C# Language Specification

The first phase

For each of the method arguments E_i:

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  If E_i is an anonymous function, an explicit parameter type inference (§7.5.2.7) is made from E_i to T_i
  
• 
  Otherwise, if E_i has a type U and x_i is a value parameter then a lower-bound inference is made from U to T_i.
  
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  Otherwise, if E_i has a type U and x_i is a ref or out parameter then an exact inference is made from U to T_i.
  
• 
  Otherwise, no inference is made for this argument.
  

2. The second phase

The second phase proceeds as follows:

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  All unfixed type variables X_i which do not depend on (§7.5.2.5) any X_j are fixed (§7.5.2.10).
  
• 
  If no such type variables exist, all unfixed type variables X_i are fixed for which all of the following hold:
  
  • 
    There is at least one type variable X_j that depends on X_i
    
  • 
    X_i has a non-empty set of bounds
    
• 
  If no such type variables exist and there are still unfixed type variables, type inference fails.
  
• 
  Otherwise, if no further unfixed type variables exist, type inference succeeds.
  
• 
  Otherwise, for all arguments E_i with corresponding parameter type T_i where the output types (§7.5.2.4) contain unfixed type variables X_j but the input types (§7.5.2.3) do not, an output type inference (§7.5.2.6) is made from E_i to T_i. Then the second phase is repeated.
  

3.