diff --git a/lib/Sema/TypeCheckAccess.cpp b/lib/Sema/TypeCheckAccess.cpp
index 492aba619df3d..4a844cf7b2e0f 100644
--- a/lib/Sema/TypeCheckAccess.cpp
+++ b/lib/Sema/TypeCheckAccess.cpp
@@ -2124,7 +2124,21 @@ swift::getDisallowedOriginKind(const Decl *decl,
           return DisallowedOriginKind::None;
         }
       }
+
+      // Allow SPI available use in an extension to an SPI available type.
+      // This is a narrow workaround for rdar://159292698 as this should be
+      // handled as part of the `where.isSPI()` above, but that context check
+      // is currently too permissive. It allows SPI use in SPI available which
+      // can break swiftinterfaces. The SPI availability logic likely need to be
+      // separated from normal SPI and treated more like availability.
+      auto ext = dyn_cast_or_null<ExtensionDecl>(where.getDeclContext());
+      if (ext) {
+        auto nominal = ext->getExtendedNominal();
+        if (nominal && nominal->isAvailableAsSPI())
+          return DisallowedOriginKind::None;
+      }
     }
+
     // SPI can only be exported in SPI.
     return where.getDeclContext()->getParentModule() == M ?
       DisallowedOriginKind::SPILocal :
diff --git a/test/SPI/spi_and_spi_available.swift b/test/SPI/spi_and_spi_available.swift
new file mode 100644
index 0000000000000..72c05e70bdd80
--- /dev/null
+++ b/test/SPI/spi_and_spi_available.swift
@@ -0,0 +1,60 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-typecheck-verify-swift \
+// RUN:   -enable-library-evolution -swift-version 5 \
+// RUN:   -library-level=api -require-explicit-availability=ignore
+
+// REQUIRES: OS=macosx
+
+@_spi_available(macOS, introduced: 12.0) @available(iOS 12.0, *)
+public struct SPIAvailableType { // expected-note {{struct declared here}}
+    public init() {}
+}
+
+@_spi(S)
+public struct NormalSPIType {
+    public init() {}
+}
+
+// SPI available in SPI available should be accepted.
+
+@_spi_available(macOS, introduced: 12.0) @available(iOS 12.0, *)
+public struct OtherSPIAvailableType {
+    public func foo(s: SPIAvailableType) {}
+}
+
+extension OtherSPIAvailableType {
+    public func bar(s: SPIAvailableType) {} // expected-error {{cannot use struct 'SPIAvailableType' here; it is SPI}} // FIXME We should allow this.
+}
+
+// Normal SPI in normal SPI should be accepted.
+
+@_spi(S)
+public struct OtherNormalSPIType {
+    public func foo(s: SPIAvailableType) {}
+}
+
+extension OtherNormalSPIType {
+    public func bar2(s: SPIAvailableType) {}
+}
+
+// Normal SPI in SPI available should be rejected.
+
+@_spi_available(macOS, introduced: 12.0) @available(iOS 12.0, *)
+public func SPIAvailableToSPI(s: NormalSPIType) {} // FIXME This should be an error
+
+@inlinable
+@_spi_available(macOS, introduced: 12.0) @available(iOS 12.0, *)
+public func inlinableSPIAvailable() {
+ let _: NormalSPIType = NormalSPIType() // FIXME There should be many errors here
+}
+
+// SPI available in normal SPI is currently accepted.
+
+@_spi(S)
+public func SPIToSPIAvailable(s: NormalSPIType) {}
+
+@inlinable
+@_spi(S)
+public func inlinableSPI() {
+ let _: SPIAvailableType = SPIAvailableType()
+}