Parser: recover on unfinished cons patterns

docs/release-notes/.FSharp.Compiler.Service/8.0.400.md

@@ -19,6 +19,7 @@
 ### Added
 
 * Generate new `Equals` overload to avoid boxing for structural comparison ([PR #16857](https://github.com/dotnet/fsharp/pull/16857))
+* Parser: better recovery for unfinished patterns ([PR #17231](https://github.com/dotnet/fsharp/pull/17231))
 
 ### Changed
 * Enforce `AttributeTargets.Interface` ([PR #17173](https://github.com/dotnet/fsharp/pull/17173))

src/Compiler/pars.fsy

@@ -3454,6 +3454,17 @@ headBindingPattern:
       { let mColonColon = rhs parseState 2
         SynPat.ListCons($1, $3, rhs2 parseState 1 3, { ColonColonRange = mColonColon }) }
 
+  | headBindingPattern COLON_COLON recover
+      { let mColonColon = rhs parseState 2
+        let pat2 = SynPat.Wild(mColonColon.EndRange)
+        SynPat.ListCons($1, pat2, rhs2 parseState 1 2, { ColonColonRange = mColonColon }) }
+
+  | headBindingPattern COLON_COLON
+      { let mColonColon = rhs parseState 2
+        reportParseErrorAt mColonColon (FSComp.SR.parsExpectingPattern ())
+        let pat2 = SynPat.Wild(mColonColon.EndRange)
+        SynPat.ListCons($1, pat2, rhs2 parseState 1 2, { ColonColonRange = mColonColon }) }
+
   | tuplePatternElements %prec pat_tuple
       { let pats, commas = $1
         let pats, commas = normalizeTuplePat pats commas
@@ -3760,6 +3771,17 @@ parenPattern:
       { let mColonColon = rhs parseState 2
         SynPat.ListCons($1, $3, rhs2 parseState 1 3, { ColonColonRange = mColonColon }) }
 
+  | parenPattern COLON_COLON recover
+      { let mColonColon = rhs parseState 2
+        let pat2 = SynPat.Wild(mColonColon.EndRange)
+        SynPat.ListCons($1, pat2, rhs2 parseState 1 2, { ColonColonRange = mColonColon }) }
+
+  | parenPattern COLON_COLON
+      { let mColonColon = rhs parseState 2
+        reportParseErrorAt mColonColon (FSComp.SR.parsExpectingPattern ())
+        let pat2 = SynPat.Wild(mColonColon.EndRange)
+        SynPat.ListCons($1, pat2, rhs2 parseState 1 2, { ColonColonRange = mColonColon }) }
+
   | constrPattern { $1 }
 
 tupleParenPatternElements:

tests/service/PatternMatchCompilationTests.fs

@@ -756,7 +756,7 @@ let z as =
         "(10,7--10,9): Unexpected keyword 'as' in binding";
         "(10,5--10,6): Expecting pattern";
         "(11,10--11,12): Unexpected keyword 'as' in binding. Expected '=' or other token.";
-        "(12,9--12,11): Unexpected keyword 'as' in binding";
+        "(12,6--12,8): Expecting pattern";
         "(13,8--13,10): Unexpected keyword 'as' in binding";
         "(14,8--14,10): Unexpected keyword 'as' in binding";
         "(15,8--15,10): Unexpected keyword 'as' in pattern. Expected ')' or other token.";
@@ -776,6 +776,8 @@ let z as =
         "(8,29--8,30): This expression was expected to have type\u001d    'unit'    \u001dbut here has type\u001d    'int'";
         "(9,26--9,27): This expression was expected to have type\u001d    'unit'    \u001dbut here has type\u001d    'int'";
         "(10,14--10,15): This expression was expected to have type\u001d    ''a * 'b'    \u001dbut here has type\u001d    'int'";
+        "(12,16--12,18): This expression was expected to have type\u001d    ''a list'    \u001dbut here has type\u001d    'int'";
+        "(12,4--12,13): Incomplete pattern matches on this expression. For example, the value '[]' may indicate a case not covered by the pattern(s).";
         "(15,4--15,5): The pattern discriminator 'r' is not defined.";
         "(15,4--15,12): Incomplete pattern matches on this expression."
     ]
@@ -1176,7 +1178,7 @@ let as :? z =
         "(10,7--10,9): Unexpected keyword 'as' in binding";
         "(10,5--10,6): Expecting pattern";
         "(11,10--11,12): Unexpected keyword 'as' in binding. Expected '=' or other token.";
-        "(12,9--12,11): Unexpected keyword 'as' in binding";
+        "(12,6--12,8): Expecting pattern";
         "(13,8--13,10): Unexpected keyword 'as' in binding";
         "(14,8--14,10): Unexpected keyword 'as' in binding";
         "(15,13--15,15): Unexpected keyword 'as' in pattern. Expected '(' or other token.";
@@ -1204,6 +1206,8 @@ let as :? z =
         "(9,22--9,26): The type 'unit' does not have any proper subtypes and cannot be used as the source of a type test or runtime coercion.";
         "(10,13--10,14): The type 'i' is not defined.";
         "(10,10--10,14): The type ''a * 'b' does not have any proper subtypes and cannot be used as the source of a type test or runtime coercion.";
+        "(12,15--12,16): The type 'm' is not defined.";
+        "(12,12--12,16): The type ''a list' does not have any proper subtypes and cannot be used as the source of a type test or runtime coercion.";
         "(16,4--16,5): The pattern discriminator 't' is not defined.";
         "(16,14--16,15): The type 'u' is not defined.";
         "(16,11--16,15): This runtime coercion or type test from type\u001d    'a    \u001d to \u001d    'b    \u001dinvolves an indeterminate type based on information prior to this program point. Runtime type tests are not allowed on some types. Further type annotations are needed."

tests/service/data/SyntaxTree/Pattern/Cons 01.fs

@@ -0,0 +1,6 @@
+module Module
+
+match () with
+| _ :: _ -> ()
+
+()

tests/service/data/SyntaxTree/Pattern/Cons 01.fs.bsl

@@ -0,0 +1,22 @@
+ImplFile
+  (ParsedImplFileInput
+     ("/root/Pattern/Cons 01.fs", false, QualifiedNameOfFile Module, [], [],
+      [SynModuleOrNamespace
+         ([Module], false, NamedModule,
+          [Expr
+             (Match
+                (Yes (3,0--3,13), Const (Unit, (3,6--3,8)),
+                 [SynMatchClause
+                    (ListCons
+                       (Wild (4,2--4,3), Wild (4,7--4,8), (4,2--4,8),
+                        { ColonColonRange = (4,4--4,6) }), None,
+                     Const (Unit, (4,12--4,14)), (4,2--4,14), Yes,
+                     { ArrowRange = Some (4,9--4,11)
+                       BarRange = Some (4,0--4,1) })], (3,0--4,14),
+                 { MatchKeyword = (3,0--3,5)
+                   WithKeyword = (3,9--3,13) }), (3,0--4,14));
+           Expr (Const (Unit, (6,0--6,2)), (6,0--6,2))],
+          PreXmlDoc ((1,0), FSharp.Compiler.Xml.XmlDocCollector), [], None,
+          (1,0--6,2), { LeadingKeyword = Module (1,0--1,6) })], (true, true),
+      { ConditionalDirectives = []
+        CodeComments = [] }, set []))

tests/service/data/SyntaxTree/Pattern/Cons 02.fs

@@ -0,0 +1,6 @@
+module Module
+
+match () with
+| _ :: -> ()
+
+()

tests/service/data/SyntaxTree/Pattern/Cons 02.fs.bsl

@@ -0,0 +1,24 @@
+ImplFile
+  (ParsedImplFileInput
+     ("/root/Pattern/Cons 02.fs", false, QualifiedNameOfFile Module, [], [],
+      [SynModuleOrNamespace
+         ([Module], false, NamedModule,
+          [Expr
+             (Match
+                (Yes (3,0--3,13), Const (Unit, (3,6--3,8)),
+                 [SynMatchClause
+                    (ListCons
+                       (Wild (4,2--4,3), Wild (4,6--4,6), (4,2--4,6),
+                        { ColonColonRange = (4,4--4,6) }), None,
+                     Const (Unit, (4,10--4,12)), (4,2--4,12), Yes,
+                     { ArrowRange = Some (4,7--4,9)
+                       BarRange = Some (4,0--4,1) })], (3,0--4,12),
+                 { MatchKeyword = (3,0--3,5)
+                   WithKeyword = (3,9--3,13) }), (3,0--4,12));
+           Expr (Const (Unit, (6,0--6,2)), (6,0--6,2))],
+          PreXmlDoc ((1,0), FSharp.Compiler.Xml.XmlDocCollector), [], None,
+          (1,0--6,2), { LeadingKeyword = Module (1,0--1,6) })], (true, true),
+      { ConditionalDirectives = []
+        CodeComments = [] }, set []))
+
+(4,4)-(4,6) parse error Expecting pattern

tests/service/data/SyntaxTree/Pattern/Cons 03.fs

@@ -0,0 +1,6 @@
+module Module
+
+match () with
+| _ ::
+
+()

tests/service/data/SyntaxTree/Pattern/Cons 03.fs.bsl

@@ -0,0 +1,25 @@
+ImplFile
+  (ParsedImplFileInput
+     ("/root/Pattern/Cons 03.fs", false, QualifiedNameOfFile Module, [], [],
+      [SynModuleOrNamespace
+         ([Module], false, NamedModule,
+          [Expr
+             (Match
+                (Yes (3,0--3,13), Const (Unit, (3,6--3,8)),
+                 [SynMatchClause
+                    (ListCons
+                       (Wild (4,2--4,3), Wild (4,6--4,6), (4,2--4,6),
+                        { ColonColonRange = (4,4--4,6) }), None,
+                     ArbitraryAfterError ("patternClauses2", (4,6--4,6)),
+                     (4,2--4,6), Yes, { ArrowRange = None
+                                        BarRange = Some (4,0--4,1) })],
+                 (3,0--4,6), { MatchKeyword = (3,0--3,5)
+                               WithKeyword = (3,9--3,13) }), (3,0--4,6));
+           Expr (Const (Unit, (6,0--6,2)), (6,0--6,2))],
+          PreXmlDoc ((1,0), FSharp.Compiler.Xml.XmlDocCollector), [], None,
+          (1,0--6,2), { LeadingKeyword = Module (1,0--1,6) })], (true, true),
+      { ConditionalDirectives = []
+        CodeComments = [] }, set []))
+
+(4,4)-(4,6) parse error Expecting pattern
+(6,0)-(6,1) parse error Incomplete structured construct at or before this point in pattern matching. Expected '->' or other token.

tests/service/data/SyntaxTree/Pattern/Cons 04.fs

@@ -0,0 +1,7 @@
+module Module
+
+match () with
+| _ ::
+| _ -> ()
+
+()

tests/service/data/SyntaxTree/Pattern/Cons 04.fs.bsl

@@ -0,0 +1,26 @@
+ImplFile
+  (ParsedImplFileInput
+     ("/root/Pattern/Cons 04.fs", false, QualifiedNameOfFile Module, [], [],
+      [SynModuleOrNamespace
+         ([Module], false, NamedModule,
+          [Expr
+             (Match
+                (Yes (3,0--3,13), Const (Unit, (3,6--3,8)),
+                 [SynMatchClause
+                    (Or
+                       (ListCons
+                          (Wild (4,2--4,3), Wild (4,6--4,6), (4,2--4,6),
+                           { ColonColonRange = (4,4--4,6) }), Wild (5,2--5,3),
+                        (4,2--5,3), { BarRange = (5,0--5,1) }), None,
+                     Const (Unit, (5,7--5,9)), (4,2--5,9), Yes,
+                     { ArrowRange = Some (5,4--5,6)
+                       BarRange = Some (4,0--4,1) })], (3,0--5,9),
+                 { MatchKeyword = (3,0--3,5)
+                   WithKeyword = (3,9--3,13) }), (3,0--5,9));
+           Expr (Const (Unit, (7,0--7,2)), (7,0--7,2))],
+          PreXmlDoc ((1,0), FSharp.Compiler.Xml.XmlDocCollector), [], None,
+          (1,0--7,2), { LeadingKeyword = Module (1,0--1,6) })], (true, true),
+      { ConditionalDirectives = []
+        CodeComments = [] }, set []))
+
+(4,4)-(4,6) parse error Expecting pattern