INJECTIVE_FOR_INFERENCE silently ignored on non-functions

[Andreas Abel]

Pragma INJECTIVE_FOR_INFERENCE is currently silently ignored if applied to a non-function.
We would like to be warned about the UselessInjectiveForInferencePragma.

agda/src/full/Agda/TypeChecking/Rules/Decl.hs

Line 778 in 185d9d4

 A.InjectiveForInferencePragma x -> markFirstOrder x 

Notably, the implementation is framed by cases that make more effort (StaticPragma, NotProjectionLikePragma, InlinePragma):

agda/src/full/Agda/TypeChecking/Rules/Decl.hs

Lines 772 to 791 in 185d9d4

 A.StaticPragma x -> do 
   def <- getConstInfo x 
   case theDef def of 
     Function{} -> markStatic x 
     _          -> typeError $ GenericError "STATIC directive only works on functions" 
 A.InjectivePragma x -> markInjective x 
 A.InjectiveForInferencePragma x -> markFirstOrder x 
 A.NotProjectionLikePragma qn -> do 
   def <- getConstInfo qn 
   case theDef def of 
     it@Function{} -> 
       modifyGlobalDefinition qn $ \def -> def { theDef = it { funProjection = Left NeverProjection } } 
     _ -> typeError $ GenericError "NOT_PROJECTION_LIKE directive only applies to functions" 
 A.InlinePragma b x -> do 
   def <- getConstInfo x 
   case theDef def of 
     Function{} -> markInline b x 
     d@Constructor{ conSrcCon } | copatternMatchingAllowed conSrcCon 
       -> modifyGlobalDefinition x $ set lensTheDef d{ conInline = b } 
     _ -> typeError $ GenericError $ applyUnless b ("NO" ++) "INLINE directive only works on functions or constructors of records that allow copattern matching" 

I am saying "more effort" here because these cases have some deficiencies:

• use of GenericError
• error instead of warning
• unclear what happens for abstract (because getConstInfo might return AbstractDefn); there should be comments showing that the implementor was aware of abstract and the current implementation works as expected for abstract

ATTN: @WhatisRT @UlfNorell