certain un-annotated declarations or incorrect let annotation → compiler thread blocked indefinitely in an MVar operation

(There are probably actually different underlying type inference problems involving let and recursion with a similar outcome, but multiple individual cases are often crammed into one issue, so distinction is hard)

• incorrect record field setting with let https://github.com/elm/compiler/issues/2206
• correct record field setting with let https://github.com/elm/compiler/issues/1890
• record destructuring with let https://github.com/elm/compiler/issues/1748
• (very simple example) with let https://github.com/elm/compiler/issues/1839
• (very simple example) with let https://github.com/elm/compiler/issues/1793
• (very simple example) with let https://github.com/elm/compiler/issues/2301
• argument type incorrect with let https://github.com/elm/compiler/issues/2013
• argument type incorrect with let https://github.com/elm/compiler/issues/1974
• (simple example) argument type incorrectly different full type with let https://github.com/elm/compiler/issues/1975
• (simple examples) argument type incorrectly different type variable with let https://github.com/elm/compiler/issues/2014
• (non-minimal example) incorrectly typed tuple with let https://github.com/elm/compiler/issues/2338
• (simple example) types correct, parametric types with let https://github.com/elm/compiler/issues/2001
• (simple example) let result type incorrect https://github.com/elm/compiler/issues/2100
• (simple example) correct type with let and phantom types https://github.com/elm/compiler/issues/2185
• (simple example) let declaration recursion directly into themselves with too many and too widely typed arguments https://github.com/elm/compiler
  /issues/2184
• (simple example) un-annotated let in un-annotated declaration https://github.com/elm/compiler/issues/2296
• (long example) with let https://github.com/elm/compiler/issues/2146
• (long example) with let and records https://github.com/elm/compiler/issues/2148
• multiple examples https://github.com/elm/compiler/issues/1789
• (short example) mutual recursion with correct type https://github.com/elm/compiler/issues/2119
• (simple example) recursion with incorrect type https://github.com/elm/compiler/issues/2153
• (no simple example) https://github.com/elm/compiler/issues/2035

Start by reproducing one of the linked minimal examples involving let bindings, recursion, or type inference, then compare it with the related issues listed in the report. Done means the compiler no longer remains blocked indefinitely in the MVar operation and the affected cases have a confirmed result.