Forbid !<integer> preceding or succeeding '='.

source/text.cpp

@@ -601,6 +601,11 @@ spv_result_t encodeInstructionStartingWithImmediate(
       return error;
     }
 
+    if (operandValue == "=") {
+      DIAGNOSTIC << firstWord << " not allowed before =.";
+      return SPV_ERROR_INVALID_TEXT;
+    }
+
     // Needed to pass to spvTextEncodeOpcode(), but it shouldn't ever be
     // expanded.
     spv_operand_pattern_t dummyExpectedOperands;

syntax.md

@@ -138,7 +138,7 @@ encountered, it begins a new instruction and parsing returns to normal.  (If a
 subsequent OpCode is never found, then this alternate parsing mode handles all
 the remaining words in the program.  If a subsequent OpCode is in an
 [assignment form](#assignment-form), the ID preceding it begins a new
-instruction, even if that ID is itself a `!<integer>`.)
+instruction.)
 
 The assembler processes the words encountered in alternate parsing mode as
 follows:
@@ -169,6 +169,10 @@ Note that this has some interesting consequences, including:
   by the alternate parsing mode.  They must be replaced by `!<integer>` for
   successful assembly.
 
+* The `<result-id>` on the left-hand side of an assignment cannot be
+  a`!<integer>`.  But it can be a number prefixed by `%`, which still gives the
+  user control over its value.
+
 * The `=` sign cannot be processed by the alternate parsing mode if the OpCode
   following it is a `!<integer>`.
 

test/ImmediateInt.cpp

@@ -78,19 +78,27 @@ TEST_F(ImmediateIntTest, AnyWordInSimpleStatement) {
   // EXPECT_EQ(original, CompileSuccessfully("!0x0004002B !1 !2 !123", kCAF));
 }
 
-TEST_F(ImmediateIntTest, AnyWordInAssignmentStatement) {
+TEST_F(ImmediateIntTest, AnyWordAfterEqualsAndOpCode) {
   const SpirvVector original =
       CompileSuccessfully("%2 = OpArrayLength %12 %1 123");
-  // TODO(deki): uncomment assertions below and make them pass.
-  // EXPECT_EQ(original, CompileSuccessfully("!2 = OpArrayLength %12 %1 123"));
-  // EXPECT_EQ(original, CompileSuccessfully("%2 = !0x00040044 %12 %1 123"));
   EXPECT_EQ(original, CompileSuccessfully("%2 = OpArrayLength !12 %1 123"));
   EXPECT_EQ(original, CompileSuccessfully("%2 = OpArrayLength %12 !1 123"));
   EXPECT_EQ(original, CompileSuccessfully("%2 = OpArrayLength %12 %1 !123"));
-  // Instead of checking all possible multiple-! combinations, only probe a few.
   EXPECT_EQ(original, CompileSuccessfully("%2 = OpArrayLength %12 !1 !123"));
-  // EXPECT_EQ(original, CompileSuccessfully("%2 = !0x00040044 !12 !1 !123"));
-  // EXPECT_EQ(original, CompileSuccessfully("!2 = !0x00040044 %12 %1 123"));
+  EXPECT_EQ(original, CompileSuccessfully("%2 = OpArrayLength !12 !1 123"));
+  EXPECT_EQ(original, CompileSuccessfully("%2 = OpArrayLength !12 !1 !123"));
+}
+
+TEST_F(ImmediateIntTest, ResultIdInAssignment) {
+  EXPECT_EQ("!2 not allowed before =.",
+            CompileFailure("!2 = OpArrayLength %12 %1 123"));
+  EXPECT_EQ("!2 not allowed before =.",
+            CompileFailure("!2 = !0x00040044 %12 %1 123"));
+}
+
+TEST_F(ImmediateIntTest, OpCodeInAssignment) {
+  EXPECT_EQ("Invalid Opcode prefix '!0x00040044'.",
+            CompileFailure("%2 = !0x00040044 %12 %1 123"));
 }
 
 // Literal integers after !<integer> are handled correctly.