swift/host/plugins#/Library/Developer/Developer/usr/bin/swift-plugin-server -external-plugin-path /Library/Developer/Developer/usr/local/lib/swift/host/plugins#/Library/Developer/Developer/usr/bin/swift-plugin-server -plugin-path /Library/Developer/CommandLineTools/usr/lib/swift/host/plugins -plugin-path /Library/Developer/CommandLineTools/usr/local/lib/swift/host/plugins -emit-module-doc-path /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/Modules/BugRepro.swiftdoc -emit-module-source-info-path /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/Modules/BugRepro.swiftsourceinfo -emit-objc-header-path /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/BugRepro.build/BugRepro-Swift.h -emit-dependencies-path /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/BugRepro.build/BugRepro.emit-module.d -parse-as-library -o /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/Modules/BugRepro.swiftmodule -emit-abi-descriptor-path /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/Modules/BugRepro.abi.json
/Library/Developer/CommandLineTools/usr/bin/swift-frontend -frontend -c -primary-file /Users/alexander/dev/apple/BugRepro/Sources/BugRepro/BugRepro.swift -emit-dependencies-path /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/BugRepro.build/BugRepro.d -emit-reference-dependencies-path /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/BugRepro.build/BugRepro.swiftdeps -target arm64-apple-macosx10.13 -Xllvm -aarch64-use-tbi -enable-objc-interop -sdk /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -I /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/Modules -F /Library/Developer/CommandLineTools/Library/Developer/Frameworks -color-diagnostics -enable-testing -g -debug-info-format=dwarf -dwarf-version=4 -module-cache-path /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/ModuleCache -swift-version 6 -Onone -D SWIFT_PACKAGE -D DEBUG -new-driver-path /Library/Developer/CommandLineTools/usr/bin/swift-driver -empty-abi-descriptor -resource-dir /Library/Developer/CommandLineTools/usr/lib/swift -enable-anonymous-context-mangled-names -file-compilation-dir /Users/alexander/dev/apple/BugRepro -Xcc -fmodule-map-file=/Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/MyCLibrary.build/module.modulemap -Xcc -I -Xcc /Users/alexander/dev/apple/BugRepro/Sources/MyCLibrary/include -Xcc -isysroot -Xcc /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -Xcc -fPIC -Xcc -g -module-name BugRepro -package-name bugrepro -disable-clang-spi -target-sdk-version 15.2 -target-sdk-name macosx15.2 -external-plugin-path /Library/Developer/Developer/usr/lib/swift/host/plugins#/Library/Developer/Developer/usr/bin/swift-plugin-server -external-plugin-path /Library/Developer/Developer/usr/local/lib/swift/host/plugins#/Library/Developer/Developer/usr/bin/swift-plugin-server -plugin-path /Library/Developer/CommandLineTools/usr/lib/swift/host/plugins -plugin-path /Library/Developer/CommandLineTools/usr/local/lib/swift/host/plugins -parse-as-library -o /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/BugRepro.build/BugRepro.swift.o -index-store-path /Users/alexander/dev/apple/BugRepro/.build/arm64-apple-macosx/debug/index/store -index-system-modules
error: emit-module command failed with exit code 1 (use -v to see invocation)
<module-includes>:1:9: note: in file included from <module-includes>:1:
1 | #import "/Users/alexander/dev/apple/BugRepro/Sources/MyCLibrary/include/MyCLibrary.h"
  |         `- note: in file included from <module-includes>:1:
2 |

/Users/alexander/dev/apple/BugRepro/Sources/MyCLibrary/include/MyCLibrary.h:4:10: note: in file included from /Users/alexander/dev/apple/BugRepro/Sources/MyCLibrary/include/MyCLibrary.h:4:
2 | #pragma once
3 |
4 | #include "../headers/custom_header.h"
  |          `- note: in file included from /Users/alexander/dev/apple/BugRepro/Sources/MyCLibrary/include/MyCLibrary.h:4:

/Users/alexander/dev/apple/BugRepro/Sources/MyCLibrary/include/../headers/custom_header.h:2:10: error: 'cmath' file not found
1 |
2 | #include <cmath>
  |          `- error: 'cmath' file not found
3 |
4 | int some_function(int a, int b) {

/Users/alexander/dev/apple/BugRepro/Sources/BugRepro/BugRepro.swift:4:8: error: could not build Objective-C module 'MyCLibrary'
2 | // https://docs.swift.org/swift-book
3 |
4 | import MyCLibrary
  |        `- error: could not build Objective-C module 'MyCLibrary'
5 |
6 | public func bugRepro() {
<module-includes>:1:9: note: in file included from <module-includes>:1:
1 | #import "/Users/alexander/dev/apple/BugRepro/Sources/MyCLibrary/include/MyCLibrary.h"
  |         `- note: in file included from <module-includes>:1:
2 |

/Users/alexander/dev/apple/BugRepro/Sources/MyCLibrary/include/MyCLibrary.h:4:10: note: in file included from /Users/alexander/dev/apple/BugRepro/Sources/MyCLibrary/include/MyCLibrary.h:4:
2 | #pragma once
3 |
4 | #include "../headers/custom_header.h"
  |          `- note: in file included from /Users/alexander/dev/apple/BugRepro/Sources/MyCLibrary/include/MyCLibrary.h:4:

/Users/alexander/dev/apple/BugRepro/Sources/MyCLibrary/include/../headers/custom_header.h:2:10: error: 'cmath' file not found
1 |
2 | #include <cmath>
  |          `- error: 'cmath' file not found
3 |
4 | int some_function(int a, int b) {

/Users/alexander/dev/apple/BugRepro/Sources/BugRepro/BugRepro.swift:4:8: error: could not build Objective-C module 'MyCLibrary'
2 | // https://docs.swift.org/swift-book
3 |
4 | import MyCLibrary
  |        `- error: could not build Objective-C module 'MyCLibrary'
5 |
6 | public func bugRepro() {

Steps to reproduce

Run swift build

Swift Package Manager version/commit hash

Swift Package Manager - Swift 6.0.3

Swift & OS version (output of swift --version ; uname -a)

swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
Target: arm64-apple-macosx15.0
Darwin MacBook-Pro.local 24.1.0 Darwin Kernel Version 24.1.0: Thu Oct 10 21:00:32 PDT 2024; root:xnu-11215.41.3~2/RELEASE_ARM64_T6030 arm64

[rjmansfield]

.interoperabilityMode(.Cxx)] needs to go on the target that uses the C++ library.

--- Package.swift.orig	2025-01-13 09:52:44
+++ Package.swift	2025-01-13 09:59:29
@@ -15,10 +15,10 @@
         // Targets are the basic building blocks of a package, defining a module or a test suite.
         // Targets can depend on other targets in this package and products from dependencies.
         .target(
-            name: "MyCLibrary",
-            swiftSettings: [.interoperabilityMode(.Cxx)]),
+            name: "MyCLibrary"),
         .target(
             name: "BugRepro",
-            dependencies: ["MyCLibrary"])
+            dependencies: ["MyCLibrary"],
+            swiftSettings: [.interoperabilityMode(.Cxx)]),
     ]
 )

Then it should build and you should be able to call the some_function().

print("MyCLibrary: \(MyCLibrary.some_function(1, 2))")

See https://www.swift.org/documentation/cxx-interop/project-build-setup/#mixing-swift-and-c-using-swift-package-manager for more information and examples.