It makes no sense to call isPixelFormatSrgb() on a depth/stencil format.

Doing so usually points to some error in user code, so directly disallow
it, instead of returning false. Such behavior is consistent with
pixelFormatChannelCount() and other helper APIs.

src/Magnum/PixelFormat.cpp

@@ -352,6 +352,7 @@ bool isPixelFormatSrgb(const PixelFormat format) {
         case PixelFormat::RG32F:
         case PixelFormat::RGB32F:
         case PixelFormat::RGBA32F:
+            return false;
         case PixelFormat::Depth16Unorm:
         case PixelFormat::Depth24Unorm:
         case PixelFormat::Depth32F:
@@ -359,7 +360,7 @@ bool isPixelFormatSrgb(const PixelFormat format) {
         case PixelFormat::Depth16UnormStencil8UI:
         case PixelFormat::Depth24UnormStencil8UI:
         case PixelFormat::Depth32FStencil8UI:
-            return false;
+            CORRADE_ASSERT_UNREACHABLE("isPixelFormatSrgb(): can't determine colorspace of" << format, {});
     }
     #ifdef CORRADE_TARGET_GCC
     #pragma GCC diagnostic pop

src/Magnum/PixelFormat.h

@@ -814,7 +814,8 @@ MAGNUM_EXPORT UnsignedInt pixelFormatChannelCount(PixelFormat format);
 @m_since_latest
 
 Returns @cpp true @ce for `*Srgb` formats, @cpp false @ce otherwise. Expects
-that the pixel format is *not* implementation-specific.
+that the pixel format is *not* implementation-specific and not a
+depth/stencil format.
 @see @ref isPixelFormatImplementationSpecific()
 */
 MAGNUM_EXPORT bool isPixelFormatSrgb(PixelFormat format);

src/Magnum/Test/PixelFormatTest.cpp

@@ -50,7 +50,7 @@ struct PixelFormatTest: TestSuite::Tester {
 
     void isSrgb();
     void isSrgbInvalid();
-    void isSrgbImplementationSpecific();
+    void isSrgbDepthStencilImplementationSpecific();
 
     void isDepthOrStencil();
     void isDepthOrStencilInvalid();
@@ -100,7 +100,7 @@ PixelFormatTest::PixelFormatTest() {
 
               &PixelFormatTest::isSrgb,
               &PixelFormatTest::isSrgbInvalid,
-              &PixelFormatTest::isSrgbImplementationSpecific,
+              &PixelFormatTest::isSrgbDepthStencilImplementationSpecific,
 
               &PixelFormatTest::isDepthOrStencil,
               &PixelFormatTest::isDepthOrStencilInvalid,
@@ -319,8 +319,8 @@ void PixelFormatTest::channelFormatCountDepthStencilImplementationSpecific() {
 
 void PixelFormatTest::isSrgb() {
     CORRADE_VERIFY(isPixelFormatSrgb(PixelFormat::RG8Srgb));
+    CORRADE_VERIFY(!isPixelFormatSrgb(PixelFormat::RG8Snorm));
     CORRADE_VERIFY(!isPixelFormatSrgb(PixelFormat::RGB16F));
-    CORRADE_VERIFY(!isPixelFormatSrgb(PixelFormat::Stencil8UI));
 }
 
 void PixelFormatTest::isSrgbInvalid() {
@@ -335,14 +335,16 @@ void PixelFormatTest::isSrgbInvalid() {
         "isPixelFormatSrgb(): invalid format PixelFormat(0xdead)\n");
 }
 
-void PixelFormatTest::isSrgbImplementationSpecific() {
+void PixelFormatTest::isSrgbDepthStencilImplementationSpecific() {
     CORRADE_SKIP_IF_NO_ASSERT();
 
     std::ostringstream out;
     Error redirectError{&out};
     isPixelFormatSrgb(pixelFormatWrap(0xdead));
+    isPixelFormatSrgb(PixelFormat::Depth16Unorm);
     CORRADE_COMPARE(out.str(),
-        "isPixelFormatSrgb(): can't determine colorspace of an implementation-specific format 0xdead\n");
+        "isPixelFormatSrgb(): can't determine colorspace of an implementation-specific format 0xdead\n"
+        "isPixelFormatSrgb(): can't determine colorspace of PixelFormat::Depth16Unorm\n");
 }
 
 void PixelFormatTest::isDepthOrStencil() {