[2.x] fix: Respect explicit platform settings in dependency resolution (#8697)

**Problem**
When platform is set, it incorrectly adds the platform suffix to implicit Scala library dependencies even though they explicitly set platform to jvm. This causes resolution errors.

**Solution**
Modified addPlatformSuffix to prioritize explicit platform settings on dependencies. If a dependency has an explicit platform, use that instead of the project platform. The project platform should only apply to dependencies without an explicit platform.

Fixes #8665
Generated-by: Claude Sonnet 4.5

lm-coursier/src/main/scala/lmcoursier/FromSbt.scala

@@ -55,9 +55,11 @@ object FromSbt {
         case _          => s"${name}_$platformName"
       }
     (platformOpt, projectPlatform) match {
-      case (Some(p), None) => addSuffix(p)
-      case (_, Some(p))    => addSuffix(p)
-      case _               => name
+      case (Some(p), _) =>
+        addSuffix(p) // Use explicit platform if set (don't override with project platform)
+      case (None, Some(p)) =>
+        addSuffix(p) // Only use project platform if dependency has no explicit platform
+      case _ => name
     }
   }
 

lm-coursier/src/test/scala/lmcoursier/FromSbtPlatformSpec.scala

@@ -0,0 +1,110 @@
+package lmcoursier
+
+import org.scalatest.matchers.should.Matchers
+import org.scalatest.propspec.AnyPropSpec
+import sbt.librarymanagement.*
+
+final class FromSbtPlatformSpec extends AnyPropSpec with Matchers {
+
+  property("explicit platform should not be overridden by project platform") {
+    // Test case for issue #8665: Auto-injected Scala library with explicit .platform(Platform.jvm)
+    // should not get project platform applied
+
+    val scala3Library = ModuleID("org.scala-lang", "scala3-library", "3.7.4")
+      .withCrossVersion(Binary())
+      .platform(Platform.jvm) // Explicit platform set to jvm
+
+    val projectPlatform = Some("native0.5") // Project has native0.5 platform
+
+    // When converting to Coursier module, the name should NOT include native0.5 suffix
+    val (module, _) = FromSbt.moduleVersion(
+      scala3Library,
+      scalaVersion = "3.7.4",
+      scalaBinaryVersion = "3",
+      optionalCrossVer = false,
+      projectPlatform = projectPlatform
+    )
+
+    // The module name should be scala3-library_3, NOT scala3-library_native0.5_3
+    module.name.value shouldBe "scala3-library_3"
+    module.name.value should not contain "native0.5"
+  }
+
+  property("project platform should apply to dependencies without explicit platform") {
+    // Dependencies without explicit platform should get project platform
+    val regularDep = ModuleID("com.example", "foo", "1.0.0")
+      .withCrossVersion(Binary())
+    // No explicit platform set
+
+    val projectPlatform = Some("native0.5")
+
+    val (module, _) = FromSbt.moduleVersion(
+      regularDep,
+      scalaVersion = "2.13.18",
+      scalaBinaryVersion = "2.13",
+      optionalCrossVer = false,
+      projectPlatform = projectPlatform
+    )
+
+    // Should get the project platform suffix
+    module.name.value shouldBe "foo_native0.5_2.13"
+  }
+
+  property("explicit platform should take priority over project platform") {
+    // When a dependency has an explicit platform, it should be used even if project has a different platform
+    val depWithExplicitPlatform = ModuleID("com.example", "bar", "1.0.0")
+      .withCrossVersion(Binary())
+      .platform("js1") // Explicit platform set to js1
+
+    val projectPlatform = Some("native0.5") // Project has different platform
+
+    val (module, _) = FromSbt.moduleVersion(
+      depWithExplicitPlatform,
+      scalaVersion = "2.13.18",
+      scalaBinaryVersion = "2.13",
+      optionalCrossVer = false,
+      projectPlatform = projectPlatform
+    )
+
+    // Should use explicit platform (js1), not project platform (native0.5)
+    module.name.value shouldBe "bar_js1_2.13"
+    module.name.value should not contain "native0.5"
+  }
+
+  property("jvm platform should not add suffix") {
+    // JVM platform (explicit or project) should not add suffix
+    val dep = ModuleID("com.example", "baz", "1.0.0")
+      .withCrossVersion(Binary())
+      .platform(Platform.jvm)
+
+    val (module, _) = FromSbt.moduleVersion(
+      dep,
+      scalaVersion = "2.13.18",
+      scalaBinaryVersion = "2.13",
+      optionalCrossVer = false,
+      projectPlatform = Some("native0.5")
+    )
+
+    // JVM platform should not add suffix, even with project platform set
+    module.name.value shouldBe "baz_2.13"
+    module.name.value should not contain "jvm"
+    module.name.value should not contain "native0.5"
+  }
+
+  property("no platform when both are None") {
+    val dep = ModuleID("com.example", "qux", "1.0.0")
+      .withCrossVersion(Binary())
+    // No explicit platform
+
+    val (module, _) = FromSbt.moduleVersion(
+      dep,
+      scalaVersion = "2.13.18",
+      scalaBinaryVersion = "2.13",
+      optionalCrossVer = false,
+      projectPlatform = None
+    )
+
+    // Should just have cross-version suffix, no platform
+    module.name.value shouldBe "qux_2.13"
+  }
+}

sbt-app/src/sbt-test/dependency-management/platform-scala3-library/build.sbt

@@ -0,0 +1,40 @@
+lazy val check = taskKey[Unit]("Runs the check")
+
+// Reproduces issue #8665: platform should not be applied to auto-injected Scala library
+scalaVersion := "3.7.4"
+platform := "native0.5"
+
+// autoScalaLibrary is true by default, which auto-injects scala3-library
+// The auto-injected library has .platform(Platform.jvm) explicitly set
+// It should NOT get the native0.5 platform suffix
+
+TaskKey[Unit]("check") := {
+  val ur = update.value
+  
+  // The auto-injected scala3-library should be resolved as scala3-library_3, NOT scala3-library_native0.5_3
+  val scala3LibraryFiles = ur.matching(
+    moduleFilter(organization = "org.scala-lang", name = "scala3-library_3", revision = "*")
+  )
+  assert(
+    scala3LibraryFiles.nonEmpty,
+    s"scala3-library_3 (without platform suffix) was not found in update report. " +
+    s"This indicates the platform was incorrectly applied to the auto-injected library. " +
+    s"Update report: $ur"
+  )
+  
+  // Verify that scala3-library_native0.5_3 does NOT exist (it shouldn't)
+  val wrongFiles = ur.matching(
+    moduleFilter(organization = "org.scala-lang", name = "scala3-library_native0.5_3", revision = "*")
+  )
+  assert(
+    wrongFiles.isEmpty,
+    s"scala3-library_native0.5_3 was incorrectly found in update report. " +
+    s"The auto-injected Scala library should not get the project platform suffix. " +
+    s"Update report: $ur"
+  )
+  
+  streams.value.log.info("✓ Auto-injected scala3-library correctly resolved without platform suffix")
+}
+
+csrCacheDirectory := baseDirectory.value / "coursier-cache"
+

sbt-app/src/sbt-test/dependency-management/platform-scala3-library/test

@@ -0,0 +1,2 @@
+> check
+