diff --git a/Package.swift b/Package.swift
index 249551a3..5c4679ed 100644
--- a/Package.swift
+++ b/Package.swift
@@ -1,4 +1,4 @@
-// swift-tools-version:5.0
+// swift-tools-version:5.4
 //===--- Package.swift ----------------------------------------*- swift -*-===//
 //
 // This source file is part of the Swift Numerics open source project
@@ -82,13 +82,13 @@ let package = Package(
     ),
     
     // MARK: - Test executables
-    .target(
+    .executableTarget(
       name: "ComplexLog",
       dependencies: ["Numerics", "_TestSupport"],
       path: "Tests/Executable/ComplexLog"
     ),
     
-    .target(
+    .executableTarget(
       name: "ComplexLog1p",
       dependencies: ["Numerics", "_TestSupport"],
       path: "Tests/Executable/ComplexLog1p"
diff --git a/Sources/ComplexModule/Complex+Differentiable.swift b/Sources/ComplexModule/Complex+Differentiable.swift
index c38d4503..eea3816d 100644
--- a/Sources/ComplexModule/Complex+Differentiable.swift
+++ b/Sources/ComplexModule/Complex+Differentiable.swift
@@ -9,7 +9,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if swift(>=5.3) && canImport(_Differentiation)
+// STC April 2022: experimentally remove differentiable conformance to work
+// around CI config failure.
+#if canImport(_Differentiation) && !os(macOS)
 import _Differentiation
 
 extension Complex: Differentiable
diff --git a/Sources/RealModule/Float16+Real.swift b/Sources/RealModule/Float16+Real.swift
index c83f4947..6a991026 100644
--- a/Sources/RealModule/Float16+Real.swift
+++ b/Sources/RealModule/Float16+Real.swift
@@ -11,9 +11,8 @@
 
 import _NumericsShims
 
-// When building with a Swift 5.4 or later toolchain, Float16 is available on
-// non-x86 macOS from 11.0 onward.
-#if swift(>=5.4) && !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
+// Float16 is only available on macOS when targeting arm64.
+#if !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
 
 @available(macOS 11.0, iOS 14.0, tvOS 14.0, watchOS 7.0, *)
 extension Float16: Real {
@@ -175,169 +174,4 @@ extension Float16: Real {
   #endif
 }
 
-// When building with older Swift toolchains for macOS, Float16 is not
-// available. We will drop support for these older toolchains at some
-// future point and remove this duplication.
-#elseif swift(>=5.3) && !(os(macOS) || targetEnvironment(macCatalyst))
-
-@available(iOS 14.0, tvOS 14.0, watchOS 7.0, *)
-extension Float16: Real {
-  @_transparent
-  public static func cos(_ x: Float16) -> Float16 {
-    Float16(.cos(Float(x)))
-  }
-  
-  @_transparent
-  public static func sin(_ x: Float16) -> Float16 {
-    Float16(.sin(Float(x)))
-  }
-  
-  @_transparent
-  public static func tan(_ x: Float16) -> Float16 {
-    Float16(.tan(Float(x)))
-  }
-  
-  @_transparent
-  public static func acos(_ x: Float16) -> Float16 {
-    Float16(.acos(Float(x)))
-  }
-  
-  @_transparent
-  public static func asin(_ x: Float16) -> Float16 {
-    Float16(.asin(Float(x)))
-  }
-  
-  @_transparent
-  public static func atan(_ x: Float16) -> Float16 {
-    Float16(.atan(Float(x)))
-  }
-  
-  @_transparent
-  public static func cosh(_ x: Float16) -> Float16 {
-    Float16(.cosh(Float(x)))
-  }
-  
-  @_transparent
-  public static func sinh(_ x: Float16) -> Float16 {
-    Float16(.sinh(Float(x)))
-  }
-  
-  @_transparent
-  public static func tanh(_ x: Float16) -> Float16 {
-    Float16(.tanh(Float(x)))
-  }
-  
-  @_transparent
-  public static func acosh(_ x: Float16) -> Float16 {
-    Float16(.acosh(Float(x)))
-  }
-  
-  @_transparent
-  public static func asinh(_ x: Float16) -> Float16 {
-    Float16(.asinh(Float(x)))
-  }
-  
-  @_transparent
-  public static func atanh(_ x: Float16) -> Float16 {
-    Float16(.atanh(Float(x)))
-  }
-  
-  @_transparent
-  public static func exp(_ x: Float16) -> Float16 {
-    Float16(.exp(Float(x)))
-  }
-  
-  @_transparent
-  public static func expMinusOne(_ x: Float16) -> Float16 {
-    Float16(.expMinusOne(Float(x)))
-  }
-  
-  @_transparent
-  public static func log(_ x: Float16) -> Float16 {
-    Float16(.log(Float(x)))
-  }
-  
-  @_transparent
-  public static func log(onePlus x: Float16) -> Float16 {
-    Float16(.log(onePlus: Float(x)))
-  }
-  
-  @_transparent
-  public static func erf(_ x: Float16) -> Float16 {
-    Float16(.erf(Float(x)))
-  }
-  
-  @_transparent
-  public static func erfc(_ x: Float16) -> Float16 {
-    Float16(.erfc(Float(x)))
-  }
-  
-  @_transparent
-  public static func exp2(_ x: Float16) -> Float16 {
-    Float16(.exp2(Float(x)))
-  }
-  
-  @_transparent
-  public static func exp10(_ x: Float16) -> Float16 {
-    Float16(.exp10(Float(x)))
-  }
-  
-  @_transparent
-  public static func hypot(_ x: Float16, _ y: Float16) -> Float16 {
-    if x.isInfinite || y.isInfinite { return .infinity }
-    let xf = Float(x)
-    let yf = Float(y)
-    return Float16(.sqrt(xf*xf + yf*yf))
-  }
-  
-  @_transparent
-  public static func gamma(_ x: Float16) -> Float16 {
-    Float16(.gamma(Float(x)))
-  }
-  
-  @_transparent
-  public static func log2(_ x: Float16) -> Float16 {
-    Float16(.log2(Float(x)))
-  }
-  
-  @_transparent
-  public static func log10(_ x: Float16) -> Float16 {
-    Float16(.log10(Float(x)))
-  }
-  
-  @_transparent
-  public static func pow(_ x: Float16, _ y: Float16) -> Float16 {
-    Float16(.pow(Float(x), Float(y)))
-  }
-  
-  @_transparent
-  public static func pow(_ x: Float16, _ n: Int) -> Float16 {
-    // Float16 is simpler than Float or Double, because the range of
-    // "interesting" exponents is pretty small; anything outside of
-    // -22707 ... 34061 simply overflows or underflows for every
-    // x that isn't zero or one. This whole range is representable
-    // as Float, so we can just use powf as long as we're a little
-    // bit (get it?) careful to preserve parity.
-    let clamped = min(max(n, -0x10000), 0x10000) | (n & 1)
-    return Float16(libm_powf(Float(x), Float(clamped)))
-  }
-  
-  @_transparent
-  public static func root(_ x: Float16, _ n: Int) -> Float16 {
-    Float16(.root(Float(x), n))
-  }
-  
-  @_transparent
-  public static func atan2(y: Float16, x: Float16) -> Float16 {
-    Float16(.atan2(y: Float(y), x: Float(x)))
-  }
-  
-  #if !os(Windows)
-  @_transparent
-  public static func logGamma(_ x: Float16) -> Float16 {
-    Float16(.logGamma(Float(x)))
-  }
-  #endif
-}
-
 #endif
diff --git a/Sources/_TestSupport/RealTestSupport.swift b/Sources/_TestSupport/RealTestSupport.swift
index ccabeaeb..ad90efec 100644
--- a/Sources/_TestSupport/RealTestSupport.swift
+++ b/Sources/_TestSupport/RealTestSupport.swift
@@ -15,7 +15,7 @@ public protocol FixedWidthFloatingPoint: BinaryFloatingPoint
 where Exponent: FixedWidthInteger,
       RawSignificand: FixedWidthInteger { }
 
-#if swift(>=5.4) && !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
+#if !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
 @available(macOS 11.0, iOS 14.0, watchOS 14.0, tvOS 7.0, *)
 extension Float16: FixedWidthFloatingPoint { }
 #endif
diff --git a/Tests/ComplexTests/DifferentiableTests.swift b/Tests/ComplexTests/DifferentiableTests.swift
index 36373098..31632bb9 100644
--- a/Tests/ComplexTests/DifferentiableTests.swift
+++ b/Tests/ComplexTests/DifferentiableTests.swift
@@ -9,7 +9,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if swift(>=5.3) && canImport(_Differentiation)
+// STC April 2022: experimentally remove differentiable conformance to work
+// around CI config failure.
+#if canImport(_Differentiation) && !os(macOS)
 
 import XCTest
 import ComplexModule
diff --git a/Tests/RealTests/ElementaryFunctionChecks.swift b/Tests/RealTests/ElementaryFunctionChecks.swift
index 7dfd7caf..4a5ffa4f 100644
--- a/Tests/RealTests/ElementaryFunctionChecks.swift
+++ b/Tests/RealTests/ElementaryFunctionChecks.swift
@@ -130,7 +130,7 @@ internal extension Real where Self: BinaryFloatingPoint {
 
 final class ElementaryFunctionChecks: XCTestCase {
   
-  #if swift(>=5.4) && !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
+  #if !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
   func testFloat16() {
     if #available(macOS 11.0, iOS 14.0, watchOS 14.0, tvOS 7.0, *) {
       Float16.elementaryFunctionChecks()
diff --git a/Tests/RealTests/IntegerExponentTests.swift b/Tests/RealTests/IntegerExponentTests.swift
index 2d4a3a7d..8cc32d98 100644
--- a/Tests/RealTests/IntegerExponentTests.swift
+++ b/Tests/RealTests/IntegerExponentTests.swift
@@ -77,7 +77,7 @@ internal extension Real where Self: FixedWidthFloatingPoint {
   }
 }
 
-#if swift(>=5.4) && !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
+#if !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
 @available(macOS 11.0, iOS 14.0, watchOS 14.0, tvOS 7.0, *)
 extension Float16 {
   static func testIntegerExponent() {
@@ -174,7 +174,7 @@ extension Double {
 
 final class IntegerExponentTests: XCTestCase {
   
-  #if swift(>=5.4) && !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
+  #if !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
   func testFloat16() {
     if #available(macOS 11.0, iOS 14.0, watchOS 14.0, tvOS 7.0, *) {
       Float16.testIntegerExponent()
diff --git a/Tests/WindowsMain.swift b/Tests/WindowsMain.swift
index 520722af..259a0454 100644
--- a/Tests/WindowsMain.swift
+++ b/Tests/WindowsMain.swift
@@ -35,7 +35,7 @@ extension RealTests.ApproximateEqualityTests {
   ])
 }
 
-#if swift(>=5.4) && !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
+#if !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
 extension ElementaryFunctionChecks {
   static var all = testCase([
     ("testFloat16", ElementaryFunctionChecks.testFloat16),
@@ -161,7 +161,7 @@ var testCases = [
   IntegerUtilitiesTests.DoubleWidthTests.all,
 ]
 
-#if swift(>=5.3) && canImport(_Differentiation)
+#if canImport(_Differentiation)
 extension DifferentiableTests {
   static var all = testCase([
     ("testComponentGetter", DifferentiableTests.testComponentGetter),