More partial application warnings

Changes

@@ -43,6 +43,8 @@ Working version
 
 ### Compiler user-interface and warnings:
 
+- #????: Report partial application warnings on type errors in applications.
+  (Stephen Dolan, report and testcase by whitequark, review by ??)
 
 ### Internal/compiler-libs changes:
 

testsuite/tests/typing-warnings/application.ml

@@ -84,3 +84,23 @@ Line 1, characters 19-20:
 Warning 20: this argument will not be used by the function.
 Exception: Stdlib.Exit.
 |}]
+
+let f a b = a + b;;
+[%%expect {|
+val f : int -> int -> int = <fun>
+|}]
+let g x = x + 1
+let _ = g (f 1);;
+[%%expect {|
+val g : int -> int = <fun>
+Line 2, characters 10-15:
+2 | let _ = g (f 1);;
+              ^^^^^
+Warning 5: this function application is partial,
+maybe some arguments are missing.
+Line 2, characters 10-15:
+2 | let _ = g (f 1);;
+              ^^^^^
+Error: This expression has type int -> int
+       but an expression was expected of type int
+|}]

typing/typecore.ml

@@ -2709,12 +2709,18 @@ and type_expect_
       let (args, ty_res) = type_application env funct sargs in
       end_def ();
       unify_var env (newvar()) funct.exp_type;
-      rue {
-        exp_desc = Texp_apply(funct, args);
-        exp_loc = loc; exp_extra = [];
-        exp_type = ty_res;
-        exp_attributes = sexp.pexp_attributes;
-        exp_env = env }
+      let exp =
+        { exp_desc = Texp_apply(funct, args);
+          exp_loc = loc; exp_extra = [];
+          exp_type = ty_res;
+          exp_attributes = sexp.pexp_attributes;
+          exp_env = env } in
+      begin
+        try rue exp
+        with Error (_, _, Expr_type_clash _) as err ->
+          check_partial_application false exp;
+          raise err
+      end
   | Pexp_match(sarg, caselist) ->
       begin_def ();
       let arg = type_exp env sarg in