Give better error message when IF Statement has context requirements (#3150)

* Give better error message when IF Statement has context requirements

* simplify code

* Update TypeChecker.fs

* something has a type

* Adding a test

* Transitive IF context

* Another test - fixes #3146

* fix wording

src/fsharp/CompileOps.fs

@@ -628,6 +628,7 @@ let OutputPhasedErrorR (os:StringBuilder) (err:PhasedDiagnostic) =
           let t1, t2, _cxs = NicePrint.minimalStringsOfTwoTypes denv t1 t2
           
           match contextInfo with
+          | ContextInfo.IfExpression range when range = m -> os.Append(FSComp.SR.ifExpression(t1,t2)) |> ignore
           | ContextInfo.OmittedElseBranch range when range = m -> os.Append(FSComp.SR.missingElseBranch(t2)) |> ignore
           | ContextInfo.ElseBranchResult range when range = m -> os.Append(FSComp.SR.elseBranchHasWrongType(t1,t2)) |> ignore
           | _ -> os.Append(ConstraintSolverTypesNotInEqualityRelation1E().Format t1 t2 )  |> ignore
@@ -639,6 +640,7 @@ let OutputPhasedErrorR (os:StringBuilder) (err:PhasedDiagnostic) =
           let t1, t2, _cxs = NicePrint.minimalStringsOfTwoTypes denv t1 t2
           
           match contextInfo with
+          | ContextInfo.IfExpression range when range = m -> os.Append(FSComp.SR.ifExpression(t1,t2)) |> ignore
           | ContextInfo.OmittedElseBranch range when range = m -> os.Append(FSComp.SR.missingElseBranch(t2)) |> ignore
           | ContextInfo.ElseBranchResult range when range = m -> os.Append(FSComp.SR.elseBranchHasWrongType(t1,t2)) |> ignore
           | _ -> os.Append(ConstraintSolverTypesNotInEqualityRelation2E().Format t1 t2) |> ignore
@@ -664,6 +666,7 @@ let OutputPhasedErrorR (os:StringBuilder) (err:PhasedDiagnostic) =
          &&   typeEquiv g t2 t2' ->
           let t1,t2,tpcs = NicePrint.minimalStringsOfTwoTypes denv t1 t2
           match contextInfo with
+          | ContextInfo.IfExpression range when range = m -> os.Append(FSComp.SR.ifExpression(t1,t2)) |> ignore
           | ContextInfo.OmittedElseBranch range when range = m -> os.Append(FSComp.SR.missingElseBranch(t2)) |> ignore
           | ContextInfo.ElseBranchResult range when range = m -> os.Append(FSComp.SR.elseBranchHasWrongType(t1,t2)) |> ignore
           | ContextInfo.TupleInRecordFields ->

src/fsharp/ConstraintSolver.fs

@@ -117,6 +117,8 @@ let FreshenMethInfo m (minfo:MethInfo) =
 type ContextInfo =
 /// No context was given.
 | NoContext
+/// The type equation comes from an IF expression.
+| IfExpression of range
 /// The type equation comes from an omitted else branch.
 | OmittedElseBranch of range
 /// The type equation comes from a type check of the result of an else branch.

src/fsharp/ConstraintSolver.fsi

@@ -52,6 +52,8 @@ val FreshenMethInfo : range -> MethInfo -> TType list
 type ContextInfo =
 /// No context was given.
 | NoContext
+/// The type equation comes from an IF expression.
+| IfExpression of range
 /// The type equation comes from an omitted else branch.
 | OmittedElseBranch of range
 /// The type equation comes from a type check of the result of an else branch.

src/fsharp/FSComp.txt

@@ -17,6 +17,7 @@ undefinedNameTypeParameter,"The type parameter %s is not defined."
 undefinedNamePatternDiscriminator,"The pattern discriminator '%s' is not defined."
 replaceWithSuggestion,"Replace with '%s'"
 missingElseBranch,"The 'if' expression is missing an 'else' branch. The 'then' branch has type '%s'. Because 'if' is an expression, and not a statement, add an 'else' branch which returns a value of the same type."
+ifExpression,"The 'if' expression needs to have type '%s' to satisfy context type requirements. It currently has type '%s'."
 elseBranchHasWrongType,"All branches of an 'if' expression must return the same type. This expression was expected to have type '%s' but here has type '%s'."
 commaInsteadOfSemicolonInRecord,"A ';' is used to separate field values in records. Consider replacing ',' with ';'."
 derefInsteadOfNot,"The '!' operator is used to dereference a ref cell. Consider using 'not expr' here."

src/fsharp/TypeChecker.fs

@@ -5872,12 +5872,19 @@ and TcExprUndelayed cenv overallTy env tpenv (expr: SynExpr) =
     | SynExpr.IfThenElse (e1,e2,e3opt,spIfToThen,isRecovery,mIfToThen,m) ->
         let e1',tpenv = TcExprThatCantBeCtorBody cenv cenv.g.bool_ty env tpenv e1
         let e2',tpenv =
+            let env =
+                match env.eContextInfo with
+                | ContextInfo.ElseBranchResult _ ->  { env with eContextInfo = ContextInfo.ElseBranchResult e2.Range }
+                | _ -> 
+                    match e3opt with
+                    | None -> { env with eContextInfo = ContextInfo.OmittedElseBranch e2.Range }
+                    | _ -> { env with eContextInfo = ContextInfo.IfExpression e2.Range }
+
             if not isRecovery && Option.isNone e3opt then
-                let env = { env with eContextInfo = ContextInfo.OmittedElseBranch e2.Range}
                 UnifyTypes cenv env m cenv.g.unit_ty overallTy
-                TcExprThatCanBeCtorBody cenv overallTy env tpenv e2
-            else
-                TcExprThatCanBeCtorBody cenv overallTy env tpenv e2
+
+            TcExprThatCanBeCtorBody cenv overallTy env tpenv e2
+
         let e3',sp2,tpenv = 
             match e3opt with 
             | None ->
@@ -5886,6 +5893,7 @@ and TcExprUndelayed cenv overallTy env tpenv (expr: SynExpr) =
                 let env = { env with eContextInfo = ContextInfo.ElseBranchResult e3.Range }
                 let e3',tpenv = TcExprThatCanBeCtorBody cenv overallTy env tpenv e3 
                 e3',SequencePointAtTarget,tpenv
+
         primMkCond spIfToThen SequencePointAtTarget sp2 m overallTy e1' e2' e3', tpenv
 
     // This is for internal use in the libraries only 

tests/fsharpqa/Source/Warnings/ElseBranchHasWrongContextType.fs

+// #Warnings
+//<Expects status="Error" id="FS0001">The 'if' expression needs to have type 'bool'</Expects>
+
+let x = 1
+let y : bool = 
+    if x = 2 then "A"
+    else "B"
+    
+exit 0
\ No newline at end of file

tests/fsharpqa/Source/Warnings/NestedElseBranchHasWrongType.fs

+// #Warnings
+//<Expects status="Error" id="FS0001">All branches of an 'if' expression must return the same type.</Expects>
+
+let x = 1
+if x = 1 then true
+else
+    if x = 2 then "A"
+    else "B"
+    
+exit 0
\ No newline at end of file

tests/fsharpqa/Source/Warnings/env.lst

@@ -45,6 +45,8 @@
 	SOURCE=SuggestDoubleBacktickUnions.fs SCFLAGS="--vserrors" # SuggestDoubleBacktickUnions.fs
 	SOURCE=ElseBranchHasWrongType.fs # ElseBranchHasWrongType.fs
 	SOURCE=ElseBranchHasWrongType2.fs # ElseBranchHasWrongType2.fs
+	SOURCE=NestedElseBranchHasWrongType.fs # NestedElseBranchHasWrongType.fs
+	SOURCE=ElseBranchHasWrongContextType.fs # ElseBranchHasWrongContextType.fs
 	SOURCE=ElseBranchContextDoesntPropagateInAppl.fs # ElseBranchContextDoesntPropagateInAppl.fs
 	SOURCE=ElseBranchContextDoesntPropagateInAppl2.fs # ElseBranchContextDoesntPropagateInAppl2.fs
 	SOURCE=ElseBranchContextDoesntPropagateInForLoop.fs # ElseBranchContextDoesntPropagateInForLoop.fs