diff --git a/src/Magnum/Test/ImageTest.cpp b/src/Magnum/Test/ImageTest.cpp
index 4d268bcaa..a4805eead 100644
--- a/src/Magnum/Test/ImageTest.cpp
+++ b/src/Magnum/Test/ImageTest.cpp
@@ -172,6 +172,9 @@ namespace GL {
     }
 
     enum class CompressedPixelFormat { RGBS3tcDxt1 = 21 };
+    /* Clang -Wmissing-prototypes warns otherwise, even though this is in an
+       anonymous namespace */
+    Vector3i compressedPixelFormatBlockSize(CompressedPixelFormat);
     Vector3i compressedPixelFormatBlockSize(CompressedPixelFormat format) {
         #ifdef CORRADE_NO_ASSERT
         static_cast<void>(format);
@@ -179,6 +182,7 @@ namespace GL {
         CORRADE_INTERNAL_ASSERT(format == CompressedPixelFormat::RGBS3tcDxt1);
         return {4, 4, 1};
     }
+    UnsignedInt compressedPixelFormatBlockDataSize(CompressedPixelFormat);
     UnsignedInt compressedPixelFormatBlockDataSize(CompressedPixelFormat format) {
         #ifdef CORRADE_NO_ASSERT
         static_cast<void>(format);
diff --git a/src/Magnum/Test/ImageViewTest.cpp b/src/Magnum/Test/ImageViewTest.cpp
index dcb8fcdf7..0e8db025b 100644
--- a/src/Magnum/Test/ImageViewTest.cpp
+++ b/src/Magnum/Test/ImageViewTest.cpp
@@ -164,6 +164,9 @@ namespace GL {
     }
 
     enum class CompressedPixelFormat { RGBS3tcDxt1 = 21 };
+    /* Clang -Wmissing-prototypes warns otherwise, even though this is in an
+       anonymous namespace */
+    Vector3i compressedPixelFormatBlockSize(CompressedPixelFormat);
     Vector3i compressedPixelFormatBlockSize(CompressedPixelFormat format) {
         #ifdef CORRADE_NO_ASSERT
         static_cast<void>(format);
@@ -171,6 +174,7 @@ namespace GL {
         CORRADE_INTERNAL_ASSERT(format == CompressedPixelFormat::RGBS3tcDxt1);
         return {4, 4, 1};
     }
+    UnsignedInt compressedPixelFormatBlockDataSize(CompressedPixelFormat);
     UnsignedInt compressedPixelFormatBlockDataSize(CompressedPixelFormat format) {
         #ifdef CORRADE_NO_ASSERT
         static_cast<void>(format);
diff --git a/src/Magnum/Trade/Test/ImageDataTest.cpp b/src/Magnum/Trade/Test/ImageDataTest.cpp
index 019333c9a..c755e48ac 100644
--- a/src/Magnum/Trade/Test/ImageDataTest.cpp
+++ b/src/Magnum/Trade/Test/ImageDataTest.cpp
@@ -204,6 +204,9 @@ namespace GL {
     }
 
     enum class CompressedPixelFormat { RGBS3tcDxt1 = 21 };
+    /* Clang -Wmissing-prototypes warns otherwise, even though this is in an
+       anonymous namespace */
+    Vector3i compressedPixelFormatBlockSize(CompressedPixelFormat);
     Vector3i compressedPixelFormatBlockSize(CompressedPixelFormat format) {
         CORRADE_INTERNAL_ASSERT(format == CompressedPixelFormat::RGBS3tcDxt1);
         #ifdef CORRADE_NO_ASSERT
@@ -211,6 +214,7 @@ namespace GL {
         #endif
         return {4, 4, 1};
     }
+    UnsignedInt compressedPixelFormatBlockDataSize(CompressedPixelFormat);
     UnsignedInt compressedPixelFormatBlockDataSize(CompressedPixelFormat format) {
         CORRADE_INTERNAL_ASSERT(format == CompressedPixelFormat::RGBS3tcDxt1);
         #ifdef CORRADE_NO_ASSERT
@@ -234,6 +238,9 @@ namespace Vk {
     }
 
     enum class CompressedPixelFormat { Astc5x5x4RGBAF = 111 };
+    /* Clang -Wmissing-prototypes warns otherwise, even though this is in an
+       anonymous namespace */
+    Vector3i compressedPixelFormatBlockSize(CompressedPixelFormat);
     Vector3i compressedPixelFormatBlockSize(CompressedPixelFormat format) {
         #ifdef CORRADE_NO_ASSERT
         static_cast<void>(format);
@@ -241,6 +248,7 @@ namespace Vk {
         CORRADE_INTERNAL_ASSERT(format == CompressedPixelFormat::Astc5x5x4RGBAF);
         return {5, 5, 4};
     }
+    UnsignedInt compressedPixelFormatBlockDataSize(CompressedPixelFormat);
     UnsignedInt compressedPixelFormatBlockDataSize(CompressedPixelFormat format) {
         #ifdef CORRADE_NO_ASSERT
         static_cast<void>(format);