From fd1d77767e234586dce5188ce7fff79c53e47ecd Mon Sep 17 00:00:00 2001
From: whitequark <whitequark@whitequark.org>
Date: Mon, 21 Nov 2016 13:08:03 +0000
Subject: [PATCH] inferencer: significantly improve the op-assignment
 diagnostic.

Before this commit, it displayed incorrect output if an error
appeared on 2nd run and beyond, and displayed messages for trying
to do "num32 -= num64" that made very little sense.
---
 artiq/compiler/transforms/inferencer.py   | 41 +++++++++++++----------
 artiq/test/lit/inferencer/error_coerce.py |  4 +--
 2 files changed, 25 insertions(+), 20 deletions(-)

diff --git a/artiq/compiler/transforms/inferencer.py b/artiq/compiler/transforms/inferencer.py
index 920343b44..3a1dbe32f 100644
--- a/artiq/compiler/transforms/inferencer.py
+++ b/artiq/compiler/transforms/inferencer.py
@@ -1049,22 +1049,10 @@ class Inferencer(algorithm.Visitor):
         if coerced:
             return_type, target_type, value_type = coerced
 
-            try:
-                node.target.type.unify(target_type)
-            except types.UnificationError as e:
-                printer = types.TypePrinter()
-                note = diagnostic.Diagnostic("note",
-                    "expression of type {typec}",
-                    {"typec": printer.name(node.value.type)},
-                    node.value.loc)
-                diag = diagnostic.Diagnostic("error",
-                    "expression of type {typea} has to be coerced to {typeb}, "
-                    "which makes assignment invalid",
-                    {"typea": printer.name(node.target.type),
-                     "typeb": printer.name(target_type)},
-                    node.op.loc, [node.target.loc], [note])
-                self.engine.process(diag)
-                return
+            if isinstance(node.value, asttyped.CoerceT):
+                orig_value_type = node.value.value.type
+            else:
+                orig_value_type = node.value.type
 
             try:
                 node.target.type.unify(return_type)
@@ -1072,17 +1060,34 @@ class Inferencer(algorithm.Visitor):
                 printer = types.TypePrinter()
                 note = diagnostic.Diagnostic("note",
                     "expression of type {typec}",
-                    {"typec": printer.name(node.value.type)},
+                    {"typec": printer.name(orig_value_type)},
                     node.value.loc)
                 diag = diagnostic.Diagnostic("error",
                     "the result of this operation has type {typeb}, "
-                    "which makes assignment to a slot of type {typea} invalid",
+                    "which cannot be assigned to a left-hand side of type {typea}",
                     {"typea": printer.name(node.target.type),
                      "typeb": printer.name(return_type)},
                     node.op.loc, [node.target.loc], [note])
                 self.engine.process(diag)
                 return
 
+            try:
+                node.target.type.unify(target_type)
+            except types.UnificationError as e:
+                printer = types.TypePrinter()
+                note = diagnostic.Diagnostic("note",
+                    "expression of type {typec}",
+                    {"typec": printer.name(orig_value_type)},
+                    node.value.loc)
+                diag = diagnostic.Diagnostic("error",
+                    "this operation requires the left-hand side of type {typea} "
+                    "to be coerced to {typeb}, which cannot be done",
+                    {"typea": printer.name(node.target.type),
+                     "typeb": printer.name(target_type)},
+                    node.op.loc, [node.target.loc], [note])
+                self.engine.process(diag)
+                return
+
             node.value = self._coerce_one(value_type, node.value, other_node=node.target)
 
     def visit_ForT(self, node):
diff --git a/artiq/test/lit/inferencer/error_coerce.py b/artiq/test/lit/inferencer/error_coerce.py
index 8afb83e6b..c4f732e7e 100644
--- a/artiq/test/lit/inferencer/error_coerce.py
+++ b/artiq/test/lit/inferencer/error_coerce.py
@@ -28,10 +28,10 @@
 # CHECK-L: ${LINE:+1}: error: cannot coerce list(elt='a) to a numeric type
 [] - 1.0
 
-# CHECK-L: ${LINE:+2}: error: expression of type numpy.int? has to be coerced to float, which makes assignment invalid
+# CHECK-L: ${LINE:+2}: error: the result of this operation has type float, which cannot be assigned to a left-hand side of type numpy.int?
 # CHECK-L: ${LINE:+1}: note: expression of type float
 a = 1; a += 1.0
 
-# CHECK-L: ${LINE:+2}: error: the result of this operation has type (numpy.int?, float), which makes assignment to a slot of type (numpy.int?,) invalid
+# CHECK-L: ${LINE:+2}: error: the result of this operation has type (numpy.int?, float), which cannot be assigned to a left-hand side of type (numpy.int?,)
 # CHECK-L: ${LINE:+1}: note: expression of type (float,)
 b = (1,); b += (1.0,)