Math: enforce that the interpolation phase in select() is unitless.

Math::lerp() enforces that already (the code won't even compile in that
case), here it accidentally worked, leading to confusion.

doc/changelog.dox

@@ -1644,7 +1644,11 @@ See also:
     pointer, which was deemed a more useful property.
 -   @ref Math::sign() now always returns a unitless type instead of the
     input type itself, so it's possible to e.g. extract a sign of an angle
-    value and multiply other angles with it.
+    value and multiply other angles with it. In a similar spirit,
+    @ref Math::select() now enforces the interpolation phase to be a unitless
+    type. That was the case with @ref Math::lerp() already, where using angles
+    for `t` didn't even compile, this makes the behavior consistently strict
+    for both.
 -   @ref MeshTools::primitiveCount() now requires the element count to follow
     rules defined by a particular primitive to be consistent with requirements
     of @ref MeshTools::generateIndices() and related APIs. Before it was just

src/Magnum/Math/Functions.h

@@ -598,6 +598,7 @@ A constant interpolation counterpart to @ref lerp(): @f[
 Equivalent to calling @cpp Math::lerp(a, b, t >= U(1)) @ce.
 */
 template<class T, class U> constexpr T select(const T& a, const T& b, U t) {
+    static_assert(IsUnitless<U>::value, "expecting an unitless type for the interpolation phase");
     return lerp(a, b, t >= U(1));
 }
 

src/Magnum/Math/Test/FunctionsTest.cpp

@@ -428,16 +428,13 @@ void FunctionsTest::select() {
     CORRADE_COMPARE(Math::select(a, b, Vector3(0.25f, 1.5f, 1.0f)), Vector3(-1.0f, -2.0f, 11.0f));
 
     /* Wrapped types */
-    CORRADE_COMPARE(Math::select(2.0_degf, 5.0_degf, 0.5_degf), 2.0_degf);
+    CORRADE_COMPARE(Math::select(2.0_degf, 5.0_degf, 0.5f), 2.0_degf);
 }
 
 void FunctionsTest::selectBool() {
     CORRADE_COMPARE(Math::select(true, false, 0.5f), true);
     CORRADE_COMPARE(Math::select(Math::BitVector<4>{0xa}, Math::BitVector<4>{0x5}, 1.1f), Math::BitVector<4>{0x5});
     CORRADE_COMPARE(Math::select(Math::BitVector<4>{0xa}, Math::BitVector<4>{0x5}, Vector4{1.1f, -1.0f, 1.3f, 0.5f}), Math::BitVector<4>{0xf});
-
-    /* Wrapped types */
-    CORRADE_COMPARE(Math::select(true, false, 0.5_degf), true);
 }
 
 void FunctionsTest::fma() {