From 8ecd6c4d8c021f7778b4959bdf75204dfd2d1946 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Tue, 26 Nov 2019 19:50:52 -0500
Subject: [PATCH] add compiler note for bad int coercion

closes #3724
---
 src/ir.cpp              | 27 +++++++++++++++++++++++++++
 test/compile_errors.zig |  1 +
 2 files changed, 28 insertions(+)

diff --git a/src/ir.cpp b/src/ir.cpp
index db135092e..18d7698c7 100644
--- a/src/ir.cpp
+++ b/src/ir.cpp
@@ -71,6 +71,7 @@ enum ConstCastResultId {
     ConstCastResultIdPtrLens,
     ConstCastResultIdCV,
     ConstCastResultIdPtrSentinel,
+    ConstCastResultIdIntShorten,
 };
 
 struct ConstCastOnly;
@@ -97,6 +98,7 @@ struct ConstCastBadAllowsZero;
 struct ConstCastBadNullTermArrays;
 struct ConstCastBadCV;
 struct ConstCastPtrSentinel;
+struct ConstCastIntShorten;
 
 struct ConstCastOnly {
     ConstCastResultId id;
@@ -117,6 +119,7 @@ struct ConstCastOnly {
         ConstCastBadNullTermArrays *sentinel_arrays;
         ConstCastBadCV *bad_cv;
         ConstCastPtrSentinel *bad_ptr_sentinel;
+        ConstCastIntShorten *int_shorten;
     } data;
 };
 
@@ -186,6 +189,11 @@ struct ConstCastPtrSentinel {
     ZigType *actual_type;
 };
 
+struct ConstCastIntShorten {
+    ZigType *wanted_type;
+    ZigType *actual_type;
+};
+
 static IrInstruction *ir_gen_node(IrBuilder *irb, AstNode *node, Scope *scope);
 static IrInstruction *ir_gen_node_extra(IrBuilder *irb, AstNode *node, Scope *scope, LVal lval,
         ResultLoc *result_loc);
@@ -10213,6 +10221,14 @@ static ConstCastOnly types_match_const_cast_only(IrAnalyze *ira, ZigType *wanted
         return result;
     }
 
+    if (wanted_type->id == ZigTypeIdInt && actual_type->id == ZigTypeIdInt) {
+        result.id = ConstCastResultIdIntShorten;
+        result.data.int_shorten = allocate_nonzero<ConstCastIntShorten>(1);
+        result.data.int_shorten->wanted_type = wanted_type;
+        result.data.int_shorten->actual_type = actual_type;
+        return result;
+    }
+
     result.id = ConstCastResultIdType;
     result.data.type_mismatch = allocate_nonzero<ConstCastTypeMismatch>(1);
     result.data.type_mismatch->wanted_type = wanted_type;
@@ -12733,6 +12749,17 @@ static void report_recursive_error(IrAnalyze *ira, AstNode *source_node, ConstCa
             add_error_note(ira->codegen, parent_msg, source_node,
                     buf_sprintf("calling convention mismatch"));
             break;
+        case ConstCastResultIdIntShorten: {
+            ZigType *wanted_type = cast_result->data.int_shorten->wanted_type;
+            ZigType *actual_type = cast_result->data.int_shorten->actual_type;
+            const char *wanted_signed = wanted_type->data.integral.is_signed ? "signed" : "unsigned";
+            const char *actual_signed = wanted_type->data.integral.is_signed ? "signed" : "unsigned";
+            add_error_note(ira->codegen, parent_msg, source_node,
+                buf_sprintf("%s %" PRIu32 "-bit int cannot represent all possible %s %" PRIu32 "-bit values",
+                    wanted_signed, wanted_type->data.integral.bit_count,
+                    actual_signed, actual_type->data.integral.bit_count));
+            break;
+        }
         case ConstCastResultIdFnAlign: // TODO
         case ConstCastResultIdFnVarArgs: // TODO
         case ConstCastResultIdFnReturnType: // TODO
diff --git a/test/compile_errors.zig b/test/compile_errors.zig
index c1e2d579a..f2600d2d7 100644
--- a/test/compile_errors.zig
+++ b/test/compile_errors.zig
@@ -1648,6 +1648,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         "tmp.zig:3:31: error: integer value 300 cannot be coerced to type 'u8'",
         "tmp.zig:7:22: error: integer value 300 cannot be coerced to type 'u8'",
         "tmp.zig:11:20: error: expected type 'u8', found 'u16'",
+        "tmp.zig:11:20: note: unsigned 8-bit int cannot represent all possible unsigned 16-bit values",
     );
 
     cases.add(