MethodError: no method matching isless(::Int64, ::QuadExpr) / isless(::VariableRef, ::QuadExpr)

[LebedevRI]

using JuMP, SCIP

model = Model(SCIP.Optimizer)

@variable(model, a)
@variable(model, b)
@variable(model, c)

@variable(model, d)
@variable(model, e)

@constraint(model, (a*b >= c) => {d == e})

print(model)

MethodError: no method matching isless(::VariableRef, ::QuadExpr)

Closest candidates are:
  isless(::Any, ::Missing)
   @ Base missing.jl:88
  isless(::Missing, ::Any)
   @ Base missing.jl:87


Stacktrace:
 [1] <(x::VariableRef, y::QuadExpr)
   @ Base ./operators.jl:343
 [2] <=(x::VariableRef, y::QuadExpr)
   @ Base ./operators.jl:392
 [3] >=(x::QuadExpr, y::VariableRef)
   @ Base ./operators.jl:416
 [4] macro expansion
   @ ~/.julia/packages/JuMP/D44Aq/src/macros.jl:1345 [inlined]
 [5] top-level scope
   @ In[30]:12

using JuMP, SCIP

model = Model(SCIP.Optimizer)

@variable(model, a)
@variable(model, b)

@variable(model, d)
@variable(model, e)

@constraint(model, (a*b >= 42) => {d == e})

print(model)

MethodError: no method matching isless(::Int64, ::QuadExpr)

Closest candidates are:
  isless(::Integer, ::ForwardDiff.Dual{Ty}) where Ty
   @ ForwardDiff ~/.julia/packages/ForwardDiff/PcZ48/src/dual.jl:145
  isless(::Real, ::AbstractFloat)
   @ Base operators.jl:178
  isless(::Real, ::ForwardDiff.Dual{Ty}) where Ty
   @ ForwardDiff ~/.julia/packages/ForwardDiff/PcZ48/src/dual.jl:145
  ...


Stacktrace:
 [1] <(x::Int64, y::QuadExpr)
   @ Base ./operators.jl:343
 [2] <=(x::Int64, y::QuadExpr)
   @ Base ./operators.jl:392
 [3] >=(x::QuadExpr, y::Int64)
   @ Base ./operators.jl:416
 [4] macro expansion
   @ ~/.julia/packages/JuMP/D44Aq/src/macros.jl:1345 [inlined]
 [5] top-level scope
   @ In[33]:11

If i launder a*b >= c through an expression, #3580 happens.

[odow]

You cannot use inequalities in the left-hand side of an indicator constraint. It just be a single binary variable:
https://jump.dev/JuMP.jl/stable/manual/constraints/#Indicator-constraints

We should add a better error message for this.