Downgrade the error about a _compiler_version("1.N.2") to a warning

The error about only a `*` being meaningful in the second position is
a warning in the compiler, so match that behavior.

Author: DougGregor

Sources/SwiftIfConfig/IfConfigError.swift

@@ -118,7 +118,8 @@ extension IfConfigError: DiagnosticMessage {
 
   var severity: SwiftDiagnostics.DiagnosticSeverity {
     switch self {
-    case .ignoredTrailingComponents, .likelySimulatorPlatform: return .warning
+    case .compilerVersionSecondComponentNotWildcard, .ignoredTrailingComponents, .likelySimulatorPlatform:
+      return .warning
     default: return .error
     }
   }

Sources/SwiftIfConfig/IfConfigEvaluation.swift

@@ -354,7 +354,11 @@ func evaluateIfConfig(
       let versionString = stringSegment.content.text
       let expectedVersion: VersionTuple
       do {
-        expectedVersion = try VersionTuple(parsingCompilerBuildVersion: versionString, argExpr)
+        expectedVersion = try VersionTuple.parseCompilerBuildVersion(
+          versionString,
+          argExpr,
+          extraDiagnostics: &extraDiagnostics
+        )
       } catch {
         return recordError(error, at: stringSegment.content)
       }

Sources/SwiftIfConfig/VersionTuple+Parsing.swift

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+import SwiftDiagnostics
 import SwiftSyntax
 
 extension VersionTuple {
@@ -20,11 +21,35 @@ extension VersionTuple {
   ///   we are parsing.
   ///   - versionSyntax: The syntax node that contains the version string, used
   ///   only for diagnostic purposes.
-  init(
-    parsingCompilerBuildVersion versionString: String,
+  static func parseCompilerBuildVersion(
+    _ versionString: String,
     _ versionSyntax: ExprSyntax
-  ) throws {
-    components = []
+  ) -> (version: VersionTuple?, diagnostics: [Diagnostic]) {
+    var extraDiagnostics: [Diagnostic] = []
+    let version: VersionTuple?
+    do {
+      version = try parseCompilerBuildVersion(versionString, versionSyntax, extraDiagnostics: &extraDiagnostics)
+    } catch {
+      version = nil
+      extraDiagnostics.append(contentsOf: error.asDiagnostics(at: versionSyntax))
+    }
+
+    return (version, extraDiagnostics)
+  }
+
+  /// Parse a compiler build version of the form "5007.*.1.2.3*", which is
+  /// used by an older if configuration form `_compiler_version("...")`.
+  /// - Parameters:
+  ///   - versionString: The version string for the compiler build version that
+  ///   we are parsing.
+  ///   - versionSyntax: The syntax node that contains the version string, used
+  ///   only for diagnostic purposes.
+  static func parseCompilerBuildVersion(
+    _ versionString: String,
+    _ versionSyntax: ExprSyntax,
+    extraDiagnostics: inout [Diagnostic]
+  ) throws -> VersionTuple {
+    var components: [Int] = []
 
     // Version value are separated by periods.
     let componentStrings = versionString.split(separator: ".", omittingEmptySubsequences: false)
@@ -49,7 +74,9 @@ extension VersionTuple {
       // The second component is always "*", and is never used for comparison.
       if index == 1 {
         if componentString != "*" {
-          throw IfConfigError.compilerVersionSecondComponentNotWildcard(syntax: versionSyntax)
+          extraDiagnostics.append(
+            IfConfigError.compilerVersionSecondComponentNotWildcard(syntax: versionSyntax).asDiagnostic
+          )
         }
         try recordComponent(0)
         continue
@@ -102,5 +129,7 @@ extension VersionTuple {
       }
       components[0] = components[0] / 1000
     }
+
+    return VersionTuple(components: components)
   }
 }

Tests/SwiftIfConfigTest/EvaluateTests.swift

@@ -230,6 +230,19 @@ public class EvaluateTests: XCTestCase {
         )
       ]
     )
+
+    assertIfConfig(
+      #"_compiler_version("5.7.100")"#,
+      .active,
+      diagnostics: [
+        DiagnosticSpec(
+          message: "the second version component is not used for comparison in legacy compiler versions",
+          line: 1,
+          column: 19,
+          severity: .warning
+        )
+      ]
+    )
   }
 
   func testCanImport() throws {