Better error message for missing unit

Author: MilesCranmer

src/symbolic_dimensions.jl

@@ -42,7 +42,8 @@ end
 static_fieldnames(::Type{<:SymbolicDimensions}) = ALL_SYMBOLS
 function Base.getproperty(d::SymbolicDimensions{R}, s::Symbol) where {R}
     nzdims = getfield(d, :nzdims)
-    i = ALL_MAPPING[s]
+    i = get(ALL_MAPPING, s, INDEX_TYPE(0))
+    iszero(i) && error("$s is not available as a symbol in SymbolicDimensions. Symbols available: $(ALL_SYMBOLS).")
     ii = searchsortedfirst(nzdims, i)
     if ii <= length(nzdims) && nzdims[ii] == i
         return getfield(d, :nzvals)[ii]

test/unittests.jl

@@ -580,6 +580,11 @@ end
 
     # Test conversion
     @test typeof(SymbolicDimensions{Rational{Int}}(dimension(us"km/s"))) == SymbolicDimensions{Rational{Int}}
+
+    # Helpful error if symbol not found:
+    sym5 = dimension(us"km/s")
+    VERSION >= v"1.8" &&
+        @test_throws "rad is not available as a symbol" sym5.rad
 end
 
 @testset "Test ambiguities" begin