Add Aqua.jl as a CI job and fix remaining errors

.github/workflows/aqua.yml

@@ -0,0 +1,23 @@
+name: aqua-lint
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    types: [opened, synchronize, reopened]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: julia-actions/setup-julia@latest
+        with:
+          version: '1'
+      - uses: actions/checkout@v1
+      - name: Aqua
+        shell: julia --color=yes {0}
+        run: |
+          using Pkg
+          Pkg.add(PackageSpec(name="Aqua"))
+          Pkg.develop(PackageSpec(path=pwd()))
+          using MathOptInterface, Aqua
+          Aqua.test_all(MathOptInterface; ambiguities = false)

src/Utilities/cachingoptimizer.jl

@@ -54,7 +54,7 @@ A `CachingOptimizer` may be in one of three possible states
 * `ATTACHED_OPTIMIZER`: The CachingOptimizer has an optimizer, and it is
   synchronized with the cached model.
 
-### modes
+### Modes
 
 A `CachingOptimizer` has two modes of operation (`CachingOptimizerMode`):
 

src/Utilities/functions.jl

@@ -2193,8 +2193,15 @@ function operate(
     return operate!(op, T, copy(f), g)
 end
 
-function Base.:+(args::VectorLike{T}...) where {T}
-    return operate(+, T, args...)
+function Base.:+(arg::VectorLike, args::VectorLike...)
+    T = _eltype(arg, args)
+    if T === nothing
+        error(
+            "Cannot add VectorOfVariables together without a coefficient " *
+            "type. Convert one argument to a VectorAffineFunction first.",
+        )
+    end
+    return operate(+, T, arg, args...)
 end
 
 # Base.:+(α::Vector{T}, f::VectorLike{T}...) is too general as it also covers
@@ -2207,8 +2214,15 @@ function Base.:+(f::VectorLike{T}, α::Vector{T}) where {T}
     return operate(+, T, f, α)
 end
 
-function Base.:-(args::VectorLike{T}...) where {T}
-    return operate(-, T, args...)
+function Base.:-(arg::VectorLike, args::VectorLike...)
+    T = _eltype(arg, args)
+    if T === nothing
+        error(
+            "Cannot subtract VectorOfVariables without a coefficient " *
+            "type. Convert one argument to a VectorAffineFunction first.",
+        )
+    end
+    return operate(-, T, arg, args...)
 end
 
 function Base.:-(f::VectorLike{T}, α::Vector{T}) where {T}